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.