[Haskell-beginners] Defining a containing function on polymorphic list

[Raeck Zhao]

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 
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