A pretty-printing for Haskell in Haskell. Almost same as http://code.haskell.org/haskell-src-exts/examples/Prettify.hs .

Prettify.hs

-- references
-- http://code.haskell.org/haskell-src-exts/Test/examples/HaskellParser.hs
-- http://code.haskell.org/haskell-src-exts/examples/Prettify.hs

import qualified Language.Haskell.Exts.Annotated as P
import qualified Language.Haskell.Exts.Extension as Ext
import Language.Haskell.Exts.Pretty (prettyPrint)
import System.Environment (getArgs)

parse originalFileName input = P.parseModuleWithMode parseMode input
  where
    parseMode :: P.ParseMode
    parseMode = P.defaultParseMode 
      { P.parseFilename = originalFileName
      , P.extensions = Ext.glasgowExts ++ 
         [ Ext.ExplicitForAll
         , Ext.DoAndIfThenElse
         ]
      }

main = 
  do args <- getArgs
     case args of
       [] -> putStrLn usage
       (_:_:_) -> fail "too many command-line arguments"
       [fileName] ->
         do s <- readFile fileName
            let x = parse fileName s
            putStrLn $ case x of
              P.ParseOk h -> 
                prettyPrint h
              P.ParseFailed srcLoc message ->
                unlines [ prettyPrint srcLoc
                        , message
                        ]
  where usage = "Prettify.hs <source.hs>"