There are two ways to fix this. Let me see if I can get my syntax right this
time :)
1.) Let GHC work out the Eq instance:
data Shape = Square | Triangle | Circle deriving Eq
2.) Tell GHC how to do it explicitly:
data Shape = Square | Triangle | Circle
instance Eq Shape where
Square == Square = True
Triangle == Triangle = True
Circle == Circle = True
_ == _ = False
Note that the last line here means that any other comparisons are false.
On Mon, Dec 22, 2008 at 9:35 AM, Raeck Zhao
Thank you very much for your reply! It is really helpful!
But I just found another 'problem', I just realize that the list does not support the user-defined data type? the list is also depending on the Eq function?
For example,
data Shape = Square | Triangle | Circle
when I type either
[Square, Triangle, Circle]
or
Square == Square
there are errors!
So there is no way to construct a truly polymorphic List? any way to extend the list to support some user-defined data type?
Or... I define the Shape in a wrong way actually?
Thanks
Raeck
------------------------------ Date: Mon, 22 Dec 2008 09:02:53 -0500 From: wagner.andrew@gmail.com To: raeck@msn.com Subject: Re: [Haskell-cafe] Defining a containing function on polymorphic list CC: haskell-cafe@haskell.org; beginners@haskell.org
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 Eqhttp://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#t:Eqa where (==)http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v:%3D%3D:: a -> a -> Boolhttp://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t:B... (/=)http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v:/%3D:: a -> a -> Boolhttp://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t:B... 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
contain :: a -> [a] -> Bool contain x [] = False contain x (y:ys) = if x == y then True else contain x ys it seems that
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: 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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