forked from cadence/breezewiki
55 lines
2.1 KiB
Racket
55 lines
2.1 KiB
Racket
|
#lang racket/base
|
||
|
|
(require (for-syntax racket/base))
|
||
|
|
|
||
|
|
(provide
|
||
|
|
; help make a nested if where the false results are the same
|
||
|
|
if/out)
|
||
|
|
|
||
|
|
(module+ test
|
||
|
|
(require rackunit)
|
||
|
|
(define (check-syntax-equal? s1 s2)
|
||
|
|
(check-equal? (syntax->datum s1)
|
||
|
|
(syntax->datum s2))))
|
||
|
|
|
||
|
|
;; actual transforming goes on in here.
|
||
|
|
;; it's in a submodule so that it can be required in both levels, for testing
|
||
|
|
|
||
|
|
(module transform racket/base
|
||
|
|
(provide transform-if/out)
|
||
|
|
(define (transform-if/out stx)
|
||
|
|
(define tree (cdr (syntax->datum stx))) ; condition true false
|
||
|
|
(define else (cddr tree)) ; the else branch cons cell
|
||
|
|
(define result
|
||
|
|
(let walk ([node tree])
|
||
|
|
(cond
|
||
|
|
; normally, node should be a full cons cell (a pair) but it might be something else.
|
||
|
|
; situation: reached the end of a list, empty cons cell
|
||
|
|
[(null? node) node]
|
||
|
|
; situation: reached the end of a list, cons cdr was non-list
|
||
|
|
[(symbol? node) node]
|
||
|
|
; normal situation, full cons cell
|
||
|
|
; -- don't go replacing through nested if/out
|
||
|
|
[(and (pair? node) (eq? 'if/out (car node))) node]
|
||
|
|
; -- replace if/in
|
||
|
|
[(and (pair? node) (eq? 'if/in (car node)))
|
||
|
|
(append '(if) (cdr node) else)]
|
||
|
|
; recurse down pair head and tail
|
||
|
|
[(pair? node) (cons (walk (car node)) (walk (cdr node)))]
|
||
|
|
; something else that can't be recursed into, so pass it through
|
||
|
|
[#t node])))
|
||
|
|
(datum->syntax stx (cons 'if result))))
|
||
|
|
|
||
|
|
;; the syntax definitions and their tests go below here
|
||
|
|
|
||
|
|
(require 'transform (for-syntax 'transform))
|
||
|
|
|
||
|
|
(define-syntax (if/out stx)
|
||
|
|
(transform-if/out stx))
|
||
|
|
(module+ test
|
||
|
|
(check-syntax-equal? (transform-if/out #'(if/out (condition 1) (if/in (condition 2) (do-yes)) (do-no)))
|
||
|
|
#'(if (condition 1) (if (condition 2) (do-yes) (do-no)) (do-no)))
|
||
|
|
(check-equal? (if/out #t (if/in #t 'yes) 'no) 'yes)
|
||
|
|
(check-equal? (if/out #f (if/in #t 'yes) 'no) 'no)
|
||
|
|
(check-equal? (if/out #t (if/in #f 'yes) 'no) 'no)
|
||
|
|
(check-equal? (if/out #f (if/in #f 'yes) 'no) 'no))
|