Ability patterns (or Request patterns)

An ability pattern only appears in an ability handler and has one of two forms (see Abilities and ability handlers for details):

1. {C p1 p2 … pn -> k} where C is the name of an ability constructor in scope, and p1 through pn are patterns such that n is the arity of C. Note that n may be zero. This pattern matches if the scrutinee reduces to a fully applied invocation of the ability constructor C and the patterns p1 through pn match the arguments to the constructor. The scrutinee must be of type Request A T for some ability {A} and type T. The variable k will be bound to the continuation of the program. If the scrutinee has type Request A T and C has type X ->{A} Y, then k has type Y -> {A} T.
2. {p} where p is a pattern. This matches the case where the computation is pure (the value of type Request A T calls none of the constructors of the ability {A}). A pattern match on a Request is not complete unless this case is handled.

See the section on abilities and ability handlers for examples of ability patterns.