Parsing a Haskell source file without an explicit 'module M where' header with GHC 7

The guys at the Budapest Functional Programming meetup group finally convinced me to port Tandoori to GHC 7. Things are turning 开发者_StackOverflow中文版out OK, but I'm having a problem parsing .hs source files with no module M where header.

Here's a standalone version of the code that calls GHC's internal parser to parse source files:

module Main where

import SrcLoc
import FastString
import Outputable (showSDoc)
import Parser (parseModule)
import Lexer (unP, mkPState, ParseResult(..))
import StringBuffer (hGetStringBuffer)
import DynFlags (defaultDynFlags)
import System.Environment (getArgs)

parseMod src_filename = do 
  buf <- hGetStringBuffer src_filename
  let loc = mkSrcLoc (mkFastString src_filename) 1 0
      dflags = defaultDynFlags
  case unP Parser.parseModule (mkPState dflags buf loc) of
    POk pst rdr_module -> return rdr_module
    PFailed srcspan message -> error $ showSDoc message

main = do 
  args <- getArgs
  case args of
    [src_filename] -> parseMod src_filename
    _ -> error "Usage: parse-test filename.hs"

here's an input file that works:

foo = 1

here's another one that works:

module M where

foo = 1 
bar = 2

and here's one that doesn't:

foo = 1
bar = 2

For this one, the output of the program above is:

*** Exception: parse error on input `='

Note that I'm using GHC's parser instead of something like haskell-src because the real code also uses the renamer from GHC.

We simply prepend a module header line to the source we want to get parsed by the parser of GHC. Would this help you?