Explaining Haskell IO without Monads

This tutorial explains how to perform IO in Haskell, without attempting to give any understanding of monads. We start with the simplest example of IO, then build up to more complex examples. You can either read the tutorial to the end, or stop at the end of any section - each additional section will let you tackle new problems. We assume basic familiarity with Haskell, such as the material covered in chapters 1 to 6 of Programming in Haskell by Graham Hutton.

IO Functions

In this tutorial I use four standard IO functions:

• readFile :: FilePath -> IO String -- read in a file


• writeFile :: FilePath -> String -> IO () -- write out a file


• getArgs :: IO [String] -- get the command line arguments, from the module System.Environment


• putStrLn :: String -> IO () -- write out a string, followed by a new line, to the console

Simple IO

The simplest useful form of IO is to read a file, do something, then write out a file.

main :: IO ()
main = do
   src <- readFile "file.in"
   writeFile "file.out" (operate src)

operate :: String -> String
operate = ... -- your code here

This program gets the contents of file.in, runs the operate function on it, then writes the result to file.out. The main function contains all the IO operations, while operate is entirely pure. When writing operate you do not need to understand any details of IO. This pattern of IO was sufficient for my first two years of programming Haskell.

Action List

If the pattern described in Simple IO is insufficient, the next step is a list of actions. A main function can be written as:

main :: IO ()
main = do
    x1 <- expr1
    x2 <- expr2
    ...
    xN <- exprN
    return ()

The main function starts with do, then has a sequence of xI <- exprI statements, and ends with return (). Each statement has a pattern on the left of the arrow (often just a variable), and an expression on the right. If the expression is not of type IO, then you must write xI <- return (exprI). The return function takes a value, and wraps it in the IO type.

As a simple example we can write a program that gets the command line arguments, reads the file given by the first argument, operates on it, then writes out to the file given by the second argument:

main :: IO ()
main = do
    [arg1,arg2] <- getArgs
    src <- readFile arg1
    res <- return (operate src)
    _ <- writeFile arg2 res
    return ()

As before, operate is a pure function. The first line after the do uses a pattern match to extract the command line arguments. The second line reads the file specified by the first argument. The third line uses return to wrap a pure value. The fourth line provides no useful result, so we ignore it by writing _ <-.

Simplifying IO

The action list pattern is very rigid, and people usually simplify the code using the following three rules:

1. _ <- x can be rewritten as x.


2. If the penultimate line doesn't have a binding arrow (<-) and is of type IO (), then the return () can be removed.


3. x <- return y can be rewritten as let x = y (provided you don't reuse variable names).

With these rules we can rewrite our example as:

main :: IO ()
main = do
    [arg1,arg2] <- getArgs
    src <- readFile arg1
    let res = operate src
    writeFile arg2 res

Nested IO

So far only the main function has been of type IO, but we can create other IO functions, to wrap up common patterns. For example, we can write a utility function to print nice looking titles:

title :: String -> IO ()
title str = do
    putStrLn str
    putStrLn (replicate (length str) '-')
    putStrLn ""

We can use this title function multiple times within main:

main :: IO ()
main = do
    title "Hello"
    title "Goodbye"

Returning IO Values

The functions we've written so far have all been of type IO (), which lets us perform IO actions, but not give back interesting results. To give back the value x, we write return x as the final line of the do block. Unlike the imperative language return statement, this return must be on the final line.

readArgs :: IO (String,String)
readArgs = do
    xs <- getArgs
    let x1 = if length xs > 0 then xs !! 0 else "file.in"
    let x2 = if length xs > 1 then xs !! 1 else "file.out"
    return (x1,x2)

This function returns the first two command line arguments, or supplies default values if fewer arguments are given. We can now use this in the main program from before:

main :: IO ()
main = do
    (arg1,arg2) <- readArgs
    src <- readFile arg1
    let res = operate src
    writeFile arg2 res

Now, if less than two arguments are given, the program will use default file names instead of crashing.

Optional IO

So far we've only seen a static list of IO statements, executed in order. Using if, we can choose what IO to perform. For example, if the user enters no arguments we can tell them:

main :: IO ()
main = do
    xs <- getArgs
    if null xs then do
        putStrLn "You entered no arguments"
     else do
        putStrLn ("You entered " ++ show xs)

For optional IO you make the final statement of the do block an if, then under each branch continue the do. The only subtle point is that the else must be indented by one more space than the if. This caveat is widely considered to be a bug in the definition of Haskell, but for the moment, the extra space before the else is required.

Break Time

If you've gone from understanding no IO to this point in the tutorial, I suggest you take a break (a cookie is recommended). The IO presented above is all that imperative languages provide, and is a useful starting point. Just as functional programming provides much more powerful ways of working with functions by treating them as values, it also allows IO to be treated as values, which we explore in the rest of the tutorial.

Working with IO Values

The next stage is to work with IO as values. Until now, all IO statements have been executed immediately, but we can also create variables of type IO. Using our title function from above we can write:

main :: IO ()
main = do
    let x = title "Welcome"
    x
    x
    x

Instead of running the IO with x <-, we have placed the IO value in the variable x, without running it. The type of x is IO (), so we can now write x on a line to execute the action. By writing the x three times we perform the action three times.

Passing IO Arguments

We can also pass IO values as arguments to functions. In the previous example we ran the IO action three times, but how would we run it fifty times? We can write a function that takes an IO action, and a number, and runs the action that number of times:

replicateM_ :: Int -> IO () -> IO ()
replicateM_ n act = do
    if n == 0 then do
        return ()
     else do
        act
        replicateM_ (n-1) act

This definition makes use of optional IO to decide when to stop, and recursion to continue performing the IO. We can now rewrite the previous example as:

main :: IO ()
main = do
    let x = title "Welcome"
    replicateM_ 3 x

In an imperative language the replicateM_ function is built in as a for statement, but the flexibility of Haskell allows us to define new control flow statements - a very powerful feature. The replicateM_ function defined in Control.Monad is like ours, but more general, and can be used instead.

IO in Structures

We've seen IO values being passed as arguments, so it's natural that we can also put IO in structures such as lists and tuples. The function sequence_ takes a list of IO actions, and executes each action in turn:

sequence_ :: [IO ()] -> IO ()
sequence_ xs = do
    if null xs then do
        return ()
     else do
        head xs
        sequence_ (tail xs)

If there are no elements in the list then sequence_ stops, with return (). If there are elements in the list then sequence_ gets the first action (with head xs) and executes it, then calls sequence_ on the remaining actions. As before, sequence_ is available in Control.Monad, but in a more general form. It is now simple to rewrite replicateM_ in terms of sequence_:

replicateM_ :: Int -> IO () -> IO ()
replicateM_ n act = sequence_ (replicate n act)

Pattern Matching

A much more natural definition of sequence_, rather than using null/head/tail, is to make use of Haskell's pattern matching. If there is exactly one statement in a do block, you can remove the do. Rewriting sequence_ we can eliminate the do after the equals sign, and the do after the then keyword.

sequence_ :: [IO ()] -> IO ()
sequence_ xs =
    if null xs then
        return ()
     else do
        head xs
        sequence_ (tail xs)

Now we can replace the if with pattern matching, without needing to consider the IO:

sequence_ :: [IO ()] -> IO ()
sequence_ [] = return ()
sequence_ (x:xs) = do
    x
    sequence_ xs

Final Example

As a final example, imagine we wish to perform some operation on every file given at the command line. Using what we have already learnt, we can write:

main :: IO ()
main = do
    xs <- getArgs
    sequence_ (map operateFile xs)

operateFile :: FilePath -> IO ()
operateFile x = do
    src <- readFile x
    writeFile (x ++ ".out") (operate src)

operate :: String -> String
operate = ...

IO Design

A Haskell program usually consists of an outer IO shell calling pure functions. In the previous example main and operateFile are part of the IO shell, while operate and everything it uses are pure. As a general design principle, keep the IO layer small. The IO layer should concisely perform the necessary IO, then delegate to the pure part. Use of explicit IO in Haskell is necessary, but should be kept to a minimum - pure Haskell is where the beauty lies.

Where to go now

You should now be equipped to do all the IO you need. To become more proficient I recommend any of the following:

• Write lots of Haskell code.


• Read chapters 8 and 9 of Programming in Haskell by Graham Hutton. You should expect to spend about 6 hours thinking and contemplating on sections 8.1 to 8.4 (I recommend going to a hospital A&E department with a minor injury).


• Read Monads as Containers, an excellent introduction to monads.


• Look at the documentation on the monad laws, and find where I've used them in this tutorial.


• Read through all the functions in Control.Monad, try to define them, and then use them when writing programs.


• Implement and use a state monad.

Haskell DLL's on Windows

The current section of the GHC manual on creating DLL's on Windows is fairly confusing to read, and has some bugs (i.e. 3605). Since I got tripped up by the current documentation, I offered to rewrite sections 11.6.2 and 11.6.3 (merging them in the process). Creating Windows DLL's with GHC is surprisingly easy, and my revised manual section includes an example which can be called from both Microsoft Word (using VBA) and C++. I've pasted the revised manual section as the rest of this blog post. I'll shortly be submitting it to the GHC team, so any feedback is welcome.

