[Haskell-cafe] Problems with Language.Haskell.Interpreter and errors

[Daniel Gorín]

On Sep 30, 2009, at 2:20 AM, Martin Hofmann wrote:

> Thanks a lot.
>
>> You ought to be able to add a Control.Monad.CatchIO.catch clause to
>> your interpreter to catch this kind of errors, if you want.
>
> I forgot to mention that this didn't work for me either.
>
>> Thanks for the report!
>
> You are welcome. If you come up with a work around or a fix, I would  
> appreciate if you let me know.
>
> Cheers,
>
> Martin

Apologies for a very very very late follow-up on this thread (http://thread.gmane.org/gmane.comp.lang.haskell.cafe/64013 
).

It turns out that Control.Monad.CatchIO.catch was the right thing to  
use; you were probably bitten, just like me, by the fact that "eval"  
builds a thunk and returns it, but does not execute it. The following  
works fine for me:

import Prelude hiding ( catch )
import Language.Haskell.Interpreter
import Control.Monad.CatchIO ( catch )
import Control.Exception.Extensible hiding ( catch )

main :: IO ()
main = print =<< (runInterpreter (code `catch` handler))
     where s    = "let lst [a] = a in lst []"
           code = do setImports ["Prelude"]
                     forceM $ eval s
           handler (PatternMatchFail _) = return "catched!"

forceM :: Monad m => m a -> m a
forceM a = a >>= (\x -> return $! x)

When run, it prints  'Right "catched!"'. Notice that if you change the  
line 'forceM $ eval s' by an 'eval s', then the offending thunk is  
reduced by the print statement and the exception is thrown outside the  
catch.

Hope this helps

Daniel