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.