---

11.6.2. Making DLLs to be called from other languages

This section describes how to create DLLs to be called from other languages, such as Visual Basic or C++. This is a special case of Section 8.2.1.2, "Making a Haskell library that can be called from foreign code"; we'll deal with the DLL-specific issues that arise below. Here's an example:

Use foreign export declarations to export the Haskell functions you want to call from the outside. For example:

-- Adder.hs
{-# LANGUAGE ForeignFunctionInterface #-}
module Adder where

adder :: Int -> Int -> IO Int  -- gratuitous use of IO
adder x y = return (x+y)

foreign export stdcall adder :: Int -> Int -> IO Int

Add some helper code that starts up and shuts down the Haskell RTS:

// StartEnd.c
#include <Rts.h>

extern void __stginit_Adder(void);

void HsStart()
{
   int argc = 1;
   char* argv[] = {"ghcDll", NULL}; // argv must end with NULL

   // Initialize Haskell runtime
   char** args = argv;
   hs_init(&argc, &args);

   // Tell Haskell about all root modules
   hs_add_root(__stginit_Adder);
}

void HsEnd()
{
   hs_exit();
}

Here, Adder is the name of the root module in the module tree (as mentioned above, there must be a single root module, and hence a single module tree in the DLL). Compile everything up:

$ ghc -c Adder.hs
$ ghc -c StartEnd.c
$ ghc -shared -o Adder.dll Adder.o Adder_stub.o StartEnd.o

Now the file Adder.dll can be used from other programming languages. Before calling any functions in Adder it is necessary to call HsStart, and at the very end call HsEnd.

NOTE: It may appear tempting to use DllMain to call hs_init/hs_exit, but this won’t work (particularly if you compile with -threaded).

11.6.2.1. Using from VBA

An example of using Adder.dll from VBA is:

Private Declare Function Adder Lib "Adder.dll" Alias "adder@8" _
      (ByVal x As Long, ByVal y As Long) As Long

Private Declare Sub HsStart Lib "Adder.dll" ()
Private Declare Sub HsEnd Lib "Adder.dll" ()

Private Sub Document_Close()
HsEnd
End Sub

Private Sub Document_Open()
HsStart
End Sub

Public Sub Test()
MsgBox "12 + 5 = " & Adder(12, 5)
End Sub

This example uses the Document_Open/Close functions of Microsoft Word, but provided HsStart is called before the first function, and HsEnd after the last, then it will work fine.

11.6.2.2. Using from C++

An example of using Adder.dll from C++ is:

// Tester.cpp
#include "HsFFI.h"
#include "Adder_stub.h"
#include <stdio.h>

extern "C" {
    void HsStart();
    void HsEnd();
}

int main()
{
    HsStart();
    // can now safely call functions from the DLL
    printf("12 + 5 = %i\n", adder(12,5))    ;
    HsEnd();
    return 0;
}

This can be compiled and run with:

$ ghc -o tester Tester.cpp Adder.dll.a
$ tester
12 + 5 = 17

Please give feedback in the comments.

Reviewing View Patterns

View Patterns are an interesting extension to the pattern matching capabilities of Haskell, implemented in GHC 6.10 and above. After using view patterns in real world programs, including HLint, I've come to like them. I use view patterns in 10 of the 27 modules in HLint.

View Pattern Overview

My intuitive understanding of view patterns is given in my Approaches and Applications of Inductive Programming 2009 paper, which describes the view pattern translation as:

f (sort -> min:ascending) = ...
    ==
f v_1 | min:ascending <- sort v_1 = ...
    ==
f v_1 | case v_2 of _:_ -> True ; _ -> False = ...
    where  v_2 = sort v_1 ; min:ascending = v_2

The view pattern on the first line sorts the list elements, then binds the lowest element to min and the remaining elements to ascending. If there are no elements in the list then the pattern will not match. This can be translated to a pattern guard, which can then be translated to a case expression. This translation does not preserve the scoping behaviour of the variables, but is sufficient for all my uses of view patterns. It is important to note that the translation from view patterns to pattern guards is fairly simple, and mainly eliminates one redundant intermediate variable. However, the translation from pattern guards to case expressions and guards is substantially harder.

How I Use View Patterns

My uses of view patterns seem to fall into a few distinct categories. Here are some example code snippets (mainly from HLint), along with explanation.

1) Complex/Nested Matching

uglyEta (fromParen -> App f (fromParen -> App g x)) (fromParen -> App h y) = g == h

Every operation/match pair in a pattern guard requires a separate pattern guard, whereas view patterns can be nested very naturally. Here the abstract syntax tree for expressions has brackets, and the fromParen function unwraps any brackets to find the interesting term inside. View patterns allow us to perform nested matches, which would have required three separate pattern guards.

2) Matching on a Different Structure

isAppend (view -> App2 op _ _) = op ~= "++"

The expression tree I use in HLint has lots of expressions which apply a function to two arguments - for example App (App (Var f) x) y and InfixOp x f y. I have a type class View that maps expressions into the data type data App2 = NoApp2 | App2 String Exp Exp, allowing easy matching on a range of expressions.

3) Safe Normalisation

dismogrify (simplify -> x) = .... x ....

While working with Yhc Core for the Catch and Supero tools I often wanted to process a syntax tree after simplifying it. If you name the original tree x, and the simplified tree y, then it's an easy (and type-safe) mistake to use x instead of y. To avoid this I wrote:

dismogrify bad_x = .... x ....
   where x = simplify bad_x

Using bad_x in the expression makes the mistake easy for a human to spot. Using a view pattern makes the mistake impossible.

4) Mapping

classify (Ident (getRank -> x)) = ...

Sometimes I want to take a variable in one domain, and work with it in another. In the above example getRank converts a String to a Rank enumeration. Within the classify function I only wish to work with the rank as an enumeration, so it's convenient to never bind the string. This pattern is similar to safe normalisation, but it's purpose isn't safety - just making things a little neater.

5) Abstraction

The view pattern example in the GHC manual is all about abstraction. I have mainly used HLint in programs which don't use abstract data types, just algebraic data types which are intended to be manipulated directly. I don't think there are many data types which are both abstract and have a structural view, so I suspect this use will be less common (Data.Sequence is the only type that comes to mind).

Improvements I Suggest

I think there are three improvements that should be made to the view patterns in GHC 6.10.4:

1) Warnings

In GHC 6.10 all view patterns are incorrectly considered overlapping (see bug #2395), so all users of view patterns need to supply -fno-warn-overlapping-patterns. This problem has been fixed in GHC 6.12, which is great news.

2) Scoping

The current scoping behaviour seems undesirable:

apply (f -> y) = ...
    where f = ...

Here the f in the view pattern isn't the f bound at the where. I suggest that the lhs of the -> can use variables from the where, in a similar manner to pattern guards. (It's possible this suggestion is misguided, as the scoping rules can be quite subtle.)

3) Implicit Patterns

The original view patterns wiki document asks what should become of (-> ...), and proposes it become (view -> ...). I like this idea as HLint already contains 12 instances of (view -> ...). The only question is which view should be used? I think there are two possible answers:

a) The view currently in scope

If the desugaring is simply to view, then people can select their imports appropriately to choose their view function. This proposal is similar to the rebindable syntax already supported, but in this case may be a legitimate default, due to several possible view interpretations. If one day everyone starts using Data.View.view, then the default could be switched. As an example (in combination with proposal 2) we could have:

uglyEta (-> App f (-> App g x)) (-> App h y) = g == h
    where view = fromParen

b) Data.View.view

In HLint I have used:

class View a b where
    view :: a -> b

I haven't needed any functional dependencies, as the matching always constrains the types sufficiently. I have mapped one source type (i.e. Exp) to several matching types (App2 and App1), but never mapped multiple source types onto one matching type. If I was to add a dependency it should be that b uniquely determines a, as usually b will have a pattern on the RHS which will constrain b already.

I think my preference is for using Data.view.view, primarily because all other Haskell syntax is bound to a fixed name, rather than using the name currently in scope. However, my opinions on functional dependencies should be taken with skepticism - I'm not normally a user of functional dependencies.

4) Rejected Suggestions

I do not support the idea of implicit view patterns without some leading syntax (see bug 3583) - view patterns are nice, but I don't think they are important enough to be first-class, like they are in F# (note that F# interoperates with OO languages, so first-class view patterns are much more reasonable there).

I also do not support the idea of implicit Maybe in view patterns - Maybe should not be special, and this suggestion doesn't seem to fit with the rest of Haskell.

Conclusion

View patterns are a nice enhancement to pattern guards, increasing their compositionality and reducing the need for redundant intermediate variables. I could live without view patterns, but I don't think I should have to - the design is good, and they fit in nicely with the language. As for pattern guards, I consider them an essential part of the Haskell language that really makes a substantially difference to some pieces of code that would otherwise be rather ugly.

Edit: Fix as per Christophe's comment.

