Re: GHC as a library - getting output from GHCI

6 May 2007

      Hi Pepe

Pepe Iborra wrote:
...
Mads
On 04/05/2007, at 19:19, Mads Lindstrøm wrote:
...
Hi Pepe
I would have liked something cross-platform.
Take a look at the unix-compat[1] package by Bjorn Bringert, although
it looks like it won't help you. Maybe it can be extended.
...
Also, if stmt contains an error, wrapStmt will not be evaluated, and
the
resulting error is written to standard output or maybe to standard
error. Whichever way, I am back to the same problem.
What else should be the behaviour in the case stmt contains an error?
It could behave like most other IO actions which produce values - return
them to the caller. For instance readFile :: FilePath -> IO String .
runStmt could have the following type:

runStmt :: Session -> String -> IO (RunResult, String)

where the returned String would contain either the error or the result
from evaluating the statement.
...
You can capture errors and stop them being written out very easily. 
Take a look at this snippet (not mine, it's beschmi's code) from
Shim[2]:
Yes, that works for me.

Greetings,

Mads
...
\begin{code}
load' :: FilePath -> Maybe String -> SHM
(SuccessFlag,[CompileNote],Session)
load' sourcefile source = do
  source' <- addTime source
  ses <- getSessionFor sourcefile
  dflags0 <- io $ GHC.getSessionDynFlags ses
  ref <- io $ MVar.newMVar []
  let dflags1 = dflags0{ log_action = logMsg ref }
  io $ GHC.setSessionDynFlags ses dflags1
  io $ GHC.setTargets ses [Target (TargetFile sourcefile Nothing)
source']
  loadResult <- io $ GHC.load ses LoadAllTargets
  cnotes <- io $ reverse `liftM` MVar.readMVar ref
  case loadResult of
       Succeeded -> do -- GHC takes care of setting the right context
         modq <- io $ findModuleInFile ses sourcefile
         io $ GHC.setContext ses [modq] []
         return (Succeeded,cnotes,ses)
       Failed    -> do   -- We take care of getting at least the
Prelude
         io(GHC.setContext ses [] =<< atomM (getPrelude ses))
         return (Failed,cnotes,ses)
  where atomM = liftM (:[])
        logMsg ref severity' srcSpan' style' msg' = do
          dir <- getCurrentDirectory
          logS ('\n':show ((mkLocMessage srcSpan' msg') style'))
          MVar.modifyMVar_ ref
            (\l -> return $ (CompileNote severity' srcSpan'
                                         style' msg' dir):l)
\end{code}
Here, DynFlags.logAction is being filled with a handler that writes to
a MVar instead of the default handler that writes to stdout.
pepe
[1] - http://hackage.haskell.org/cgi-bin/hackage
-scripts/package/unix-compat-0.1
[2] - http://shim.haskellco.de/shim/Shim/Hsinfo.hs