[Haskell-cafe] Problem with reloading modules in GHC API

Hongmin Fan hongmin.fan.lists at gmail.com
Mon Jul 19 23:12:38 EDT 2010


I'm using GHC API to dynamically load some module, and evaluate it; and
later change the content of the module, and re-evaluate it. But I found
unless I delete the object file created by previous compilation, the module
seems not reloaded. I have set ghcLink = LinkInMemory as an older post

To illustrate what I'm saying, here is a piece of code (sorry for any
naivety in the code, new to Haskell too)

import System.IO (IOMode(..),hClose,hPutStr,openFile)
import Directory (removeFile)
import GHC
import GHC.Paths
import DynFlags
import Unsafe.Coerce

src_file = "Target.hs"
obj_file = "Target.o"

main = do
    writeTarget "arg"
    func0 <- compileTarget
    putStrLn $ show $ func0 2

    writeTarget "arg*2"
    func1 <- compileTarget
    putStrLn $ show $ func1 2

writeTarget input = do
--    removeFile obj_file `catch` (const $ return ()) -- uncomment this line
to have correct results
    h <- openFile src_file WriteMode
    hPutStr h "module Target (Target.target) where\n"
    hPutStr h "target::Double -> Double\n"
    hPutStr h "target arg = \n    "
    hPutStr h input
    hClose h

compileTarget =
    defaultErrorHandler defaultDynFlags $ do
        func <- runGhc (Just libdir) $ do
            -- setup dynflags
            dflags <- getSessionDynFlags
            setSessionDynFlags dflags { ghcLink = LinkInMemory }

            -- load target module
            target <- guessTarget src_file Nothing
            setTargets [target]
            r <- load LoadAllTargets
            case r of
                Failed -> error "Compilation failed"
                Succeeded -> do
                    m <- findModule (mkModuleName "Target") Nothing
                    -- set context and compile
                    setContext [] [m]
                    value <- compileExpr ("Target.target")
                        let value' = (unsafeCoerce value) :: Double ->
                        return value'
        return func

The code basically write to a Haskell source file twice with different
content, and hoping to get different results, but unless I uncomment the
line with removeFile, the output of 2 runs are the same; using 'touch' to
touch the source file being written between 2 runs also gives the correct
results. So maybe caused by some caching mechanism?

I'm using GHC 6.12.1 in Ubuntu 10.04. I have this workaround of deleting the
obj file, but I'm wondering the "correct" way of doing it. Did some search
on GHC API, but never got something relevant.

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