Thanks for the answer, but I'm trying to avoid type classes. By the way, I'm not trying to embed OO into Haskell. I'm trying to solve this issue: https://github.com/haskell-opengl/OpenGLRaw/issues/15 The binding to OpenGL declares GLenum as CUInt and GLboolean as CUChar, meaning I can't use a GLenum as a GLboolean or vice-versa. The C spec allows the use of GLboolean values where GLenums are expected. Maybe I'm taking the wrong approach... 2013/1/16 Felipe Almeida Lessa

For your particular constraints, it can be as easy as:

class IsA a where toA :: a -> A

foo' :: IsA a => a -> C foo' = foo . toA

However, you may asking the wrong question since it smells like you're trying to embed OO into Haskell =).

Cheers,

On Wed, Jan 16, 2013 at 1:03 PM, Thiago Negri wrote:

...

Hello.

How do I achieve type hierarchy in Haskell?

Suppose we have the following code:

foo :: A -> C bar :: B -> C

I want something that allow me to say that B is a subtype of A, meaning: 1. I can use a value of type A where a value of type A is needed. 2. I can use a value of type B where a value of type B is needed. 3. I can use a value of type B where a value of type A is needed. 4. I can't use a value of type A where a value of type B is needed.

What are my options?

I've thought in type classes and data types with an attribute representing the extension. Any other way to do this?

Thanks, Thiago.

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-- Felipe.