71 lines
2.6 KiB
Racket
71 lines
2.6 KiB
Racket
#lang racket/base
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;; Macros for defining weak and extensible mappings between sets of values
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;;
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;;; Copyright 2010, 2011, 2012, 2013 Tony Garnock-Jones <tonyg@ccs.neu.edu>
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;;;
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;;; This file is part of marketplace-dns.
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;;;
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;;; marketplace-dns is free software: you can redistribute it and/or
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;;; modify it under the terms of the GNU General Public License as
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;;; published by the Free Software Foundation, either version 3 of the
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;;; License, or (at your option) any later version.
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;;;
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;;; marketplace-dns is distributed in the hope that it will be useful,
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;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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;;; General Public License for more details.
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;;;
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;;; You should have received a copy of the GNU General Public License
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;;; along with marketplace-dns. If not, see
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;;; <http://www.gnu.org/licenses/>.
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(provide define-mapping)
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;; Internal. Extracts macro keywords from a list of arguments.
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(define-syntax check-defaults
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(syntax-rules ()
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((_ fn bn fd bd #:forward-default new-fd rest ...)
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(check-defaults fn bn new-fd bd rest ...))
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((_ fn bn fd bd #:backward-default new-bd rest ...)
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(check-defaults fn bn fd new-bd rest ...))
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((_ fn bn fd bd (lhs rhs) ...)
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(begin
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(define (fn l)
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(cond
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((eqv? l 'lhs) 'rhs) ...
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(else (fd l))))
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(define (bn r)
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(cond
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((eqv? r 'rhs) 'lhs) ...
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(else (bd r))))))))
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;; Symbol -> raised exn:fail:contract
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;; Used by default to complain when no specific mapping is found.
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;; The argument indicates to the user the direction of the mapping.
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(define (die-with-mapping-name n)
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(lambda (v)
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(raise (exn:fail:contract
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(format "~v: Mapping not found for ~v" n v)
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(current-continuation-marks)))))
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;; (define-mapping <identifier> <identifier>
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;; { #:forward-default <expr> }?
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;; { #:backward-default <expr> }?
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;; (<expr> <expr>) ...)
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;; Defines two functions, forward-name and backward-name, which take
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;; values from the left-hand-sides of the mappings given as "(<expr>
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;; <expr>)" to the right-hand-sides and vice versa, respectively.
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;;
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;; If specified, the #:forward-default and #:backward-default exprs
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;; should evaluate to a procedure of one argument which can be used
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;; for fallback computation of the mapping or for error
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;; reporting. They default to raising exn:fail:contract.
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(define-syntax define-mapping
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(syntax-rules ()
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((_ forward-name backward-name rest ...)
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(check-defaults forward-name
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backward-name
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(die-with-mapping-name 'forward-name)
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(die-with-mapping-name 'backward-name)
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rest ...))))
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