Capabilities

A capability states which operations a type must provide. A constrained function can use those operations without committing to one concrete type. An implementation connects the capability to a concrete type.

Fragment:

class Equal(a) {
  equal :: a -> a -> Bool.
}.

impl Equal(Int) {
  equal = \(left, right) -> left == right.
}.

Capability bodies currently contain method signatures, and implementation targets must be concrete. The compiler rejects duplicate capabilities or implementations, incorrect arity, an unsatisfied constraint, and ambiguous concrete implementations.

The bundled capabilities are Eq, Ord, Num, Integral, Fractional, Showable, and Default. Their methods and built-in implementations are documented in Prelude.

Partial: an explicit Class::method reference can dispatch when exactly one visible concrete implementation applies. Cross-module method visibility, user-visible dictionaries, default methods, superclasses, and a general overlap policy are not implemented.

See the expression grammar for declaration and constraint notation.