> On Aug 18, 2017, at 2:28 AM, Sam Waxman <samwax...@gmail.com> wrote:
>
> If I have code like this,
>
> (define-syntax-rule (identity x)
> x)
>
> ((identity identity) 1)
>
> Is there a simple way to change this so that ((identity identity) 1) actually
> does expand into (identity 1) which expands into 1? I'm guessing not, but it
> would certainly be a nice trick if there were.
You need an "identifier macro", which is a macro that will match the case where
the macro name is used in an identifier position. [1] For instance:
#lang racket
(require rackunit)
(define-syntax (identity stx)
(syntax-case stx ()
[(identity arg) #'(#%app identity arg)]
[identity (identifier? #'identity) ; guard prevents matching `(identity)`
#'(λ (x) x)]))
(check-true (procedure? identity))
(check-equal? ((identity identity) 1) 1)
(check-equal? (map identity (range 5)) (range 5))
[1]
http://docs.racket-lang.org/guide/pattern-macros.html?q=identifier%20macro#%28tech._identifier._macro%29
<http://docs.racket-lang.org/guide/pattern-macros.html?q=identifier%20macro#(tech._identifier._macro)>
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