How I Use HLint

HLint is a tool for automatically suggesting improvements to your Haskell code. This post describes how I use HLint, and provides and some background on its development. Before reading this article, if you are an active Haskell programmer who has not yet tried out HLint, I suggest you perform the following steps:

cabal update && cabal install hlint
cd your-current-project
hlint . --report
# open report.html in your web browser

The original purpose of HLint was to help teach beginners. When helping with the functional programming course at York, I used to wander round the students, looking at their code, and suggesting improvements. After three years helping with the same course, I found myself regularly suggesting the same improvements. For example, the pattern if a then True else b came up a lot, which can be written more succinctly as a || b. Of course, having turned myself into a pattern recognition tool, the obvious step was to automate myself - and HLint is the result.

I am no longer at a University, and so the way I use HLint has changed. Often on the Haskell Cafe mailing list people ask for code reviews - intermediate level Haskellers trying to gain knowledge from those around them. The suggestions resulting from a code review are often split into two categories. There are small-scale suggestions about things such as using a better library function, and large-scale suggestions about what the structure of the program should be. Often it is useful to tackle the small-scale issues, tidying and polishing what is already there, before investigating any large-scale issues. Unfortunately reviewers are often short of time, so they may not get round to making large-scale suggestions. The hope is that HLint can automate much of the small-scale suggestions, allowing clever people to use their time more effectively on the more complex problems.

Another reason to use HLint is one of developer pride. Some developers do not react well to criticism, and take comments about their code in a very personal way. Worse still, if you declare that some small syntactic pattern is the "wrong way to do it", then you can inadvertently end up just point out the failings. In contrast, if HLint is run first, then the human suggestions are typically deeper, and are design trade-offs that can be debated.

HLint is not designed as a tool to fix existing code, but more as a tool to promote learning, thus pre-emptively fixing future code. I do not intend people to slavishly apply the hints given by HLint - each hint should be carefully considered. For example, the darcs project uses HLint, but has decided that they are not interested in eta reduction hints, so have used HLint's ignoring facility.

One use of HLint is to provide an easy mechanism to start participating in an open source project. One of the largest hurdles in project participation is writing your first patch. Many projects have different conventions and requirements, plus there is usually a large code base that needs to be learnt. A good first step might be to run HLint over the code. While many of the hints suggested by HLint might be design decisions, or minor issues, there are likely to be a few more unambiguous improvements. As a simple example, taking the xmonad code base and applying HLint shows that the import Data.Maybe statements in XMonad\Core.hs could be combined. This would be a perfect first patch for a budding xmonad developer.

HLint can be used in many ways, but my two golden rules for HLint usage are:

1. Do not blindly apply the output of HLint


2. Never review code that hasn't had HLint applied

Draft paper on Derive, comments wanted

It's been a long time since I last blogged (about 3 months). Since then I've had a paper on Firstify accepted in to the Haskell Symposium (I'll post the final version to my website shortly). I've also been writing a paper on Derive to go with my invited talk at Approaches and Applications of Inductive Programming (co-located with ICFP this year). I have to submit a final version by the 22nd of June (6 days time), but any comments on this draft would be gratefully received - either add them as comments to this post or send an email to ndmitchell AT gmail DOT com.

Download link: http://community.haskell.org/~ndm/temp/derive_draft.pdf

Title: Deriving a DSL from One Example

Abstract: Given an appropriate domain specific language (DSL), it is possible to describe the relationship between Haskell data types and many generic functions, typically type class instances. While describing the relationship is possible, it is not always an easy task. There is an alternative -- simply give one example output for a carefully chosen input, and have the relationship derived.

When deriving a relationship from only one example, it is important that the derived relationship is the intended one. We identify general restrictions on the DSL, and on the provided example, to ensure a level of predictability. We then apply these restrictions in practice, to derive the relationship between Haskell data types and generic functions. We have used our scheme in the Derive tool, where over 60% of type classes are derived from a single example.

Home page: http://community.haskell.org/~ndm/derive/

Darcs repo: http://community.haskell.org/~ndm/darcs/derive

The work presented in this paper will become the basis of Derive 2.0. Many thanks for any comments!

Concise Generic Queries

A few weeks ago David Miani asked how to write concise queries over a data type. The answer is certainly generic programming, a technique that I feel is underused in the Haskell community. I suggested David look at Uniplate, but he found greater success with SYB. Sean Leather gave a solution using EMGM. One of the advantages of Uniplate is conciseness, so I decided to tackle the same problem and compare.

A full description of the task, including data type definitions, is at Sean's blog. From a data type representing structured files (tables, headings, paragraphs) find a heading with a particular name then within that heading find a paragraph starting with "Description". The rest of this post contains solutions using Uniplate, EMGM (taken from Sean) and SYB (from David). The SYB solution is slightly different from the EMGM or Uniplate solutions, but they all do roughly the same generic operations. It is entirely possible that the EMGM/SYB solutions could be improved, but that is a job for other people.

Uniplate Solution

The Uniplate solution is:

projDesc :: String -> OrgFileP -> Maybe String
projDesc name p = listToMaybe [y |
 OrgHeadingP _ x ys <- universeBi p, name == x,
 ParagraphP y <- universeBi ys, "Description" `isPrefixOf` y]

The code can be read as:

• Line 1: Type signature, given a name and a file, return the paragraph if you find one


• Line 3: Find a heading with the right name


• Line 4: Find a paragraph below that heading, whose name starts with "Description"


• Line 2: Pick the paragraph

I find this code to be a clear, concise and simple description of the problem. The thought process to come up with the solution was as follows: You want to search, or perform a query. The first question is whether this is a deep (all nodes) or shallow (just the children) query - David doesn't say but the example seems to imply deep. If it's deep use universeBi. Operations are combined with a list comprehension that finds an element, check it has the necessary properties (the name), then performs more operations. The result is the code you see above.

EMGM Solution

Sean's solution can be found at his blog:

projDesc :: String -> OrgFileP -> Maybe String
projDesc name file = do
  hdg <- G.firstr (headings name file)
  para <- firstPara hdg
  if para =~ "Description" then return para else Nothing

headings :: String -> OrgFileP -> [OrgHeadingP]
headings name = filter check . G.collect
  where
    check (OrgHeadingP _ possible _) = name == possible

firstPara :: OrgHeadingP -> Maybe String
firstPara hdg = paraStr =<< G.firstr (G.collect hdg)
  where
    paraStr (ParagraphP str) = Just str
    paraStr _                = Nothing

This solution isn't bad, but is more verbose than the Uniplate solution. Perhaps it could be rewritten with list comprehensions? It seems that G.collect is similar to universeBi - although I am not sure.

SYB Solution

David's SYB solution can be found here along with another solution using different combinators.

eitherOr :: Either a b -> Either a b -> Either a b
eitherOr x@(Right _) _ = x
eitherOr  _ y  = y

getP14Desc :: OrgElement -> Either ErrString String
getP14Desc org = everything eitherOr (Left descError `mkQ` findDesc) =<<
                 everything eitherOr (Left findError `mkQ` findP14) org
    where
      findP14 h@(Heading {headingName=name})
          | name == "Project14" = Right h
      findP14 _ = Left findError

      findDesc (Paragraph {paragraphText=text})
          | text =~ "Description" = Right text
      findDesc _ = Left findError

      descError = "Couldn't find description for project"
      findError = "Couldn't find project."

Summary

The relative merits of each solution are highly subjective, but I believe the Uniplate solution is concise. The Uniplate solution is a simple translation of the problem, without any clever steps, so hopefully other users (who didn't write the library!) will be able to achieve similar results. The Uniplate solution required only one function from the Uniplate library, so has a small learning curve. Even if you don't choose Uniplate, generic programming techniques are very useful, and can make your code concise and robust.

Website move

Today I spotted that I could no longer push to my darcs repos hosted at York University. A little more checking showed that my home page had also been removed - I guess that's what happens when you are no longer a student there (although a warning email before would have been nice...). So I am pleased to announce my new website address:

http://community.haskell.org/~ndm/

Thanks to the wondrous Haskell community for providing all the resources I needed to move my website with no human intervention at haste. Expect my darcs repos to move somewhere shortly too.

I have now submitted the final bound copies of my thesis, and have uploaded a copy to my website (I had uploaded it to York, but didn't get chance to announce it!). I should say a great thank you to everyone who helped with my work/thesis, in particular Colin Runciman for supervising me for six years, and Detlef Plump and Simon Peyton Jones for examining me and really helping improve the final document with their comments.

The thesis has four content chapters, corresponding to Uniplate, Supero, Firstify and Catch. I have submitted a paper to ICFP 09 which expands/clarifies the Firstify work, which I'll upload as a draft shortly. For the other chapters, the version in the thesis is an improvement on the version in any papers I've published.

Hoogle package search

Recently on the Haskell mailing list there has been some discussions of which packages Hoogle searches by default. One person remarked that it was unfortunate that the network package isn't searched by default. There are lots of packages on Hackage, and Hoogle needs to decide how to cope with so much choice. There are a number of questions that I need to answer in Hoogle:

1. What packages should Hoogle search by default? All of hackage? The base libraries? Only the packages a user has installed? Only packages that make it in to the Haskell Platform?


