2017-08-01 20:11 GMT+02:00 Jordan Johnson <j...@fellowhuman.com>:
> Hi all,
>
> Reading the documentation on ~datum in syntax/parse, I see:
>
> The ~datum form is useful for recognizing identifiers symbolically, in
> contrast to the ~literal form, which recognizes them by binding.
> That makes sense to me. But then I see:
>
> > (syntax-parse (let
> <http://../reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29>
> ([define
> <http://../reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29>
> 'something-else]) #'(define
> <http://../reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29>
> x y))
> [((~datum define
> <http://../reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29>
> ) var:id e:expr) 'yes]
> [_
> <http://../reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29>
> 'no])
>
> 'yes
> > (syntax-parse (let
> <http://../reference/let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29>
> ([define
> <http://../reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29>
> 'something-else]) #'(define
> <http://../reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29>
> x y))
> [((~literal define
> <http://../reference/define.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._define%29%29>
> ) var:id e:expr) 'yes]
> [_
> <http://../reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29.__%29%29>
> 'no])
>
> 'yes
>
> I’m confused by this. It seems, based on the first sentence I quoted, that
> in the second example, in which the syntax being parsed has *define* bound
> to something other than the usual, the use of *~literal* should make the
> first clause fail to match. And I’d have expected the examples to contrast
> *~datum* and *~literal*, rather than exhibit the same behavior.
>
The binding information is attached a syntax object during expansion.
In your example the #'(define x y) haven't gone through expansion,
so you do not get what your expected result.
However we can change your macro a little:
#lang racket
(require (for-syntax syntax/parse))
(define-syntax (is-define? stx)
(syntax-parse stx
[(_is-define? id)
(syntax-parse #'id
[(~literal define) #''yes]
[_ #''no])]))
(is-define? define)
(let ([define 42])
(is-define? define))
This will result in
'yes
'no
/Jens Axel
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