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 where {
  size : a -> i32;
}

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

Classes with methods use where { ... }. Marker classes with no methods use a semicolon:

class Marker a;

class Eq a where {
  == : a -> a -> Bool;
  != : a -> a -> Bool;
}

Superclasses use <= (read “requires”):

class Ord a <= Eq a where {
  < : a -> a -> Bool;
}

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

User-defined classes may take multiple type parameters. For example:

In Rex source you write:

class Convert a b where {
  convert : a -> b;
}

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

where Convert (a, b) -> ...

This matches the implementation model described in Specification.