2. What groups of packages should Hoogle have available? Each package individually? All packages which compile on Windows? All packages by a certain author? All packages whose minor version number is even?


3. What UI should Hoogle show? Should there be checkboxes for each os's package? Should their be a checkbox for each compiler/version? Should their be no UI but some documentation?

And these questions present a number of trade offs:

• The packages have to be divided under sensible and clear lines - I don't want to (and shouldn't) arbitrate divisions like "good" or "popular".


• The more packages you search, the less relevant the results will be.


• The fewer packages you search, the more chance that you miss something.


• The more UI that is added the more confusing things get.


• My development time for Hoogle derives Bounded, Finite and increasingly also derives Small.

Thoughts and suggestions are very welcome. I've set up a wiki page to track peoples thoughts, please make your view and arguments known: http://haskell.org/haskellwiki/Hoogle/Packages.

(As an aside, I recently found that dolphin friendly tuna is actually really harmful to the environment, far more harmful than dolphin unfriendly tuna. Read more here.)

Monomorphism and Defaulting

Haskell has some ugly corners - not many, but a few. One that many people consider exceptionally ugly is the monomorphism restriction. In this post I'm going to discuss three related issues - Constant Applicative Forms (CAFs), the monomorphism restriction and defaulting. But before we start, lets take a simple example.

Computing Pi

Haskell already provides the pi function which represents the value of pi, but lets assume it didn't. Taking a quick look at Wikipedia we can see that one way of computing Pi is the Gregory-Leibniz series. We can calculate pi as:

pi = (4/1) + (-4/3) + (4/5) + (-4/7) + (4/9) + (-4/11) ...

So let's write that as a Haskell program:

pie = sum $ take 1000000 $ zipWith (/) (iterate negate 4) [1,3..]

Here the constant 1000000 gives the accuracy of our approach, increasing this value will give a higher precision. As it currently stands, the Haskell library says pi = 3.14159265358979 and our program says pie = 3.14159165358977. Thirteen matching digits should be suffient for most uses of pi :-)

CAFs

The disadvantage of our pie function is that (under Hugs) it takes about 4 seconds to evaluate. If we are performing lots of calculations with pi, calculating pie each time will be a serious problem. CAFs are the solution!

A CAF is a top-level constant, which doesn't take any arguments, and will be computed at most once per program execution. As a slight subtlety, if the constant has class constraints on it (i.e. is Num a => a, instead of a) then it isn't a CAF because the class constraints act like implicit arguments. Our pie function above doesn't take any arguments, so is a CAF.

Defaulting

While pie doesn't have any class constraints, the right-hand side of pie does! Take a look in Hugs:

Main> :t sum $ take 1000000 $ zipWith (/) (iterate negate 4) [1,3..]
:: (Enum a, Fractional a) => a

Main> :t pie
:: Double

The right-hand side works for any Enum and Fractional type, for example Float, but pie is restricted to Double. The reason is the defaulting mechanism in Haskell - if a type can't be nailed down precisely, but is one of a handful of built-in classes, then it will default to a particular type. This feature is handy for working at an interactive environment, but can sometimes be a little unexpected.

Monomorphism restriction

Without defaulting the compiler would infer the type of pie as ::(Enum a, Fractional a) => a. However, such a definition would be rejected by the monomorphism restriction. The monomorphism restriction states that a function with no explicit arguments, but with class constraints, must be given a type annotation. This rejects functions like:

snub = sort . nub

To fix the problem there are two solutions:

snub i_hate_the_evil_mr = (sort . nub) i_hate_the_evil_mr

snub :: Ord a => [a] -> [a]
snub = sort . nub

For a function like pie only the second approach is applicable. The addition of dummy arguments to avoid the monomorphism restriction is sufficiently common that the HLint tool never suggests eta-reduction if the argument is named mr.

Conclusion

So why was the monomorphism restriction first introducted? For a function with no explicit arguments, the programmer might think they had written a CAF, but class constraints may substantially degrade the performance. Defaulting reduces the number of cases where the monomorphism restriction would otherwise bite, but it is still useful to be aware of the ugly corners.

There are proposals afoot to remove the monomorphism restriction and to increase the power of the default mechanism - hopefully both will be included in to Haskell'.

Small scripts with Haskell

Normally I give blog posts detailing the fun, interesting or advanced stuff I do with Haskell. But that isn't a real representation of my programming life! Most of the time I am doing small scripts that do little tasks, so I thought I'd describe one of those. This post is written as Literate Haskell, which means you can save the whole contents as a .lhs file and run it in GHCi or Hugs.

The task I had to complete was to take a directory of files, and for each file foo.txt generate the files foo_m1.txt to foo_m3.txt, where each one file is a block of lines from the original delimited by a blank line. i.e. given the file with the lines ["","1","1","","2","","3"], the numbers "1" would go in foo_m1.txt etc.

This blog post isn't how I actually wrote the original script - I didn't use literate Haskell (since I find it ugly), I didn't give explicit import lists (since they are needlessly verbose), I didn't give type signatures (but I should have) and I didn't split the IO and non-IO as well (but again, I should have). It is intended as a guide to the simple things you can easily do with Haskell. Now on to the code...

> import System.FilePath(takeExtension, dropExtension, (<.>), (</>))
> import System.Directory(getDirectoryContents)
> import Data.Char(isSpace)
> import Control.Monad

First, let's import some useful modules. To find more about a particular function just use Hoogle and search for it, but a quick summary:

takeExtension "foo.txt" = ".txt"
dropExtension "foo.txt" = "foo"
"foo" <.> "txt" = "foo.txt"
"bar" </> "foo.txt" = "bar/foo.txt"
getDirectoryContents "C:\Windows" = running "dir C:\Windows" at the command prompt
isSpace ' ' = True

Every Haskell program starts with a main function, which is an IO action. For this program, we are going to keep all the IO in main, and only use other pure functions. With most file processing applications its best to read files from one directory, and write them to another. That way, if anything goes wrong, its usually easy to recover. In this case we read from "data" and write to "res".

> main :: IO ()
> main = do
>     files <- getDirectoryContents "data"
>     forM_ files $ \file -> when (takeExtension file == ".txt") $ do
>         src <- readFile $ "data" </> file
>         forM_ (zip [1..] (splitFile src)) $ \(i,x) ->
>              writeFile ("res" </> dropExtension file ++ "_m" ++ show i <.> "txt") x

Or in some kind of pseudo-code:

main =
    set files to be the list of files in the directory "data"
    for each file in files which has the extension ".txt"
    {
        set src to be the result of reading the file
        for each numbered result of splitFile
        {
            write out the value from splitFile to the location "res/file_m#.txt"
            where # is the 1-based index into the list of results
        }
    }

We can now move on to the pure bits left over. We want a function splitFile that takes a file, and splits it in to three chunks for each of the blocks in the file. When processing text, often there will be stray blank lines, and the term "blank lines" will also apply to lines consisting only of spaces. The code is below:

> splitFile :: String -> [String]
> splitFile xs = map (tabify . unlines) [s1,s2,s3]
>     where
>         xs2 = dropWhile null $ map (dropWhile isSpace) $ lines xs
>         (s1,_:rest) = break null xs2
>         (s2,_:s3) = break null $ dropWhile null rest

And now presented more as a list of steps:

• split the text in to lines


• for each line drop all the leading spaces from it


• drop all the leading blank lines


• break on the first empty line, the bits before are chunk 1


• drop all leading blank lines for the rest


• break on the first empty line in the rest, before is chunk 2, after is chunk 3


• for each of the chunks, put the lines back together, then tabify them

The tabify requirement was added after. The person decided that all continuous runs of spaces should be converted to tabs, so the file could better be loaded in to a spread sheet. Easy enough to add, just a simple bit of recursive programming:

> tabify (' ':xs) = '\t' : tabify (dropWhile (== ' ') xs)
> tabify (x:xs) = x : tabify xs
> tabify [] = []

And again in English:

• if you encouter a space, drop it and all successive spaces, and write out a tab


• otherwise just continue onwards

Haskell is a great language for writing short scripts, and as the libraries improve it just keeps getting better.

FsCheck changes

Kurt Schelfthout has just released FsCheck 0.4, a tool similar to QuickCheck but for F#. While working at my internship for Credit Suisse I spent a little bit of time modifying FsCheck to include automatic generators (so you don't have to describe how to generate arbitrary values) and failure shrinking (so the counter-examples are smaller). Both these changes have now been incorporated in to the main FsCheck tool. It is really nice to see the work being contributed back, and that big companies are taking the time to get the necessary legal clearance etc.

I find shrinking to be a particularly potent feature. In one real-world task I struggled to debug a test failure for 8 hours, before shrinking was available. Attacking the same example with FsCheck and shrinking made the reason for the test failure immediately obvious.

mapM, mapM_ and monadic statements

In my last post on F# I mentioned that do mapM f xs; return 1 caused a space leak, and that the programmer should have written mapM_. I also proposed that monadic statements should work more like in F# where non-unit return values can't be ignored. Various people seemed to misunderstand both points, so I thought I'd elaborate.

mapM as a Space Leak

First, I should clarify what I understand as a space leak. A space leak is not a memory leak in the C sense. A space leak is when a computation retains more live memory than necessary for some period of time. One sign of a possible space leak is that lots of memory is retained by garbage collection.

