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.