Saturday, October 01, 2011

Template Haskell fights with Generic Programming

Summary: The InfixE construction in Template Haskell fits poorly with generic programming, because its type does not capture all its restrictions. This mismatch can result in confusing bugs, but there is a simple workaround.

I have often said that anyone manipulating abstract syntax trees, without using some form of generic programming, is doing it wrong. Recently I have been manipulating Template Haskell syntax trees using Uniplate, my preferred generic programming library. Consider the problem of replacing all instances of delete with deleteBy (==) - this task can be done with Template Haskell:

{-# LANGUAGE TemplateHaskell #-}
module IHateDelete where
import Data.List
import Language.Haskell.TH
import Data.Generics.Uniplate.Data

iHateDelete :: Q [Dec] -> Q [Dec]
iHateDelete = fmap (transformBi f)
f :: Exp -> Exp
f (VarE x) | x == 'delete = VarE 'deleteBy `AppE` VarE '(==)
f x = x

We can test this function with:

{-# LANGUAGE TemplateHaskell #-}
import IHateDelete
import Data.List

mapDelete x = map (delete x)
myElem x xs = length (delete x xs) /= length xs

To see the result of running iHateDelete pass the flag -ddump-splices. As far as we can tell, our iHateDelete function works perfectly. But wait - let's try another example:

myDelete x xs = x `delete` xs

In GHC 6.12, we get a GHC panic. In GHC 7.2 we get the error message:

Operator application with a non-variable operator: deleteBy (==)
(Probably resulting from a Template Haskell splice)

(I would find this message far clearer if it said "Infix application..." rather than "Operation application...")

The body of myDelete starts out as:

InfixE (Just (VarE 'x)) (VarE 'delete) (Just (VarE' xs))

After our transformation, this becomes:

InfixE (Just (VarE 'x)) (AppE (VarE 'deleteBy) ('(==))) (Just (VarE' xs))

Or, written in Haskell syntax:

x `deleteBy (==)` xs

This expression is not valid Haskell, and causes an error when spliced back in (when inserted back into the Haskell code).

The Problem

The underlying problem is called out in the Template Haskell Exp documentation:

InfixE (Maybe Exp) Exp (Maybe Exp)
-- ^ It's a bit gruesome to use an Exp as the operator, but how else can we distinguish constructors from non-constructors?
-- Maybe there should be a var-or-con type? Or maybe we should leave it to the String itself?

The operator in InfixE has a type which permits any expression, but has the restriction that when spliced back in the expression must only be a VarE or ConE. This restriction poses a problem for generic programming, where values are treated in a uniform manner. Sadly, both of the suggested fixes would also cause problems for generic programming. Perhaps the true fix is to let Haskell have arbitrary expressions for infix operators? Or perhaps Template Haskell should translate InfixE to AppE if the operator is incompatible with Haskell?

The Workaround

As a workaround, you can translate away all InfixE expressions that have a complex middle expression. I use the following function:

fixupInfix :: [Dec] -> [Dec]
fixupInfix = transformBi f
bad VarE{} = False
bad ConE{} = False
bad _ = True

f (InfixE a b c) | bad b = case (a,c) of
(Nothing, Nothing) -> b
(Just a , Nothing) -> b `AppE` a
(Nothing, Just c ) -> VarE 'flip `AppE` b `AppE` c
(Just a , Just c ) -> b `AppE` a `AppE` c
f x = x

The original iHateDelete can then be modified to become:

iHateDelete = fmap (fixupInfix . transformBi f)


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