Comparing mapM and mapM_ on the following program:

main = do
    mapM* putChar (replicate 10000 'a')
    return ()

The mapM_ variant has a maximum heap residency of 2Kb, while the mapM variant has 226Kb. Given an input list of length n, the residency of mapM_ is O(1), while mapM is O(n).

The exact reasons for the space leak are quite detailed, and I'm not going to attempt to cover them. My intuition is that the return list is wrapped in the IO monad, and therefore can't be deallocated until the IO action finishes. In summary, unless you are going to use the end result of a monadic map, always use mapM_.

Monadic Statements

In the above example it would be nice if the compiler had complained. You generated a value, but you didn't use it. Fortunately, it is a very easy fix - change the type of monadic bind (>>) :: Monad m => m a -> m b -> b to Monad m => m () -> m b -> m b. Now, if a monadic statement generates a value that isn't (), you get a type error. The above examples with mapM would be rejected by the type checker.

But what if we really wanted to call mapM? There are two options. The first is to bind the result, for example do _ <- mapM f xs; return 1. The second option, which F# favours, is do ignore $ mapM f xs ; return 1, with the auxiliary ignore :: Monad m => m a -> m (). I prefer the second option, as it clearly states that you want to ignore the result of a computation. You could even write a rule ignore . mapM f == mapM_ f.

F# from a Haskell perspective

I've recently started a full-time job at Standard Chartered. Before that I was doing an internship with Credit Suisse, where I spent a reasonable amount of time doing F# programming. Before I started F# I had 6 years of Haskell experience, plenty of C# experience, but little exposure to ML. I've now had 3 months to experiment with F#, using an old version (the one before the latest Community Technology Preview) and here are my impressions.

F# is a functional language from Microsoft, previously a Microsoft Research language, which is moving towards a fully supported language. F# is based on ML, and some (perhaps many) ML programs will compile with F#. At the same time, F# has complete access to the .NET framework and can interoperate with languages such as C#. F# is a hybrid language - at one extreme you can write purely functional ML, and at the other extreme you can write imperative C#, just using a different syntax. F# seems to be designed as a practical language - it isn't elegant or small, but does interoperate very nicely with every .NET feature.

The Language

The F# language is based on ML, with the addition of indentation based layout, and many of the weaknesses in F# come from ML. F# is certainly more verbose than Haskell: in some places you need an extra keyword (often a let); pattern matching is not nearly as complete; the indentation isn't as natural as Haskell. However there are some nice syntactic features in F# that are not in Haskell, including generalised list/array/sequence comprehensions and active patterns.

The type checker in F# is powerful, but unpredictable. I often get surprised by where type annotations need to go, particularly when working with .NET object types. The tuple type is treated specially in many cases, and this also leads to surprise - inserting or removing a pair of brackets can effect the type checker. Much of this complexity is necessary to manage the interaction with .NET, but it does complicate the language. Unfortunately, even with the advanced type features in F#, there are no type classes. The lack of type classes precludes the standard implementation of things such as Uniplate and QuickCheck. However, F# does have some nice reflection capabilities, and often entirely generic implementations can be given using reflection. There is certainly an interesting design trade-off between reflection based operations and type classes, something I have looked at in the past and hope to explore again in future.

F# is an impure language, which offers some practical benefits from Haskell, but also encourages a less functional style. In Haskell I sometimes work within a localised state monad - F# makes this much more natural. The impurity also allows simple interaction with .NET. Having programmed with an impure language I did find myself reaching for localised state much more often - and was occasionally tempted into using global state. In most cases, this state became problematic later. Before using F# I thought purity was a good thing, now I'm convinced that purity is a good thing but that impurity is often very useful!

Haskell could learn some things from F#. Every statement in F# must either be bound to a value or evaluate to (). In Haskell it is possible to write do mapM f xs; return 1. Any experienced Haskell programmer should spot that the mapM is a space leak (it should be mapM_), but the type system doesn't enforce it. In F# the type system does. The change in Haskell is simple, and in my opinion, desirable.

The Platform

F# is a fully fledged member of the .NET platform. You can write a class in VB.NET, write a derived class in F#, and then derive from that class in C#. All the languages can produce and consume the same libraries. This integration with .NET allows companies that already use the Microsoft tools to easily migrate - even on a function by function basis. However, the combination of both an imperative framework and a functional language at some times leads to confusing choices. All of the standard .NET libraries work with arrays, but for a functional program the list is a more natural type. F# provides both, and it was never clear which I should use where, leading to lots of conversions. The .NET libraries are very powerful, but often are overly imperative. For example, the XSD libraries (Xml Schema Description) are very imperative - you have to create objects, mutate properties, then make calls. However, in Haskell, I probably wouldn't have had any XSD support, certainly nothing as well-supported as in .NET.

The Tool Chain

The most impressive aspect of F# is the integration with the Visual Studio environment. F# contains a debugger, profiler, auto-completion, identifier lookup and many other tools. While other functional languages have some of these tools, the Visual Studio environment tends to have very refined and polished implementations. The integration with F# is sometimes a little fragile, or at least was in the version I was using, but the tools are already very powerful and are likely to continue to improve.

Overall

The F# language isn't the most beautiful language ever, but it's not bad. The integration with .NET is incredible, and while this requires compromises in the language, the benefits are considerable. I still prefer Haskell as a language, but for many users the tool chain is a more important consideration, and here F# excels.

This post was brought to you by Ctrl and v, as the computer I am currently using doesn't have a # key!

Update: I'd recommend reading Vesa Karvonen's comment below - he has additional perspectives on F# from a more ML perspective.

Enabling Reply To All in Outlook

Update: See http://neilmitchell.blogspot.com/2011/12/enabling-reply-to-all-in-outlook.html for an updated version of this functionality.

Some companies lock down the use of Outlook by disabling the Reply To All button. This makes it harder to manage email, and requires manually copying email addresses to get the same effect. But using a bit of Office VBA, it is possible to make a functioning Reply To All button. The following solution has been tested in Outlook 2003, but should work for older versions as well.

First, enable macros in Outlook. Go to Tools, Macro, Security and select Medium or Low security.

Second, add a Reply To All action. Go to Tools, Macro, Visual Basic Editor and put the following code in the text editor.

Option Explicit

Public Sub ReallyReplyAll()
Dim o As MailItem
Set o = Application.ActiveExplorer.Selection.Item(1)
o.ReplyAll.Display
End Sub

Finally, add a toolbar button to invoke the action. Go to Tools, Customise, Commands, Macros, and drag and drop the command Project1.ThisOutlookSession.ReallyReplyAll on to the toolbar. You can put this command exactly where you used to have Reply To All, and give it the same icon/name.

To test, select an email and click on the button you just added, it should do exactly what Reply To All would have done. There are some minor limitations to this method:

• 
  The button will not disable itself when it isn't applicable, i.e. when there are no emails selected. You will still be able to click on the button, but it won't do anything.
  
• 
  If you select a medium level of macro security, you will have to go through a security confirmation the first time you click Reply To All in an Outlook session.
  

If possible, try to educate the person in charge that Reply To All is perfectly good email etiquette, and that people should be trusted to use it responsibly. However, if that fails, the above method is a useful fallback.

General Updates

It's been a little while since I last posted. I've recently got back from ICFP 2008, and quite a few people asked me what I was doing now. I've also got a few comments on a a few other things. The following as a section of disjointed paragraphs on a variety of topics, both academic and personal.

Catch Talk

I gave a talk at the Haskell Symposium, about Catch. A video of the talk is now online.

Generics Talk

Alexey gave a talk about generic programming libraries at the Haskell Symposium. I was particularly interested in this talk as it is somewhat like a competition between libraries, where Uniplate is one of the competitors. One thing I noticed is that the Uniplate version of the SYB example in the talk can be written as one single lexeme, namely uniplateBi. The talk was much more about generics libraries, while Uniplate is probably more accurately described as a traversal library, so issues such as conciseness of code were left out. One thing I did disagree with from the talk was the assertion that Uniplate requires Template Haskell and Data/Typeable deriving. In reality Uniplate requires neither, but if they are present, then you have the option of using them to write even less code.

From a combination of the paper and the talk I think its fair to conclude that if Uniplate does what you want, its a pretty good choice. This fits well with the Uniplate philosophy of giving up a small amount of power, to allow a massive simplification, while still being powerful enough for most tasks.

PhD/Work

I've had my PhD viva (passed with minor corrections), and have nearly finished making the minor corrections. I'll update my website with a revised copy of the thesis shortly. I'm currently working at Credit Suisse on a three month internship. I'm not working on Haskell stuff, but instead am doing F# programming. To get a feel for some of the things that are done by Credit Suisse I recommend looking at Ganesh's ICFP talk/paper and Howard's CUFP talk. Disclaimer: Nothing I say on this blog, or anywhere public, has anything to do with Credit Suisse, but are my personal thoughts.

