I'm a beginner in haskell and trying to do a cipher validate function.
validateCipher :: [Char] -> Bool
validateCipher cipher =
if length $ nub cipher == length cipher
then return True
else return False
When I run it through ghci, it just told me: error: parse error on input ‘validateCipher’. Not sure what is going wrong, should I add Eq or any stuff(I don't quite sure what does it do either)?
There are two errors here:
return. In Haskell there is no return keyword, there is a return :: Monad a => a -> m a function, but that works differently; and$ results in the fact that length $ nub cipher == length cipher is parsed as length (nub cipher == length cipher), and that does not satisfy the type constraints, since nub cipher returns a [Char], whereas length cipher, will return an Int.We do not need return True, or True in the first place here, since we can just return the outcome of the condition we check. We can furthermore use parenthesis to write a correct expression:
validateCipher :: [Char] -> Bool
validateCipher cipher = length (nub cipher) == length cipher
IO, Maybe, etc. one can indeed sometimes use return. But not for non-monadic ones, like the one here.($) is an operator that has very low precedence (haskell.org/onlinereport/decls.html), it is basically used to group the part at the left, and the part at the right, and make a function application. So ... $ ..., is nearly always equivalent to (...) (...) (but without brackets).
return. In Haskell there is noreturnkeyword, there is only areturnfunction. Furthermore the$results in parsing it as `length (nub cipher == length cipher), so you should use brackets.