> On Aug 18, 2017, at 9:44 AM, Sam Waxman <samwax...@gmail.com> wrote:
>
> On Friday, August 18, 2017 at 9:31:33 AM UTC-4, Matthias Felleisen wrote:
>>> On Aug 18, 2017, at 5:28 AM, Sam Waxman <samwax...@gmail.com> wrote:
>>>
>>> If I have code like this,
>>>
>>> (define-syntax-rule (identity x)
>>> x)
>>>
>>> ((identity identity) 1)
>>>
>>> At phase 1, the (identity identity) will get transformed into identity. As
>>> macros expand outside in, however, this won't turn into (identity 1) like a
>>> function would, it will try expanding the identity macro all by itself
>>> without any "arguments" and give a bad syntax error.
>>>
>>> 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.
>>
>>
>> The explanation of the error is wrong. Run it in DrRacket and see which
>> identity it highlights for the error message. [I’d rather have you come up
>> with a proper explanation yourself. That way you will remember.]
>
> It highlights the second. I don't think I misspoke above, but please correct
> me if I'm wrong.
>
> ((identity identity) 1) first starts by receiving the entire s-expression.
> Then it tries expanding the first thing in that s-expression which is
> (identity identity)
>
> To do this, it goes to the identity rule, which states that identity is
> allowed to expand if it has exactly one argument. In this case it does, so
> #'(identity identity) becomes #'identity. Then instead of moving back to the
> first s-expression and moving onto 1, because macro-expansion goes outside-in
> (and more importantly, I suppose, depth first), the macro expander then tries
> to expand #'identity. This is where the bad syntax error comes from because
> the rule for identity says it must have an argument, but this identity is
> getting expanded with no arguments.
Sorry I misread your words. Yes, the expander re-expands the output of a
transformer (see below) and thus id is mentioned w/o arguments so the
compile-time call fails (or in old expanders produces a free var).
;; Expression = S-expression over Symbols only
;; Table = [Listof [List Symbol Transformer]]
;; Transformer = [Expression -> Expression]
;; Expression Table -> Expression
(define (expander exp syntax-table)
(displayln exp)
(match exp
[(? number? n) n]
[(? symbol? s) s]
[`(,(? symbol? token) ,t)
(cond
[(in-domain? token syntax-table)
(expander ((retrieve token syntax-table) exp) syntax-table)]
[else (error 'expander "not a macro: ~e" token)])]
[`(lambda (,x) exp-body) `(lamnbda (,x) (expander exp-body))]
[`(,f ,a) `(,(expander f syntax-table) ,(expander a syntax-table))]
[else (error 'expander "bad syntax: ~e" exp)]))
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