The problem here is even slightly deeper than you might realize. For example, what if you have a list of functions. How do you compare two functions to each other to see if they're equal? There is no good way really to do it! So, not only is == not completely polymorphic, but it CAN'T be.

There is a nice solution for this, however, and it's very simple:

contain :: Eq a -> [a] -> Bool
contain x [] = False
contain x (y:ys) = if x == y then True else contain x ys

The "Eq a" in the type signature says that 'a' must be a member of the 'Eq' typeclass. That says, in turn, that 'a' must have == defined for it. Fortunately, most types have, or can easily derive that definition. Here is the definition of the typeclass:

class Eq a where
(==) :: a -> a -> Bool
(/=) :: a -> a -> Bool

That is, for 'a' to be a member of 'Eq', it must have a == operator which can take 2 values of that type and return a Boolean, saying whether or not they're equal, and it must also have a definition for the /= operator, which is "not equal". These two are also defined in terms of each other, so if you define ==, you get /= for free, and vice versa.

That's probably more information than you needed to know, but I hope it helps.

2008/12/22 Raeck Zhao <raeck@msn.com>
I am trying to define a containing function to see if a value is one of the elements within a list which is polymorphic, but failed with the following codes:
> contain :: a -> [a] -> Bool
> contain x [] = False
> contain x (y:ys) = if x == y then True else contain x ys it seems that the problem is the 'operator' == does not support a polymorphic check?
Any way can solve the problem? or any alternative solution to achieve the purpose?
Thanks!
Raeck

It's the same Hotmail®. If by "same" you mean up to 70% faster. Get your account now.

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe