[Haskell-beginners] Question about define my own typeclass

[Michael Orlitzky]

On 03/27/2014 11:28 AM, ke dou wrote:
> Hi,
> 
> Sorry for the spam.
> 
> I am new to Haskell.
> I want to define my own typeclass which can convert from my own types
> like MyBool, MyInt, and MyString to according Haskell types.

As you've seen, this is actually a hard problem and it needs some of
GHC's more advanced machinery.

Mateusz's solution uses FunctionalDependencies; there is a similar
extension called TypeFamilies which allow you to do many of the same
things with (IMO) a nicer syntax. You can think of TypeFamilies as
allowing you to define functions between types. And then, just like you
can define functions between values in a typeclass, you can define
functions between types. This allows you to say (in the instance
declaration) which return type goes with MyBool, MyInt, etc.

Here is a simple modification of your program (2.5 lines?) using type
families. In the type class definition, the "type Return a..." line
means that each instance declaration needs to define a type associated
'a' called 'Return a'. Then in the type signature of 'conversion', we
can use that type, solving the problem that others have pointed out.


> {-# LANGUAGE TypeFamilies #-}
> 
> module Conversion where
> 
> import qualified Prelude
> 
> class Conversion a where
>   type Return a :: *
>   conversion :: a -> (Return a)
> 
> data MyBool = MyTrue | MyFalse
> 
> instance Conversion MyBool where
>   type Return MyBool = Prelude.Bool
>   conversion MyTrue = Prelude.True
>   conversion MyFalse = Prelude.False