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

20 Jul 2010


      Hongmin Fan  writes:
...
Thanks Andy! Yes, I do need the second approach you listed here. I think the plugin package actually
uses GHC API to load
too.
My problem here is more technical than theoretical. The type safety isn't the primary issue for me,
because I'm trying to
implement a certain genetic programming system (there is some papers like this one:
http://citeseerx.ist.psu.edu/viewdoc/
summary?doi=10.1.1.98.7686), which generates functions that I'm going to dynamically evaluate. I'm
hoping to use GHC API to
avoid writing my own evaluator. The func generation component actually is designed to generate
functions of the specific
type, so no worries about type safty.
I'm looking for a clean/correct way to use GHC API to reload the module, maybe there is some option
I should set but I
didn't? Being new to Haskell, the source code for GHCi isn't so accessible... Thanks!
If you want dynamic evaluation, you can try hint : Runtime Haskell interpreter (GHC API wrapper)
At http://hackage.haskell.org/package/hint

Cheers,

  -- Andy
...
On Tue, Jul 20, 2010 at 1:01 AM, Andy Stewart  wrote:
Hi Hongmin,
I think you're looking for how to hot-swap Haskell program.
There are two approach to reach target:
1) Source-Code level:
      Recompile source code to build new execute cache file, if re-compile
      successful, use executeFile to switch new entry. You perhaps need use
      Binary instance to save/restore state between re-launch new execute
      file.
2) Dynamic Linking object code.
      Compile plugins code to object file, the use .o and .hi file to
      dynamic linking object code to running Haskell application.
      Because .hi file have type/depend information, we can do type-check
      when dynamic linking .o file.
First way is simpler, but you perhaps will lost state after reboot,
    because you can't serialize state (such as FFI) sometimes.
Second way is more complicated, but you can present all state when hot-swapping.
Looks you need second way, from your code, that's wrong, you can't
    dynamic link object file without type-check, and expect everything will
    be fine.
    If you don't type-check when linking code, it's very dangerous, it willl *crash* your
    program once type mismatch or runtime error occur.
Infact, Don Stewart has implement a complete solution to dynamic
    linking in Haskell program, at
    http://hackage.haskell.org/package/plugins-1.4.1
    Unfortunately, it's broken with ghc-6.12.x
Before Don fix plugins package, i recommend you read Don's new paper
    (http://www.cse.unsw.edu.au/~dons/papers/dons-phd-thesis.pdf)
    You will know every detail that how to dynamic extension Haskell
    program.
Hope above will help you. :)
 -- Andy
Hongmin Fan  writes:
> Hi,
    >
    > 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 suggested
    >
    > 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")
    >                     do
    >                         let value' = (unsafeCoerce value) :: Double -> 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.
    >
    > Thanks,
    > Hongmin
    >
    > _______________________________________________
    > Haskell-Cafe mailing list
    > Haskell-Cafe@haskell.org
    > http://www.haskell.org/mailman/listinfo/haskell-cafe
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