racket-dns-2012/mapping.rkt

53 lines
1.8 KiB
Racket

#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 ...))))