42 lines
1.2 KiB
Plaintext
42 lines
1.2 KiB
Plaintext
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(define-syntax color (syntax-rules () ((color) 'green)))
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(define-syntax with-blue
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(lambda (exp r c)
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`(,(r 'let-syntax) ((color (,(r 'syntax-rules) ()
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((color) 'blue))))
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. ,(cdr exp))))
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(with-blue (color))
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;;; This has a problem -- WITH-BLUE is not hygenic:
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(let ((color (lambda () 'foo)))
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(with-blue (color)))
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=> blue
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;;; Let's fix this by adding a layer of indirection --
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;;; 1. (color) ==> (hidden-color)
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;;; 2. with-blue frobs the definition of *our* hidden-color
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(define-syntax hidden-color (lambda (exp r c) `(,(r 'quote) green)))
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(define-syntax color (lambda (exp r c) `(,(r 'hidden-color))))
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(define-syntax with-blue
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(lambda (exp r c)
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`(,(r 'let-syntax)
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((,(r 'hidden-color) (,(r 'syntax-rules) ()
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((,(r 'hidden-color)) (,(r 'quote) blue)))))
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. ,(cdr exp))))
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;;; Without all the renaming, the above is
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;;; (let-syntax ((hidden-color (syntax-rules () ((hidden-color 'blue)))))
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;;; body ...)
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;;; where *all* symbols on the first line are renamed, *including*
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;;; hidden-color, so we should be redefining the same hidden-color
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;;; to which (color) expands.
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;;; It doesn't work:
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(with-blue (color))
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=> green
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