FUZxxl FUZxxl - 4 months ago 14
Brainfuck Question

Why is GHC complaining about wrong type?

This little function checks a (finite) Brainfuck string for validity. It check's whether the

[
and
]
are balanced. The code is very straightforward and written to be tail-recursive:

-- checks Brainfuck for validity.
validateBrainfuck :: Monad m => String -> m String
validateBrainfuck s = maybe (return s) (fail . fromJust) (validate s 0) where
validate :: String -> Int -> Maybe String -- Here inversed: String means error
validate (']':_ ) 0 = Just "Too many closing brackets"
validate (']':xs) c = validate xs (pred c)
validate ('[':xs) c = validate xs (succ c)
validate ( x :xs) c = validate xs c
validate [] 0 = Nothing
validate [] _ = Just "Too many opening brackets"


Now, GHC complains about typing issues:

Brainfuck.hs:62:58:
Couldn't match expected type `Maybe String'
against inferred type `[Char]'
Expected type: Maybe (Maybe String)
Inferred type: Maybe String
In the third argument of `maybe', namely `(validate s 0)'
In the expression:
maybe (return s) (fail . fromJust) (validate s 0)


Maybe I'm just too silly to figure out what went wrong, but this looks very weird for me.

Answer

Look at the type of maybe and think what it should do:

maybe :: b -> (a -> b) -> Maybe a -> b

If the maybe value contains no result (i.e. Nothing), maybe returns the b argument.

Otherwise - when Just a is given - it applies the given function to the valid result. We don't need any fromJust extraction here.

Your code just becomes

maybe (return s) fail (validate s 0)
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