Shapes

A shape names behavior that different types can provide.

export let Eq [T] := shape {
  let eq (left : T, right : T) : Bool;
  law reflexive (value : T) := eq(value, value);
  law symmetric (left : T, right : T) := eq(left, right) = eq(right, left);
};

Define required behavior

export let Eq [T] := shape {
  let eq (left : T, right : T) : Bool;

  law reflexive (value : T) := eq(value, value);
  law symmetric (left : T, right : T) := eq(left, right) = eq(right, left);
};

The shape says what must exist. It does not say which fields a type has.

Keep shapes small

A small shape is easy to satisfy and easy to test. Split unrelated behavior into separate shapes.

Good shape names describe capability: Eq, Ord, Printable, Serializable.

Shapes are contracts

Code that asks for a shape can call the behavior from that shape without knowing the concrete type.

Shape members

Shape members are required operations. In Eq, the required operation is eq. Any given value for Eq[Int], Eq[String], or another type must provide that operation.

Laws live next to the required operation. They explain behavior callers rely on: equality should be reflexive and symmetric.