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