I was able to do the first half of exercise 1 (see sources at the end
of this message) --- write a macro computing some information at
run-time. The second half is to write the same macro, but computing
some information at compile-time --- I couldn't do this one. I'm
looking for some directions on how to achieve this.
(*) Context
; Syntax -> Syntax
; (define-world* name ...) defines all name ... to stand for a string
(define-syntax (define-world* stx)
(syntax-parse stx
((_ x:id ...)
#'(define-values (x ...) (values (begin 'x "world") ...)))))
(*) Exercise 1
Exercise 1. Modify define-world* so that it binds each given
identifier to its place in the sequence. We start counting at 0 so
that
(define-world* x y z)
binds x to 0, y to 1, and so on.
Consider two variants: one that compiles the values at compile time
and another one that computes them at run time. The second one might
be a bit easier, but do try your hands at both.
(*) My first solution
; Syntax -> Syntax
; (define-naturals-runtime* ...) defines all names ... each to natural
; number beginning at 0 (we use a counter computed at runtime).
(define-syntax (define-naturals-runtime* stx)
(syntax-parse stx
((_ x:id ...)
#'(begin
(define n -1)
(begin
(set! n (add1 n))
(define x n)) ...))))
(*) Compile-time version
That's my trouble. Here's what I wrote.
(define-syntax (define-naturals* stx)
(define n -1)
(syntax-parse stx
((_ x:id ...)
#`(begin
(begin
#,(set! n (add1 n))
(define x #,n)) ...))))
> (syntax->datum (expand-once '(define-naturals* a b c)))
'(begin
(begin #<void> (define a 0))
(begin #<void> (define b 0))
(begin #<void> (define c 0)))
I see (set! n (add1 n)) took place once --- at compile-time. I
expected something like:
'(begin
(begin #<void> (define a 0) (set! n 1))
(begin #<void> (define b 1) (set! n 2))
(begin #<void> (define c 2) (set! n 3)))
I think I don't understand the real definition of "...".
Thank you.
Sources:
https://summer-school.racket-lang.org/2018/plan/mon-mor-lecture.html
https://summer-school.racket-lang.org/2018/plan/mon-mor-lab.html
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