[Haskell-beginners] function parameter types after flip

[Daniel Seidel]

Robert Ziemba wrote:
> 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]

I think its due to the monomorphism restriction.
If you define it like

Prelude> let fElem x = (flip elem) x
Prelude> :t fElem
fElem :: (Eq a) => [a] -> a -> Bool

it works. For the details look at

http://haskell.org/haskellwiki/Monomorphism_restriction

... or wait for another answer (which I think will come).

Daniel.


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