#lang racket/base
;; Macros for defining weak and extensible mappings between sets of values

(provide define-mapping)

;; Internal. Extracts macro keywords from a list of arguments.
(define-syntax check-defaults
  (syntax-rules ()
    ((_ fn bn fd bd #:forward-default new-fd rest ...)
     (check-defaults fn bn new-fd bd rest ...))
    ((_ fn bn fd bd #:backward-default new-bd rest ...)
     (check-defaults fn bn fd new-bd rest ...))
    ((_ fn bn fd bd (lhs rhs) ...)
     (begin
       (define (fn l)
	 (case l
	   ((lhs) 'rhs) ...
	   (else (fd l))))
       (define (bn r)
	 (case r
	   ((rhs) 'lhs) ...
	   (else (bd r))))))))

;; Symbol -> raised exn:fail:contract
;; Used by default to complain when no specific mapping is found.
;; The argument indicates to the user the direction of the mapping.
(define (die-with-mapping-name n)
  (lambda (v)
    (raise (exn:fail:contract
	    (format "~v: Mapping not found for ~v" n v)
	    (current-continuation-marks)))))

;; (define-mapping <identifier> <identifier>
;;    { #:forward-default <expr> }?
;;    { #:backward-default <expr> }?
;;    (<expr> <expr>) ...)
;; Defines two functions, forward-name and backward-name, which take
;; values from the left-hand-sides of the mappings given as "(<expr>
;; <expr>)" to the right-hand-sides and vice versa, respectively.
;;
;; If specified, the #:forward-default and #:backward-default exprs
;; should evaluate to a procedure of one argument which can be used
;; for fallback computation of the mapping or for error
;; reporting. They default to raising exn:fail:contract.
(define-syntax define-mapping
  (syntax-rules ()
    ((_ forward-name backward-name rest ...)
     (check-defaults forward-name
		     backward-name
		     (die-with-mapping-name 'forward-name)
		     (die-with-mapping-name 'backward-name)
		     rest ...))))