> On Sep 29, 2015, at 7:20 PM, Brian Mastenbrook <[email protected]> wrote:
>
> After playing with various permutations of this, I encountered a behavior
> that seems like a clear violation of referential transparency:
>
> (define x 'outer)
>
> (define-syntax-rule (def-m m given-x)
> (define-syntax-rule (m)
> (begin
> (define given-x 'inner)
> x)))
>
> (def-m m x)
> (let ()
> (m))
>
> Under the old expander, this yields 'outer; under the new expander, it yields
> 'inner. Under both expanders, the following yields 'outer:
Up to this point, I would have said “obvious, let me explain” but the idea that
wrapping a let () around the last expression changes its expansion blew my
mind. I’d say it’s a bug.
Think of the two-directional substitution I mentioned in Manifesto. Here is the
next step:
(define x 'outer)
(define-syntax-rule
(def-m m given-x)
(define-syntax-rule
(m)
(begin
(define given-x 'inner)
x)))
(define-syntax-rule
(m)
(begin
(define x ‘inner) ;; <— the ‘x’ that is placed here is *the* x that is
define-d above
;; you want this to catch the next x because it is the *same* x and it was
explicitly passed in
x))
(let ()
(m))
So I will wait for Matthew to explain the let-wrapped expression. — Matthias
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