Personal Life

I've just moved to Cambridge, and got engaged to my girlfriend (now fiancee), Emily King. I'll be commuting to Credit Suisse for the next two months.

My Libraries/Tools

Now I'm working full-time (long hours plus a long commute), its hard for me to put the same amount of time into updating and maintaining my Haskell libraries and tools. I will still be accepting patches and answering questions, but probably not fixing too many bugs at any great speed. I'm still maintaining my bug tracker, so feel free to add bugs, fix bugs, or comment on bugs. If anyone has any particular interest in a tool, I'd consider taking on a co-maintainer to reduce some of the maintenance burden.

Well Typed

There is now a Haskell consultancy, Well Typed, comprising of Duncan Coutts and Ian Lynagh. These are two very good Haskell hackers, who are now selling their knowledge and experience. Between them, they've had substantial experience with GHC, Cabal, Hackage, ByteString, TemplateHaskell and numerous Haskell libraries. They've also taught lots of students Haskell, and helped lots of beginners on IRC and mailing lists. If I want help with Haskell, or with the general infrastructure and tools, they are usually the first people I approach. I strongly recommend that anyone needing Haskell help in a commercial environment get in contact with them - they can help you get the most out of Haskell. Disclaimer: I haven't been asked to write this section, and haven't checked with Ian/Duncan first, but I do wish them luck!

Running your own Hoogle on a Web Server

As promised, here is a guide on deploying Hoogle on a web server. Before doing so, you need to generate the necessary Hoogle databases, as described yesterday, and place them in the datadir configured with Cabal. Then:

• Move the hoogle binary to a location where it can act as a CGI binary, perhaps changing its name to index.cgi, if necessary. Configure the CGI program to run, possibly changing the program to be executable or adding settings somewhere.


• Copy the files from src/res in the darcs repo into a res directory located beside the binary.


• Create a file log.txt and give it global write permissions.

Now you should have Hoogle running on a web server! Some of the features, such as OpenSearch integration, won't work - but Hoogle should be usable. If anyone does get Hoogle running on a web server I'd love to hear, any feedback appreciated. In particular, if there are any tweaks required please let me know.

Hoogle Database Generation

Brief Annoucement: A new release of the Hoogle command line is out, including bug fixes and additional features. Upgrading is recommended.

Two interesting features of Hoogle 4 are working with mulitple function databases (from multiple packages), and running your own web server. Both these features aren't fully developed yet, and may change in their use, but can be used with care. This post covers how to generate your own databases, and how the web version databases are generated. Tomorrow I'm going to post on how to run your own Hoogle web server, but you'll need to generate your databases first! I'm going to walk through all the steps to create a database from the filepath library, as an example

Hoogle Databases

A Hoogle database is a set of searchable things, including text and type searching, and has a ".hoo" extension. A database may include the definitions from one package, or from multiple packages. Typically the Hoogle databases installed would include one database for each package (i.e. base.hoo, filepath.hoo), a default database (default.hoo) comprising of all the standard search items, and any number of custom databases (all.hoo) which comprise of different combinations of the other databases.

When using Hoogle, adding +name will include the given database in the search list, and -name will exclude the given package from the search. By default, Hoogle will use default.hoo, but if any +name commands are given then those databases will be used instead.

Hoogle looks for databases in the current directory, in the data directory specified by Cabal, and in any --include directories passed at the command line.

Step 1: Creating a Textbase

A Textbase is a textual representation of a function database. To generate a textbase you need to install the darcs version of Haddock, then use runhaskell Setup haddock --hoogle on your package. For filepath, this will create the file dist/doc/html/filepath/filepath.txt, which is a textbase.

Step 2: Converting a Textbase to a Database

To convert a textbase to a database use the command hoogle --convert=filepath.txt in the appropriate folder. If a package depends on any other packages, then adding +package will allow Hoogle to use the dependencies to generate a more accurate database. In the case of filepath, which depends on base, we use hoogle --convert=filepath.txt +base. This command requires base.hoo to be present.

Adding the dependencies is not strictly necessary, but will allow Hoogle to generate a more accurate database. For example, the base package defines type String = [Char], without the +base flag this type synonym would not be known to Hoogle.

We now have filepath.hoo, which can be used as a search database.

Step 3: Combining Databases

To generate a database comprising of both filepath and base, type hoogle --output=default.hoo --combine=filepath.hoo --combine=base.hoo. By combining databases you allow easy access to common groups of packages, and searching all these packages at once becomes faster than listing each database separately.

Web Version Databases

The web version uses the Hackage tarballs to generate documentation for most of its databases, but also has three custom databases:

• base - the base package is just too weird, and isn't even on hackage. A darcs version and some tweaking is required.


• keyword - the keyword database is a list of the keywords in Haskell, and is taken from the web page on the wiki.


• hackage - the hackage database is a list of all the packages on Hackage, indexed only by the package name.

All the code for generating the web version databases is found in data/generate in the Hoogle darcs repo at http://code.haskell.org/hoogle.

Future Improvements

There are two database related tasks that still need to be done: Cabal integration and indexing all of Hackage.

Bug 80: In the future I would like Hoogle databases to be generated by Cabal automatically on installing a package. Unfortunately, I don't have the time to implement such a feature currently, and even if I did implement it, I'm unlikely to ever use it. If anyone wants to work on this, please get in contact. This is mainly a project working with Cabal.

Bug 79: The other work is to index all the packages on Hackage. The problem here is generating the textbases, once they have been created the rest is fairly simple. However, to run Haddock 2 over a package requires that the package builds, and that all the dependencies are present. Unfortunatley my machine is not powerful enough to cope with the number of packages on Hackage. Hopefully at some point the machinery that builds Haddock documentation for Hackage will also generate textbases, however in the mean time if someone wants to take on the task of generating textbases for Hackage, please get in contact.

Bug Tracker

I'm not working on Hoogle full-time anymore, so am using my bug tracker to keep track of outstanding issues. In order to interact more effectively with my bug tracker, you might want to read this guide. It describes how to vote for bugs etc.

Hoogle New Features

I've now finished my Hoogle Summer of Code work, though I still intend to continue working on Hoogle when I get the chance. Before the coding period expired, I was able to add a number of new features to Hoogle. These features are all available at Hoogle, under http://haskell.org/hoogle/.

More Compact Text Searching

The old text search feature was very fast, using an on disk trie to navigate around the possible matches. The downside to this trie was the space it consumed, about half the database was devoted to it. Fortunately, I came up with an alternative way to get fast text searching (albeit slightly slower), in a lot more compact form.

Much smaller database files also mean much faster database generation, as the time spent in the IO routines is the main bottleneck.

Faster IO routines

I rewrote the underlying binary layer in Hoogle, to make it faster. It's not as fast as I would like, and I think that moving to memory-mapped files is probably a good idea. With these improvements, along with the compact text searching, I am able to generate databases in about 2 seconds (compared to about 20 seconds before).

Database Restricted Searches

Hoogle has been able to run database restricted searches for some time, but now the databases contain enough information to make it practical. By adding +package or -package to the search you can include or exclude certain packages. For example, to find out which map functions are in the containers package try map +containers. To find out which map functions are not in the containers or bytestring packages try map -containers -bytestring. I have also split out the GHC.* modules from base, so if you want to find some unboxed types in GHC's libraries try # +ghc. Note that not all the documentation links work from the GHC modules, I am still trying to fix this.

By default Hoogle searches the following packages: array, base, bytestring, cabal, containers, directory, filepath, haskell-src, hunit, keyword, mtl, parallel, parsec, pretty, process, quickcheck, random, stm, template-haskell, time, xhtml

The "ghc" package is also available if specified with +ghc and includes the GHC.* modules of base only.

Hoogle 3

I have now replaced the default Hoogle with Hoogle 4, but have copied Hoogle 3 to http://haskell.org/hoogle/3. Unfortunately, it doesn't yet work, as I need some admin help. But it will in the next few days, I hope. The only reason I can think of for using Hoogle 3 is Gtk2hs library searching, which I do want to add to Hoogle 4 when possible.

Give Me Feedback

There are quite a lot of enhancements to Hoogle that I still want to make. I have tried to list all these improvements in my bug tracker. If you find a bug, or want some feature, open an issue. If you have a particular interest in a bug, you can star it, to be informed on its progress and to indicate to me that you care.

I'm particularly interested in two pieces of feedback:

I don't use Hoogle 4 because ...

Do you use any type/name search engine? Do you want to still use Hoogle 3? Do you use Hayoo? If you use something else, what feature draws you to it? What do you dislike about Hoogle 4?

I use Hoogle 4, but my life would be nicer if ...

There are many things which effect Hoogle 4 users that I'm not aware of. If you open a bug saying what annoys you (or leave a comment and I'll do it for you) then I can keep track of this information. Even if you don't necessarily see any way to fix the problems, I'd still like to know them.

Thanks for everyone who has given feedback on Hoogle so far, it has been very useful.

