Thanks, that fixed it. Why was it segfaulting on "Nothing" though? Secondly, I'd like to get to a GHC session that just has, say, Prelude in scope so I can use dynCompileExpr with "show" etc, but I cannot figure out how to bring it into scope. The closest I got was to get GHC complaining that it was a package module. On Sun, Feb 27, 2011 at 9:24 AM, Anthonin Bonnefoy wrote:

Hi,

The first argument of runGhc takes the directory where GHC's library are. You can use the ghc-paths module (http://hackage.haskell.org/package/ghc-paths ) for this.

Just install ghc-paths with cabal, import Ghc.Paths and call runGhc with (Just libdir), it should get past the segfault.

On Sun, Feb 27, 2011 at 10:51 AM, Edward Amsden wrote:

...

I'm trying to run the following code. I'm not at all sure it's correct, it's based off of a bit of poking around in the ghc api. Running it with a command line argument like "show (5 + 2)" gives me a segmentation fault. Poking around with gdb and following the steps at http://hackage.haskell.org/trac/ghc/wiki/Debugging/CompiledCode yields precisely nothing, as even the "disassemble" command complains "No function contains program counter for selected frame."

Any ideas?

== code == module Main where

import GHC import DynFlags import Data.Dynamic import System

evalString :: Typeable a => String -> IO (Maybe a) evalString s = defaultErrorHandler defaultDynFlags $ runGhc Nothing $ do  dynflags <- getSessionDynFlags  setSessionDynFlags $ dynflags  target <- guessTarget "Prelude" Nothing  setTargets [target]  load LoadAllTargets

 dyn <- dynCompileExpr s  return $ fromDynamic dyn

main = do  (s:_) <- getArgs  e <- evalString s  putStrLn $ maybe "oops" id e

-- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

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-- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

That's doing what I want, but I'm not sure why you passed [(mkModule (stringToPackageId "base") (mkModuleName "Prelude"), Nothing) ] to setContext. I found that [mkModule (stringToPackageId "base") (mkModuleName "Prelude")] matches the type expected by setContext. Perhaps we are using different api versions? I'm using 6.12.3 2011/2/28 Daniel Schüssler :

Hi,

On 2011-February-27 Sunday 16:20:06 Edward Amsden wrote:

...

Secondly,

I'd like to get to a GHC session that just has, say, Prelude in scope so I can use dynCompileExpr with "show" etc, but I cannot figure out how to bring it into scope. The closest I got was to get GHC complaining that it was a package module.

I don't know if setContext is the proper/best way to do it, but it seems to work:

import GHC import GHC.Paths ( libdir ) import DynFlags ( defaultDynFlags ) import Module(stringToPackageId, mkModuleName) import Data.Dynamic(fromDynamic) import System.Environment

evalString s =    defaultErrorHandler defaultDynFlags $ do      runGhc (Just libdir) $ do        dflags <- getSessionDynFlags        setSessionDynFlags dflags        setContext [] [ (mkModule (stringToPackageId "base") (mkModuleName "Prelude")                        ,Nothing) ]

       dyn <- dynCompileExpr s        return $ fromDynamic dyn

main = do  (s:_) <- getArgs  e <- evalString s  putStrLn $ maybe "oops" id e

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-- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com