[Haskell-beginners] function parameter types after flip

[Daniel Fischer]

Am Donnerstag 13 August 2009 17:25:07 schrieb Robert Ziemba:
> Can anyone help me understand the behavior of the following
> expressions in ghci?  I was trying to construct a flipped function
> 'elem' that would accept a list of characters first but I ran into
> this problem with the type signature. Thank you.
>
> Prelude Data.List Data.Char> :t elem
> elem :: (Eq a) => a -> [a] -> Bool              -- OK
> Prelude Data.List Data.Char> :t (flip elem)
> (flip elem) :: (Eq a) => [a] -> a -> Bool       -- OK this is what I want
> Prelude Data.List Data.Char> let fElem = (flip elem)
> Prelude Data.List Data.Char> :t fElem
> fElem :: [()] -> () -> Bool                           -- ?? Function
> will not accept a [Char]

It's the monomorphism restriction.
If you bind a name by a simple pattern binding (like [let] fElem = flip elem) and don't 
give a type signature, it must have a monomorphic type and in ghci, that defaults to () 
for unconstrained type variables.

Ways to fix the behaviour:
1. give an explicit type signature (good in code files anyway, but cumbersome at the ghci 
prompt)
2. make it a function binding:
    let fElem xs x = x `elem` xs
    let fElem xs = (`elem` xs)
    let fElem xs = flip elem xs
3. run ghci without the monomorphism restriction
Prelude> :set -XNoMonomorphismRestriction
Prelude> let fElem = flip elem
Prelude> :t fElem
fElem :: (Eq a) => [a] -> a -> Bool

(that would be a good entry in your .ghci file anyway, you rarely want it unless you are a 
compiler writer)