[Haskell-cafe] hint / ghc api and reloading modules

Daniel Gorín dgorin at dc.uba.ar
Sat May 31 22:54:40 EDT 2008

(Since this can be of interest to those using the ghc-api I'm cc-ing  
the ghc users' list.)

Hi, Evan

The odd behavior you spotted happens only with hint under ghc-6.8. It  
turns out the problem was in the session initialization.

Since ghc-6.8 the newSession function no longer receives a GhcMode.  
The thing is that, apparently, if one was passing the Interactive  
mode to newSession under ghc-6.6, now you ought to set the ghcLink  
dynflag to LinkInMemory instead.

I couldn't find this documented anywhere (except for this patch  
http://www.haskell.org/pipermail/cvs-ghc/2007-April/034974.html) but  
it is what ghci is doing and after patching hint to do this the  
reloading of modules works fine.

I'll be uploading a fixed version of hint to hackage in the next days.


On May 31, 2008, at 2:46 PM, Evan Laforge wrote:

> I'm using "hint", but since it's basically a thin wrapper around the
> GHC API, this is probably a GHC api question too.  Maybe this should
> go to cvs-ghc?  Let me know and I'll go subscribe over there.
> It's my impression from the documentation that I should be able to
> load a module interpreted, make changes to it, and then reload it.
> This is, after all what ghci does.  It's also my impression that the
> other imported modules should be loaded as object files, if the .hi
> and .o exist, since this is also what ghci does.
> However, if I load a module and run code like so (using hint):
>     GHC.loadModules ["Cmd.LanguageEnviron"]
>     GHC.setTopLevelModules ["Cmd.LanguageEnviron"]
>     GHC.setImports ["Prelude"]
>     cmd_func <- GHC.interpret (mangle_code text) (GHC.as :: LangType)
> It works fine until I change LanguageEnviron.  If I make a change to a
> function, I don't see my changes in the output, as if the session is
> only getting partially reset.  If I insert a syntax error, then I do
> see it, so it is recompiling the file in some way.  However, if I
> *rename* the function and call it with the new name, I get a
> GhcException:
> During interactive linking, GHCi couldn't find the following symbol:
>   ... etc.
> So I examined the code in hint for loadModules and the code in
> ghci/InteractiveUI.hs:/loadModule, and they do look like they're doing
> basically the same things, except a call to rts_revertCAFs, which I
> called too just for good measure but it didn't help (I can't find its
> source anywhere, but the ghci docs imply it's optional, so I suspect
> it's a red herring).
> Here's a condensed summary of what hint is doing:
> -- reset
> GHC.setContext session [] []
> GHC.setTargets session []
> GHC.load session GHC.LoadAllTargets
> -- rts_revertCAFs
> -- load
> targets <- mapM (\f -> GHC.guessTarget f Nothing) fs
> GHC.setTargets session targets
> GHC.load session GHC.LoadAllTargets
> -- interpret
> let expr_typesig = "($expr) :: xyz"
> expr_val <- GHC.compileExpr session expr_typesig
> return (GHC.Exts.unsafeCorce# expr_val :: a)
> -- GHC.compileExpr
> maybe_stuff <- hscStmt hsc_env ("let __cmCompileExpr = "++expr)
> ([n],[hv]) <- (unsafeCoerce# hval) :: IO [HValue]
> return (Just hv)
> and then ghci does:
> -- load
> GHC.setTargets session []
> GHC.load session LoadAllTargets
> targets <- io (mapM (uncurry GHC.guessTarget) files')
> GHC.setTargets session targets
> GHC.load session LoadAllTargets
> rts_revertCAFs
> putStrLn "Ok, modules loaded: $modules"
> -- interpret
> GHC.runStmt session stmt step
> -- GHC.runStmt
> Just (ids, hval) <- hscStmt hsc_env' expr
> coerce hval to (IO [HValue]) and run it carefully
> So it *looks* like I'm doing basically the same thing as ghci...
> except obviously I'm not because ghci reloads modules without any
> trouble.  Before I go start trying to make hint even more identical to
> ghci, is there anything obviously wrong here that I'm doing?
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