[GHC] #11301: Using GHC's parser and rendering the results is unreasonably difficult

28 Dec 2015

      #11301: Using GHC's parser and rendering the results is unreasonably difficult
-------------------------------------+-------------------------------------
           Reporter:  bitemyapp      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  GHC API        |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 After a couple hours, 30-some browser tabs, and attempting to use ghc-
 simple to paper over some annoyances with GHC's API (dynFlags/session-
 stuff, particularly), I gave up.

 I then opened up haskell-src-exts and found the function I wanted in a
 minute or two.

 {{{#!hs

 import qualified Language.Haskell.Exts.Parser as P

 parsePls' :: String -> IO ()
 parsePls' s = print $ P.parseExp s
 }}}

 This is where I got stuck:

 {{{
 Prelude> parsePls "1 + 1"
 Too late for parseStaticFlags: call it before runGhc or runGhcT
 }}}

 This is the code I was working with before I gave up on GHC:

 {{{#!hs
 module ParsePls where

 import DynFlags
 import FastString
 import qualified GHC
 import qualified GhcMonad as GM
 import HsSyn
 import Lexer
 import Outputable
 import Parser
 import RdrName
 import SrcLoc
 import StaticFlags
 import StringBuffer

 import qualified Language.Haskell.GHC.Simple as S
 import qualified Language.Haskell.GHC.Simple.Types as ST

 import System.Environment

 main :: IO ()
 main = do
   [expr] <- getArgs
   parsePls expr

 parsePls :: String -> IO ()
 parsePls s = do
   initStaticOpts
   let sbuf = stringToStringBuffer s
       srcloc = mkRealSrcLoc (mkFastString s) 1 1
   (dynflags, _) <- S.getDynFlagsForConfig ST.defaultConfig
   -- dynflags <- undefined -- GHC.getSessionDynFlags
   -- unP :: P a -> PState -> ParseResult a

   -- mkPState :: DynFlags -> StringBuffer -> RealSrcLoc -> PState
   -- parseExpression :: P (LHsExpr RdrName)
   -- type LHsExpr id = Located (HsExpr id) -- Defined in ‘HsExpr’

   let parseResult = unP parseExpression
                     (mkPState dynflags sbuf srcloc)
   case parseResult of
     POk _ (L _ mdl) -> print $ showPpr dynflags mdl
     PFailed ss _ -> do
       putStrLn "Error occurred!"
       print ss
 }}}

 Is there a reason it has to be this difficult?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11301
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler