Question

Here http://en.wikibooks.org/wiki/Haskell/Classes_and_types in section Class inheritance, I read “A class can inherit from several other classes: just put all the ancestor classes in the parentheses before the =>.”

I am puzzled when “(…)=>” is described as “inheritance”. So far as I can see, it’s simply a class constraint. It merely says that this newly defined class (in the example: Real) applies to types which are already members (have instances for) the listed classes (Num and Ord).

In short, the “(…)=>” seems to me to act like a filter for qualities required of the types for which instances of this class may be created, and does not act to augment either the class or its instances.

Am I missing something? Is there some sense in which the “(…)=>” actually passes something along from “parent” to “child”?

Answer 1

In practice, this means that all members of the subclass necessarily provide all methods of the superclass.

So, as in the linked example, we can write a method that requires Eq, but only give it an Ord constraint, and the Eq methods are implied for us.

(Note that inheritance is probably a terrible term for this, because it carries a lot of associations that don’t make sense in our context. Nonetheless, I figured I might as well explain it.)