> On May 2, 2016, at 11:50 AM, Sam Tobin-Hochstadt <sa...@cs.indiana.edu> wrote:
>>
>> My initial statement was less precise than my second email.
>> `(syntax-local-introduce stx)` adds a single scope, using the
>> equivalent of `'flip` mode. That scope is "the current scope" for the
>> macro expansion step.
I appreciate your more detailed explanation.
The common thread in the Knauth/Butterick critique is that
`syntax-local-introduce` promises a result — syntax, locally introduced — but
in fact delivers a certain action — adding a scope — that will only achieve the
promised result if a certain other criterion is met (namely, a context-free
syntax object). This is not apparent from the docs (though I will prepare a
pull request to make it so).
Even with this clarification, some puzzling behavior awaits. For instance, you
are correct that using `(datum->syntax #f 'x)` makes my two original examples
work. But consider example #3 — where a subsequent local definition will shadow
the `syntax-local-introduce` identifier, without an error — and example #4,
which reverses the order of the two binding expressions, which then triggers an
ambiguous-binding error.
;;;;
#lang racket
(require rackunit)
(module macros racket
(provide (all-defined-out))
(define-syntax (sli-x stx)
(syntax-case stx ()
[(_) (with-syntax ([x (syntax-local-introduce (datum->syntax #f 'x))])
#'(define x 'syntax-local-introduce-x))]))
(define-syntax-rule (invoke-x) x))
(require 'macros)
;; example 1
(check-equal?
(let ()
(sli-x)
x) 'syntax-local-introduce-x)
;; example 2
(check-equal?
(let ()
(sli-x)
(invoke-x)) 'syntax-local-introduce-x)
;; example 3: local definition shadows `syntax-local-introduce` but ...
(check-equal?
(let ()
(sli-x)
(define x 'locally-defined)
x) 'locally-defined)
;; example 4 ... if the two are reordered, you get an ambiguous-binding error
#;(check-equal?
(let ()
(define x 'locally-defined)
(sli-x)
x) 'locally-defined)
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