GSoC Hoogle: Week 12

This week I've been trying to get Hoogle 4 to the point where it can replace Hoogle 3. This is the final official week of Google Summer of Code, but I'm planning to continue hacking Hoogle next week, and then as time allows after that.

The priority this week was getting the documentation links working. The problem was not with Hoogle - displaying the links is trivial - but ensuring that Cabal + Haddock + Hoogle + random build scripts combine to generate the correct databases. This work involved lots of little changes in lots of places, but is now working properly. Included in this work is dependency tracking of packages (so that all packages using base know that String = [Char] etc), and merging multiple databases to create a single one.

After the Hoogle database was generated correctly, I started looking at using some of the additional information present. I have now added Haddock documentation inline in the search results. If the documentation is too long to fit comfortably, Hoogle uses AJAX wizzy-ness (or more accurately, DHTML) to allow the user to expand and show all the documentation. I suspect that this will eliminate many cases of the user actually following to the Haddock webpages. This feature is fairly new, and I have pushed it out because its useful - there are still many small improvements that need to be made.

This week I also spent some time attempting to generate documentation for all the Hackage libraries. I had some success, but the computer I am currently using is years old and lacks the necessary processing power. I will tackle this at some point in the future, once I have purchased a new machine (which should be quite soon).

With all these changes, I find Hoogle 4 to be significantly more usable than Hoogle 3. Please give it a try, and give feedback. At this point I'm particularly interested in any issues that would cause you to use Hoogle 3 instead of Hoogle 4.

Hoogle 3: http://haskell.org/hoogle

Hoogle 4: http://haskell.org/hoogle/beta

If there are no major issues, I will be replacing Hoogle 4 as the standard Hoogle sometime next week.

Next week: I will be no longer doing Google Summer of Code :-) I plan to refine some of the existing bits of Hoogle, and ensure that anything I haven't done is in a bug tracker for later.

User visible changes: The web search engine now gives Haddock links and displays Haddock documentation inline.

GSoC Hoogle: Week 11

This week I've been releasing lots. Hoogle 4 is finally starting to come together, and should be a worthy replacement for Hoogle 3 very shortly. Rather than go into detail about the past week, I'm just going to give some of the bullet points:

• I have released 4 versions of the command line version of Hoogle, available on Hackage. Many bugs have been spotted by some very useful testers, and improvements have been made.


• I have released a web version of Hoogle 4, and encourage feedback.


• I have started to update the wiki Manual, which now contains some details of Hoogle's query syntax.


• I gave a talk at AngloHaskell 2008, which is available online, as slides and an audio stream. All of the other talks were excellent and are well worth listening to.


• I have started to build Hoogle documentation for all of Hackage. The machine I'm doing this on is very slow, so its not a quick process!

Next week: I'm hoping to work on generating better Hoogle databases, including a Hoogle database for the whole of Hackage. I also have a number of bugs to fix.

User visible changes: Users can download and use Hoogle, and the web interface is online.

Hoogle 4.0 web client preview

Since releasing a command line version of Hoogle 4 yesterday, I've had some useful feedback from a number of people. As a result, I have added a few bugs to the bug tracker, and fixed a few mistakes in the searching and ranking. The Hoogle on Hackage is currently 4.0.0.3 and is a recommended upgrade to all early testers.

I've now written a web interface to Hoogle 4, which has been uploaded to http://haskell.org/hoogle/beta/. This web interface is primarily so people can test searching/ranking without installing anything. There are a number of limitations:

• The links to documentation do not work - this is the most severe problem, and probably stops people permanently changing to the new version.


• The Haddock documentation is not present.


• Some database entries are duplicates.


• The Lambdabot says feature is missing.


• The Suggestion feature is incomplete.


• The AJAX style client features are not present.

The first three issues are fixed in Hoogle, but need various support through Haddock and Cabal to work. Other than these limitations, I am very interested in hearing what people think. As before, particularly regressions from Hoogle 3 or poor results/ranking.

Hoogle 4.0 release (beta, command line)

I am pleased to announce Hoogle 4.0, available on Hackage. A couple of things to note:

• This is a release of the command-line version only. It will have identical searching abilities to the web-based version, which I'm about to write.


• It currently only searches the same packages as Hoogle 3 (the final release will search more).


• It currently doesn't support the --info flag as previously described (problems with Haddock, not with Hoogle).

Walkthrough: Installation

If you have cabal-install available, it should be as simple as:

$ cabal update && cabal install hoogle

Otherwise, follow the standard Cabal/Hackage guidelines. Hoogle depends on about 4 packages on Hackage which are not available with a standard GHC install, so these will need to be built.

Walkthrough: A few searches

Here are some example searches. I have used --count=5 to limit the number of results displayed. If you are using a terminal with ANSI escape codes I recommend also passing --color to enable colored output.

$ hoogle map --count=5
Prelude map :: (a -> b) -> [a] -> [b]
Data.ByteString map :: (Word8 -> Word8) -> ByteString -> ByteString
Data.IntMap map :: (a -> b) -> IntMap a -> IntMap b
Data.IntSet map :: (Int -> Int) -> IntSet -> IntSet
Data.List map :: (a -> b) -> [a] -> [b]

$ hoogle "(a -> b) -> [a] -> [b]" --count=5
Prelude map :: (a -> b) -> [a] -> [b]
Data.List map :: (a -> b) -> [a] -> [b]
Control.Parallel.Strategies parMap :: Strategy b -> (a -> b) -> [a] -> [b]
Prelude fmap :: Functor f => (a -> b) -> f a -> f b
Control.Applicative <$> :: Functor f => (a -> b) -> f a -> f b

$ hoogle Data.Map.map --count=5
Data.Map map :: (a -> b) -> Map k a -> Map k b
Data.Map data Map k a
module Data.Map
Data.Map mapAccum :: (a -> b -> (a, c)) -> a -> Map k b -> (a, Map k c)
Data.Map mapAccumWithKey :: (a -> k -> b -> (a, c)) -> a -> Map k b -> (a, Map k c)

$ hoogle "Functor f => (a -> b) -> f a -> f b" --count=5
Prelude fmap :: Functor f => (a -> b) -> f a -> f b
Control.Applicative <$> :: Functor f => (a -> b) -> f a -> f b
Control.Monad fmap :: Functor f => (a -> b) -> f a -> f b
Control.Monad.Instances fmap :: Functor f => (a -> b) -> f a -> f b
Data.Traversable fmapDefault :: Traversable t => (a -> b) -> t a -> t b

How you can help

I've released a command line version of the search to solicit feedback. I'm interested in all comments, but especially ones of the form:

• I prefer the command line version of Hoogle 3 because ...


• When I search for ... I would expect result ... to appear, or to appear above result ...


• I was hoping for the feature ...


• It takes too long when I ...

I'm going to be accumulating Hoogle 4 bugs in my bug tracker, or by email (http://www-users.cs.york.ac.uk/~ndm/contact/) - whichever you find more convenient.

Now I'm going to start work on the Web search :-)

GSoC Hoogle: Week 10

This week I've been in Bristol, and am just about to head off to the Harbour Festival. Next week I'm heading off to AngloHaskell 2008, and will be talking about Hoogle type searching on the Saturday.

This week has been type search, yet again. There were issues with algorithmic complexity, combinatorial explosions and other fun stuff. However, its now finished. The type search is now fast enough (you can run Hoogle in Hugs against the core libraries) and gives good results. Rather than describe type searching, its easier to give an example. Searching for (a -> b) -> [a] -> [b] in Hoogle 3 gives:

Prelude.map :: (a -> b) -> [a] -> [b]
Data.List.map :: (a -> b) -> [a] -> [b]
Control.Parallel.S... parMap :: Strategy b -> (a -> b) -> [a] -> [b]
Prelude.scanr :: (a -> b -> b) -> b -> [a] -> [b]
Data.List.scanr :: (a -> b -> b) -> b -> [a] -> [b]
Prelude.scanl :: (a -> b -> a) -> a -> [b] -> [a]
Data.List.scanl :: (a -> b -> a) -> a -> [b] -> [a]
Prelude.concatMap :: (a -> [b]) -> [a] -> [b]

But in Hoogle 4 gives:

Prelude map :: (a -> b) -> [a] -> [b]
Data.List map :: (a -> b) -> [a] -> [b]
Prelude fmap :: Functor f => (a -> b) -> f a -> f b
Control.Applicative <$> :: Functor f => (a -> b) -> f a -> f b
Control.Monad fmap :: Functor f => (a -> b) -> f a -> f b
Control.Monad.Instances fmap :: Functor f => (a -> b) -> f a -> f b
Control.Applicative liftA :: Applicative f => (a -> b) -> f a -> f b
Data.Traversable fmapDefault :: Traversable t => (a -> b) -> t a -> t b
Control.Monad liftM :: Monad m => (a1 -> r) -> m a1 -> m r
Control.Parallel.Strategies parMap :: Strategy b -> (a -> b) -> [a] -> [b]

I think the new results are better. For more details, come to the AngloHaskell talk.

