[Haskell-cafe] Query regarding Type classes

[Arun Suresh]

Hello Ryan..

On Mon, Oct 13, 2008 at 3:33 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:

> Step 1: Forget everything you know about OO classes, then try again :)


you seem to have read my mind [:)].. i actaually hit upon this issue while
trying to "transcode" some C++ to Haskell..

>
>
> 2008/10/13 Arun Suresh <arun.suresh at gmail.com>:
> > class Foo a where
> >     fooFunc :: a -> Int
> >
> > data FooData = FData
> >
> > instance Foo FooData where
> >     fooFunc _ = 10
>
> So far so good.
>
> > class Bar a where
> >     barFunc :: (Foo b) => a -> b -> Int
>
> So now, barFunc advertises "for any type a which is in the Bar
> typeclass, and any type b which is in the Foo typeclass, I can give
> you a function from a to b to Int".  You can make an implementation of
> this that has different functionality based on "a", but it needs to be
> polymorphic in "b", taking any type "b" which is a member of Foo.
>
> > data BarData = BData
> >
> > instance Bar BarData where
> >     barFunc _ FData = 20
>
> Uh oh!  FData is of type FooData!  But we just advertised that we
> could take *anything* which is in the typeclass Foo, which can include
> any number of other types.  It doesn't matter that FooData is the only
> declared member at this point.  What that means is that if you want to
> do anything with the second argument, you can only use functions that
> accept any type which is a member of the Foo class.  Right now that
> just means generically polymorphic functions (id, const, etc.), and
> the fooFunc function inside the "Foo" typeclass.
>
> For example, this declaration would work:
>
> instance Bar BarData where
>    barFunc _ f = 10 + fooFunc f
>
> > When I compile I get this :
> >  Couldn't match expected type `b' against inferred type `FooData'
> >       `b' is a rigid type variable bound by
> >           the type signature for `barFunc' at Sample.hs:16:20
> >     In the pattern: FData
> >     In the definition of `barFunc': barFunc _ FData = 20
> >     In the definition for method `barFunc'
>
> The compiler is just telling you what I just told you: barFunc says
> that it should take any type "b", but the pattern FData constrains the
> argument to be a FooData.
>
> > Think Im missing something really big...
> > Could somebody kindly help me out here...
>
> I recommend reading http://www.haskell.org/haskellwiki/OOP_vs_type_classes


Thanks...
Arun


>
>  -- ryan
>
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