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<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#t%3AEq>a where(==)<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v%3A%3D%3D>:: a -> a -> Bool<http://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t%3ABool> (/=)<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v%3A%2F%3D>:: a -> a -> Bool<http://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t%3ABool> 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 <ra...@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(R). If by "same" you mean up to 70% faster. Get your > account > now.<http://windowslive.com/online/hotmail?ocid=TXT_TAGLM_WL_hotmail_acq_broad1_122008> > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
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