Next Week: I want to release a public beta of Hoogle 4 in command line form. I want to start on the web search engine and tweak the ranking algorithm. I'll also be writing up type search in the form of a presentation.

User Visible Changes: Type search works well and fast.

GSoC Hoogle: Week 9

I'm off camping for the next weekend in a couple of hours, so this is my early weekly summary. From next week, for a week and a half, I'll actually have an SSH connection so expect to see 200+ patches flow into the Hoogle repo in a few days.

This week I've been rewriting the type search. I spent 3 days writing code, type checking it, but not actually having enough written to run it. Late last night I finished the code, and this morning I debugged it. Amazingly (although actually quite commonly for Haskell) it worked with only minor tweaks. I now have a type search which should scale to large databases and provide fast and accurate searches.

All the basic tests work, and I can generate a Hoogle database for the array library. I still can't generate a Hoogle database for the base library, due to a stack overflow, but I think the cause of the stack overflow has changed and should be easy to debug.

Next Week: A public beta of the command line version is now overdue, and hopefully will happen next week. I aim to finish the actual search side of Hoogle, and move on to the web interface.

User Visible Changes: Type search works again, mostly.

GSoC Hoogle: Week 8

This week I've been travelling quite a bit, and rather busy with other things. Hopefully next week I'll be able to focus more time on Hoogle!

This week I fleshed out the final part of type search, including support for instances and alpha renaming of variables. After having implemented all the bits in the type search, I tried to convert the base libraries - and it failed, taking up too much time/memory to feasibly finish.

The type search is based around the idea of having nodes in a graph representing types, and then moving between these nodes, at a cost. In order to avoid a blow-up in the number of nodes in the graph, types are alpha-normalised and then alpha-renaming is performed afterwards. Instead of having 3 type nodes for (a,b), (c,d) and (a,a) there is just one named (a,b) and a 3 sets of alpha-renamings. All is good.

However, once you introduce instance restrictions, the types blow up. For example, from the type node a, you can move to Eq a => a, Ord a => a, Show a => a etc. The large (but feasible) number of type nodes, combined with even a small number of class names, gives a huge number of nodes. In fact, for every type variable in a node there are 2^n possible instance contexts it could take. All is bad.

Fortunately there is a solution - move instance checking outside the type graph. This makes the number of nodes feasible, and should work fairly well. It also has a few other benefits, including slightly better scoring and a simpler implementation in a few places. I also came up with a strategy for moving the cost associated with alpha-renaming into the graph search, which further simplifies things.

Of course, all this work takes time, so overall progress is slower than I would have liked. However, the results so far are promising, and the problems of scale seem to have been successfully addressed. The problem of fast and accurate type searching is hard, but hopefully Hoogle 4 will have a scalable solution that should be useful.

Next week: I want to finish the implementation of type searching, and check it works on the full base libraries. A release would be good, although may take place early in the following week.

User visible changes: Creating a database for the base library will now fail with a stack overflow. Hopefully next weeks changes will fix this!

GSoC Hoogle: Week 7

This week I've continued to improve the type searching, and generated Hoogle databases for the core libraries. I'm away from a computer all weekend until Tuesday evening, which has happened every ICFP contest for the last 3 years.

I've substantially refactored the type searching, basing it on a proper abstract Graph data type. Now the mechanisms for dealing with type search and graph traversal are separate it is much easier to express clearly what type search is doing. I've also fleshed out the type searching code so that it can accurately perform searches with all the necessary features. There are still a number of tasks to do before the type searching code is finished, but each is a fairly discrete unit of work with well-understood problems.

The other challenge for the week has been generating Hoogle databases for the core libraries - the base library and all the other libraries GHC ships with a release. With these libraries in place, it is feasible to use Hoogle to perform useful queries. The libraries are generated using a combination of Cabal, Haddock and Hoogle. I've made changes in both the Haddock and Hoogle layers so that the full base libraries can now be processed.

In order to deal with the full base libraries there are numerous GHC extensions that must be supported. In particular, Hoogle now supports multi-parameter type classes, higher-ranked types, type operators, unboxed types, unboxed tuples and NDP style arrays. All of these features are translated down into Haskell 98 types, but most closely approximate their behaviour in GHC, and can be used in searches.

Throughout the week I've been profiling the database creation code in Hoogle. The databases for the core libraries come to about 4.5Mb, and are highly optimised for performing searches - often at the cost of making them harder to create. I've halved the time to create databases during the week, using profiling to direct improvements. The time required to process the Core libraries takes 60 seconds, which is a certainly an acceptable timeframe, but could always be faster. Currently the biggest culprit in the profile is the hPutByte function:

hPutByte :: Handle -> Int -> IO ()
hPutByte hndl i = hPutChar hndl $ chr i

Since the underlying databases are written using lots of hTell and hSeek commands, it is not possible to use something like the Data.Binary library. However, if anyone has any suggestions on how to improve performance they would be gratefully received.

Next week: I want to finish off the remaining type search features, and then package up a command line release for Hackage. Hopefully Hoogle 4 will be ready for initial use by early testers.

User visible changes: Type search is more robust, but still not fully featured. Database creation if faster and robust. You can search the base libraries.

GSoC Hoogle: Week 6

This week I've been tackling type searching. I have just (in the last few minutes) got my first type search to work. At the moment type search is very limited, but all the ideas and scafolding are in place, so should now proceed relatively quickly.

In all previous versions on Hoogle, type searching was O(n), where n is the number of functions in the database. Hoogle compared the type search to each possible answer, computed a closeness score, then at the end wrote out the closest matches. This meant that before the first answer could be given, all functions had to be checked, i.e. the time for the first answer was O(n). As the Hoogle database is about to get massively bigger, this approach is insufficient.

The new version of Hoogle is much cleverer. It works by exploring a graph, following similar ideas to Dijkstra's algorithm, to reach more suitable results first. Typically, the best answers will be given without any search of the graph, and then as the graph is searched more results will appear with lower closeness. With the new scheme the complexity is O(m), where m is the number of results you want. I hope at some point after the SoC is finished to describe the algorithm properly, so others can understand it, and hopefully improve upon it.

Next week: Finishing off type searching, so it supports all the features planned. Build system work, and potentially a cabal pre-release.

User visible changes: Type search works to some degree, but not perfectly. Database debugging options (conversion and dumping to a text file) have been added.

GSoC Hoogle: Week 5

This week I was going to tackle type searching, but then realised I'm going to spend 6 hours on Friday on a train (hence the weekly update on Thursday), so can spend that time productively working on paper tackling type search. So instead of type search, I worked on a few other pieces, some of which make type search easier:

Haddock Database Generation More patches to get better output from Haddock. The code now handles class methods properly, and deals with some FFI bits.

Lazy Name Searching Searching for a name is now fairly lazy. When searching for a name, Hoogle can return the prefix of the results without doing too much computation to calculate all the results. This work is useful in its own right, but very necessary for the type searching, and can be reused.

Hoogle --info The biggest feature added this week is the --info flag. When this flag is given, Hoogle picks the first result and gives more details, including any Haddock documentation associated with the function. For example:


$ hoogle +tagsoup openurl --info
Text.HTML.Download openURL :: String -> IO String

This function opens a URL on the internet. Any http:// prefix is ignored.

> openURL "www.haskell.org/haskellwiki/Haskell"

Known Limitations:

* Only HTTP on port 80
* Outputs the HTTP Headers as well
* Does not work with all servers

It is hoped that a more reliable version of this function will be placed in a new HTTP library at some point!


Next week: Type searching! See last week for a description of what I hope to achieve.

User visible changes: The --info flag now exists.

GSoC Hoogle: Week 4

This week I've stayed in one place, and had lots of opportunity to get on with Hoogle. I've done a number of different things this week:

More on Haddock databases I fixed a number of issues with the Haddock generated Hoogle information. These patches have been submitted back to Haddock.

Binary Defer library I merged the binary defer library into the Hoogle sources, and modified it substantially. Some of the modifications were thanks to suggestions from the Haskell community, particularly David Roundy. The library is now more robust, and is being used as a solid foundation to build the rest of Hoogle on top of.

Text Searching You can now search for words, even multiple words, and the search will be performed. The text searching uses efficient data structures, scales excellently, and returns better results first.

Suggestions These improvements were detailed earlier in the week.

Next week: Type searching. I have various ideas on how to go about this, but it is the most tricky part of the whole project. I hope to come up with the perfect solution by the end of the week, but if not, will come up with something good enough for Hoogle 4 then revise it after the Summer is over (it could easily suck in a whole Summer of time if I am not careful!). Much of the low-level infrastructure is already present, so it is just the search algorithm.

User visible changes: Text searching works. A session with Hoogle as it currently stands:

> cabal haddock --hoogle
-- generates tagsoup.txt
> hoogle --convert=tagsoup.txt
Generating Hoogle database
Written tagsoup.hoo
> hoogle +tagsoup is open --color
Text.HTML.TagSoup.Type isTagOpen :: Tag -> Bool
Text.HTML.TagSoup.Type isTagOpenName :: String -> Tag -> Bool

.