> Expanding `y-coord` by itself leads to the error.
I've tripped over this, so just to elaborate slightly.
`syntax-id-rules` will work, but your current macro:
;;;;;;;;;;;;;;;
(define-syntax y-coord
(syntax-id-rules (map)
[(map y-coord lst) (map (lambda (p)
(point-y p))
lst)]))
;;;;;;;;;;;;;;;
assumes that it will be getting a syntax pattern that looks like `(map y-coord
lst)`.
But it will not. The pattern it gets is simply `y-coord`. (More broadly,
identifier macros made with `syntax-id-rules` will match only three kinds of
patterns: the name alone, the name after an open parenthesis, or the name
within `set!` Killer docs cross-reference: [1])
So you can repair your macro by matching to `y-coord` as a pattern:
;;;;;;;;;;;;;;;
#lang racket
(require rackunit)
(define-syntax y-coord
(syntax-id-rules ()
[y-coord point-y]))
(struct point (x y))
(define points (list (point 25 25) (point 33 54) (point 10 120)))
(check-equal? (map y-coord points) '(25 54 120))
;;;;;;;;;;;;;;;
BTW the "bad syntax" error is a generic error you get when there's no pattern
match inside a macro. Thus, it can be wise to include an `else` branch so that
you can pinpoint the source, for instance:
;;;;;;;;;;;;;;;
(define-syntax y-coord
(syntax-id-rules (map)
[(map y-coord lst) (map (lambda (p)
(point-y p))
lst)]
[else (error 'no-pattern-matched-in-your-y-coord-macro)]))
;;;;;;;;;;;;;;;
[1]
http://docs.racket-lang.org/guide/pattern-macros.html?q=identifier%20macro#%28part._.Identifier_.Macros%29
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