Multimethods

(require (planet "main.ss" ("murphy" "multimethod.plt" 1 0)))

Provides procedures with versatile dispatch on any number of their arguments.

Inheritance hierarchies for method signatures can be defined in an ad-hoc fashion or class and interface hierarchies from scheme/class can be used.

A small usage example:

(derive 'rock 'item)

(derive 'paper 'item)

(derive 'scissors 'item)

(define-multi (beats? a b) :: (vector-immutable a b))

(define-method (beats? a b) :: #(item item)

#f)

(define-method (beats? a b) :: #(rock scissors)

#t)

(define-method (beats? a b) :: #(paper rock)

#t)

(define-method (beats? a b) :: #(scissors paper)

#t)

> (derived? 'rock 'item)

#t

> (derived? 'rock 'rock)

#t

> (derived? 'rock 'paper)

#f

> (descendants 'item)

#hash((rock . #t) (paper . #t) (scissors . #t))

> (beats? 'rock 'paper)

#f

> (beats? 'rock 'scissors)

#t

> (beats? 'limestone 'scissors)

multimethod: no method for signature #(limestone scissors)

> (derive 'limestone 'rock)

> (beats? 'limestone 'scissors)

#t

> (parameterize-multi (beats?)       (define-method (beats? a b) :: #(limestone paper)         #t)       (beats? 'limestone 'paper))

#t

> (beats? 'limestone 'paper)

#f

> (parameterize ([global-hierarchy (derive (global-hierarchy)                                              'marble 'rock)])       (beats? 'marble 'scissors))

#t

> (beats? 'marble 'scissors)

multimethod: no method for signature #(marble scissors)