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A lexical use-modules?


From: Maxime Devos
Subject: A lexical use-modules?
Date: Sat, 26 Mar 2022 20:21:07 +0100
User-agent: Evolution 3.38.3-1

Hi,

[ CC'ing guix-devel@ because this functionality could be useful in Guix
package definitions ]

Currently, use-modules cannot be meaningfully used inside procedures,
unless '(current-module)' is always the module in which the procedure
is defined.

I wondered if some kind of 'lexical use-modules' was possible, with
sufficient macroology and module reflection, and it looks like it is:

(use-modules (srfi srfi-1))
(define-syntax use-module/lexical
  ;; todo: integrate into (use-modules ...)?
  (lambda (s)
    (syntax-case s ()
      ((_ foo)
       (let* ((module-name (syntax->datum #'foo))
              (interface (resolve-interface module-name)))
         (define (binding->import name variable)
           (define name-syntax (datum->syntax s name))
           #`(define-syntax #,name-syntax (identifier-syntax (@ foo
#,(datum->syntax #'irrelevant name)))))
         #`(begin #,@(module-map binding->import interface)))))))

(define &exception 'top-level)
(let ()
  (use-module/lexical (ice-9 exceptions))
  (pk 'inner &exception raise-continuable))
;;; (inner #<record-type &exception> #<procedure raise-continuable (obj)>)

(pk 'outer &exception)
;;; (outer top-level)

(pk 'unbound-variable raise-continuable)
;; a backtrace!

Limitation: things like

(define (foo)
  (use-module/lexical (platform-specific-constants))
  (if on-linux?
      CONSTANT_ONLY_DEFINED_ON_LINUX
      CONSTANT_ONLY_DEFINED_ON_HURD))

won't work in cross-compilation contexts.

Greetings,
Maxime.

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