Re: [Haskell-cafe] Query regarding Type classes

Hello Ryan.. On Mon, Oct 13, 2008 at 3:33 PM, Ryan Ingram 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 :

...

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