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Translation of SRFI-26

tweeks

I am translating SRFI-26/cut.scm into SKILL++.  (See here for a description of how it's supposed to work, and here for a test suite).

This looked about right: 

(define_syntax cut_internal
  (syntax_rules (X XXX)
    ((_ (slot_name ...) (proc arg ...))
     (lambda (slot_name ...) ((begin proc) arg ...)))
    ((_ (slot_name ...) (proc arg ...) XXX)
     (lambda (slot_name ... @rest rest_slot)
       (apply proc arg ... rest_slot)))
    ((_ (slot_name ...) (position ...) X se ...)
     (cut_internal (slot_name ... x) (position ... x) se ...))
    ((_ (slot_name ...) (position ...) nse se ...)
     (cut_internal (slot_name ...) (position ... nse) se ...))))

(define_syntax cut
  (syntax_rules ()
    ((cut slots_or_exprs ...)
     (cut_internal () () slots_or_exprs ...))))

It works on some cases: 

((cut times 2 X) 3) => 6
((cut times 2 XXX) 2 3 4) => 48
((cut list 1 X 3 XXX) 2 4 5 6) => (1 2 3 4 5 6)

But this case fails: 

((cut list 1 X 3 X 5) 2 4) => (1 2 3 2 5)

That should have been (1 2 3 4 5).

Can anyone help me fix the bug?

Parents
  • tweeks

    It appears define_syntax is not generating unique identifiers as it would for the SRFI-26 code. In particular, 

    (cut list 1 X 3 X)

    Is supposed to expand to 

    (lambda (x1 x2) ((begin list) 1 x1 3 x2))

    But instead it's expanding to 

    (lambda (x x) ((begin list) 1 x 3 x))

    Which is an error in Guile, but SKILL++ allows it, binding both occurances of x to the first argument. (Interestingly, Emacs Lisp also permits (lambda (x x) ...), but it chooses the second argument for the x's.)

    Anyway, after several hours of getting nowhere, I gave up and used defmacro with explicit gensyms: 

    (defmacro cut_internal (s p @rest r)
  (cond ((null? r)
         `(lambda (,@s) ((begin ,(car p)) ,@(cdr p))))
        ((equal? r '(XXX))
         (let ((v (gensym)))
           `(lambda (,@s @rest ,v)
              (apply ,(car p) ,@(cdr p) ,v))))
        ((eq? (car r) 'X)
         (let ((v (gensym)))
           `(cut_internal ,(append1 s v) ,(append1 p v) ,@(cdr r))))
        (t
         `(cut_internal ,s ,(append1 p (car r)) ,@(cdr r)))))

(defmacro cut (@rest slots_or_exprs)
  `(cut_internal () () ,@slots_or_exprs))

    Hygienic macros: nice idea, but they don't work in SKILL++ (yet?).

Reply
  • tweeks

    It appears define_syntax is not generating unique identifiers as it would for the SRFI-26 code. In particular, 

    (cut list 1 X 3 X)

    Is supposed to expand to 

    (lambda (x1 x2) ((begin list) 1 x1 3 x2))

    But instead it's expanding to 

    (lambda (x x) ((begin list) 1 x 3 x))

    Which is an error in Guile, but SKILL++ allows it, binding both occurances of x to the first argument. (Interestingly, Emacs Lisp also permits (lambda (x x) ...), but it chooses the second argument for the x's.)

    Anyway, after several hours of getting nowhere, I gave up and used defmacro with explicit gensyms: 

    (defmacro cut_internal (s p @rest r)
  (cond ((null? r)
         `(lambda (,@s) ((begin ,(car p)) ,@(cdr p))))
        ((equal? r '(XXX))
         (let ((v (gensym)))
           `(lambda (,@s @rest ,v)
              (apply ,(car p) ,@(cdr p) ,v))))
        ((eq? (car r) 'X)
         (let ((v (gensym)))
           `(cut_internal ,(append1 s v) ,(append1 p v) ,@(cdr r))))
        (t
         `(cut_internal ,s ,(append1 p (car r)) ,@(cdr r)))))

(defmacro cut (@rest slots_or_exprs)
  `(cut_internal () () ,@slots_or_exprs))

    Hygienic macros: nice idea, but they don't work in SKILL++ (yet?).

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