[racket-users] Macro help

[Kevin Forchione]

Hi guys,
I’ve been wracking my brains all day trying to come up with a macro that would convert this syntax:

;; (aux a (b (* 2 pi)) c (d pi))
;; => (define-values (a b c d) (values #f 6.28318530717958 #f 3.141592653589793)


I’m missing some part of the picture. The closest I’ve come is to create a list of the pairs:

#lang racket

(define-syntax (aux stx)
(syntax-case stx ()
[(_ (var val)) #'`((var ,val))]
[(_ var) #''((var #f))]
[(_ (var0 val0) var1 ...) #'(append `((var0 ,val0)) (aux var1 ...))]
[(_ var0 var1 ...) #'(append '((var0 #f)) (aux var1 ...))]))

(aux a (b (* 2 pi)) c (d pi)) ;=> '((a #f) (b 6.283185307179586) (c #f) (d 3.141592653589793))


Any help is greatly appreciated!

Kevin

[Michael Murdock MacLeod]

Does this work? It uses a helper function, `prune`, to parse the var-val
clauses.

#lang racket

(define-for-syntax (prune stx)
(syntax-case stx ()
[()
#'()]
[((var val) others ...)
(cons #'(var val)
(prune #'(others ...)))]
[(var others ...)
(cons #'(var #f)
(prune #'(others ...)))]))

(define-syntax (aux stx)
(syntax-case stx ()

[(_ terms ...)
(with-syntax ([((var val) ...) (prune #'(terms ...))])
#'(define-values (var ...) (values val ...)))]))

(aux a (b (* 2 pi)) c (d pi))

a
b
c
d

;; output shown below

#f
6.283185307179586
#f
3.141592653589793

[Matthias Felleisen]

Let me propose the use of syntax-parse as an alternative here. I think it clarifies the purpose of the maco. — Matthias

#lang racket

(require (for-syntax syntax/parse))

#; (aux a (b (* 2 pi)) c (d pi))

;; =>

#; (define-values (a b c d) (values #f 6.28318530717958 #f 3.141592653589793))

(begin-for-syntax

  (define-syntax-class optionally-initiliazed

    (pattern (lhs:id rhs:expr))

    (pattern lhs:id #:attr rhs #'#f)))

(define-syntax (aux stx)

  (syntax-parse stx

    [(_ p:optionally-initiliazed ...) #'(define-values (p.lhs ...) (values p.rhs ...))]))

(aux a (b (* 2 pi)) c (d pi))

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[Stephen Chang]

If `define-values` is not strictly required, here's a more
syntax-case-y recursive version:

(define-syntax (aux stx)
(syntax-case stx ()

[(_) #'(begin)]
[(_ (var val) . rst)
(identifier? #'var)
#'(begin
(define var val)
(aux . rst))]
[(_ var . rst)
(identifier? #'var)
#'(begin
(define var #f)
(aux . rst))]))

> To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/A0B65B5E-0244-4E7C-89E5-24ED2CDF69EB%40felleisen.org.

[Kevin Forchione]

Wow! Thanks so much guys. Each of these solutions adds another brick to the wall of my macro eduction!

Kevin

[yfzhe]

Another "syntax-case-y" version generates `define-values`:

(define-syntax-rule (aux clause ...)
  (aux-helper () (clause ...)))

(define-syntax (aux-helper stx)
  (syntax-case stx ()
    [(_ ([id val] ...) ())
     #'(define-values (id ...) (values val ...))]
    [(_ (id+val ...) ([id val] more ...))
     (identifier? #'id)
     #'(aux-helper (id+val ... [id val]) (more ...))]
    [(_ (id+val ...) (id more ...))
     (identifier? #'id)
     #'(aux-helper (id+val ... [id #f]) (more ...))]))

在 2019年5月24日星期五 UTC+8下午12:41:20，lysseus写道：