Re: Threading / Pipe Macro

[Mark H Weaver]

Hello again,

Chris Vine <address@hidden> writes:
> My version was hygienic when used in functions, but not when used as a
> macro in a macro.

Not to belabor the point, but I wanted to mention that the unhygienic
'->' macro may fail when used in procedures, even when '->' is never
used in the definition of another macro.  Here's an example, where 'cut'
and '->' are used in combination:

--8<---------------cut here---------------start------------->8---
scheme@(guile-user)> ,use (srfi srfi-26)
scheme@(guile-user)> (define-syntax ->
  (lambda (x)
    (syntax-case x ()
      [(k exp0 . exps)
       (let* ([reversed (reverse (cons (syntax->datum #'exp0)
                                       (syntax->datum #'exps)))]
              [out (let loop ([first (car reversed)]
                              [rest (cdr reversed)])
                     (if (null? rest)
                         first
                         (let ([func (car first)]
                               [args (cdr first)])
                           (append `(,func ,@args)
                                   (list (loop (car rest) (cdr rest)))))))])
         (datum->syntax #'k out))])))
scheme@(guile-user)> (define foo (cut -> <> (format #t "~A\n")))
;;; <stdin>:17:12: warning: possibly unbound variable `t-15fc270a-2d'
scheme@(guile-user)> (foo 4)
<unnamed port>:17:34: In procedure foo:
In procedure module-lookup: Unbound variable: t-15fc270a-2d

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]> ,q
scheme@(guile-user)> ,expand (cut -> <> (format #t "~A\n"))
$1 = (lambda (t-15fc270a-34-1)
  (format #t "~A\n" t-15fc270a-34))
scheme@(guile-user)> (define-syntax ->
                       (syntax-rules ()
                         ((-> exp)
                          exp)
                         ((-> exp ... (op args ...))
                          (op args ... (-> exp ...)))))
scheme@(guile-user)> (define foo (cut -> <> (format #t "~A\n")))
scheme@(guile-user)> (foo 4)
4
$2 = #t
scheme@(guile-user)> ,expand (cut -> <> (format #t "~A\n"))
$3 = (lambda (t-15fc270a-59)
  (format #t "~A\n" t-15fc270a-59))
scheme@(guile-user)> 
--8<---------------cut here---------------end--------------->8---

So, more generally, unhygienic macros may cause problems when they are
used in combination with other macros.  Since macros are so ubiquitous
in Scheme, attempting to avoid such combinations is likely to be
brittle.

      Best,
       Mark