[Haskell-cafe] error to Fail Conversion

[Daniel Fischer]

On Sunday 03 April 2011 16:31:10, Anwar Bari wrote:
> HI Cafe
>   I have problem with this function
> value :: Theorm-> [(String,Int)] -> Tem -> Int
> value _   env (Var s)      = maybe (error "Unknown variable.") id
> (lookup s env) value mdl env (OP s _ _ l) = maybe (error "Function not
> defined.") id (lookup (map (value mdl env) l) (S.toList $ maybe (error
> 
> "Uknown function.") id (fmap snd (multiLookup s (operations mdl)))))
> 
> This function is working fine, but I want to change error function to
> fail function to be able use it in my GUI application.

For that, you have to change the type.
If the result type is Int, you can only have it return a bona fide Int or 
an error.

For more graceful failing, the result type needs to support some indication 
of failure, it could be

Maybe Int
Either String Int
[Int]

or others. Normally, using some MonadError would be the most convenient.

value :: (Error e, MonadError e m) =>
            Theorem -> Environment -> Term -> m Int
 
replacing

maybe (error "foo") id

with

maybe (throwError $ strMsg "foo") return

> I can't change the output of value function to IO Int,

If you can change it at all, changing it as above and making your GUI-monad 
suitable (i.e. providing a MonadError instance for it with a suitable error 
type) would probably be the simplest.

> any idea will help. Thanks
> Nour.