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

[Andrew Wagner]

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 <raeck at msn.com> wrote:

>  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 at gmail.com
> To: raeck at msn.com
> Subject: Re: [Haskell-cafe] Defining a containing function on polymorphic
> list
> CC: haskell-cafe at haskell.org; beginners at 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 Eq<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#t:Eq>a
> where (==)<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v:%3D%3D>:: a -> a ->
> Bool<http://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t:Bool>
> (/=)<http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Eq.html#v:/%3D>:: a -> a ->
> Bool<http://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Bool.html#t: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 at 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
>
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