Type Classes: Defining Overloads

Type classes define a set of operations that can be implemented for many types.

Rex type classes are similar to Haskell’s: they are compile-time constraints with runtime dictionary resolution.

Defining a class

class Size a
  size : a -> i32

Method signatures can mention the class parameter a and any other types in scope.

You may also see:

class Size a where
  size : a -> i32

Both forms are accepted.

class Eq a
  == : a -> a -> bool
  != : a -> a -> bool

Superclasses use <= (read “requires”):

class Ord a <= Eq a
  < : a -> a -> bool

If you have an Ord a, you also must have an Eq a instance.

Some prelude classes logically take multiple type parameters, such as Indexable t a.

In Rex source you write:

class Indexable t a
  get : i32 -> t -> a

In where constraints, multi-parameter classes are written using a tuple:

where Indexable (t, a) -> ...

This matches the implementation model described in Specification.