Boilerplate module for dynamically compiling, loading, and evaluating Haskell code.

GhcAPI.hs

-- Boilerplate module for dynamic compilation, loading, and evaluation of
-- modules using the GHC API, v. 7.4.1. Note that the GHC API is subject to
-- change, so this file may not work for future versions. 
--
-- Load and play around with this file using:
--
--   ghci -package ghc -XCPP GhcAPI.hs
--
-- or
-- 
--   ghci -package ghc -package ghc-mtl -XCPP -DWITH_MTL GhcAPI.hs
--
-- References:
--
--   1. http://www.haskell.org/haskellwiki/GHC/As_a_library
--   2. http://www.bluishcoder.co.nz/2008/11/dynamic-compilation-and-loading-of.html
--
-- Some of the examples in the above references may be out-of-date with respect
-- to the 7.4.1 API.
--
-- Note the return type of `evalFromModule` is way too general to be useful.
-- You need to make sure that you constrain the type variable to the type of
-- the identifier you're accessing in the dynamically loaded module.
--
module GhcAPI where

#ifdef WITH_MTL
import Control.Monad.Ghc
import GHC hiding ( Ghc, GhcT, runGhc, runGhcT )
#else
import GHC
#endif

import Data.Dynamic

import GHC.Paths ( libdir )
import DynFlags


evalFromModule file modul ident =
  defaultErrorHandler defaultLogAction $ do
    runGhcT (Just libdir) $ do
      dflags <- getSessionDynFlags
      setSessionDynFlags dflags
      target <- guessTarget file Nothing
      setTargets [target]
      result <- load LoadAllTargets
      case result of
        Succeeded -> do
          m <- findModule (mkModuleName modul) Nothing
          setContext [IIModule m]
          value <- dynCompileExpr $ modul ++ "." ++ ident
          return $ Just value
        Failed -> return Nothing

evalFromModule' file modul ident witness = do
  mval <- evalFromModule file modul ident
  let c = flip (const :: Maybe a -> a -> Maybe a) witness 
  case mval of
    Just v  -> return $ c $ fromDynamic v
    Nothing -> return Nothing