Re: Does declaration order matter in guile?

[Maxime Devos]

On 13-02-2023 09:05, Sascha Ziemann wrote:
> You also can not ask Scheme about macros, because macros are not
> first-class-citizens.
>
>
> This might be interesting:
> 
https://matt.might.net/articles/metacircular-evaluation-and-first-class-run-time-macros/

You actually can ask (Guile)Scheme about macros, and they are 
first-class (just not procedures, and almost never actually used as 
first-class things):

;; first-class: macros are just a type of values
(let ((the-case-macro (module-ref (current-module) 'case)))
  (pk the-case-macro) ; -> #<syntax-transformer case>
  ;; This type is disjoint from procedures:
  (pk (procedure? the-case-macro)) ; -> #false
  (pk (macro? the-case-macro)) ; -> #true
  ;; You can use macros to transform syntax to new syntax at runtime:
  (pk (procedure? (macro-transformer the-case-macro))) ; -> #true
  (pk ((macro-transformer the-case-macro)
        #'(case number
           ((one) 1)
           ((two) 2)))) ; -> #<syntax: [lots of stuff]>
  ;; You can make macros at runtime (though usually you would just
  ;; pass syntax-transforming procedures instead of the macro wrapper
  ;; type):
  (pk (make-syntax-transformer 'pick-a-name 'macro (lambda (s) #'0)))
  (values))

While unconventional, in principle there is nothing stopping you 
(besides cross-Scheme compatibility) from using a combination of 
'let-syntax-syntax', 'syntax-case' and 'syntax-local-binding' to make 
let macros accept macros as arguments.  Example:

(use-modules (system syntax))
(define-syntax call-macro-for-each
  (lambda (s)
    (syntax-case s ()
      ((_ macro-identifier arg ...)
       (call-with-values
         (lambda ()
           ;; Note: syntax-local-binding implicitly calls macro-transformer
           (syntax-local-binding #'macro-identifier))
         (lambda (type value)
           (unless (eq? type 'macro)
             (error "first argument to call-macro-for-each must be an 
identifier of a macro"))
           (let loop ((arguments #'(arg ...)))
             (syntax-case arguments ()
               ((last) (value #'last))
               ((stuff . more-stuff)
                #`(begin #,(value #'stuff)
                         #,(loop #'more-stuff)))))))))))

(let-syntax ((pk+quote
               (lambda (s)
                 #`(pk '#,s '-> #,s))))
  (call-macro-for-each pk+quote (+ 1 1) (+ 1 2) (+ 1 3)))
;; Output:
;;; ((+ 1 1) -> 2)

;;; ((+ 1 2) -> 3)

;;; ((+ 1 3) -> 4)
$1 = 4

The only snag here is that each macro you want to pass to a 
'higher-order macro', you need to give an identifier.  As you can always 
do that with 'let-syntax', that doesn't make it non-first class IMO.

Greetings,
Maxime.

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