Is there a Haskell code formatter?

Question

I used to write

data A = A {
      a :: Double
    }
    deriving(Eq, Show)

but now i prefer

data A = A {
      a :: Double
    } deriving(Eq, Show)

I think the answer will be no, but i ask anyway: is there a code formatter for Haskell?

Answer 1

New answer

I have now written hindent, which is written in terms of haskell-src-exts. It has Emacs and Vim support.

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Old answer

There is haskell-src-exts which will parse your code and it has a pretty printing module for printing the AST to a string. E.g.

import Language.Haskell.Exts

main = interact codeFormat

codeFormat = check . fmap reformat . parseModuleWithComments where
  reformat = prettyPrint
  check r = case r of
              ParseOk a -> a
              ParseFailed loc err -> error $ show (loc,err)

Example:

λ> putStrLn $ codeFormat "module X where x = 1 where { y 1 = 2; y _ = 2 }"
module X where
x = 1
  where y 1 = 2
        y _ = 2

Alternatively you can write a pretty printer yourself (even based on the above if you just want to specialise), and then you can have whatever style you want. Replace prettyPrint with your own. The AST is very straight-forward.

Then you can hook it up with Emacs to reformat every time you hit save or something.

Answer 2

There's stylish-haskell which can do precisely what you want.

Answer 3

I've written a small script for that same purpose: https://github.com/djv/small/blob/master/tidy.hs I call it from vim to reformat my code.