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Re: [PATCH] Print backtraces for syntax errors too.

From: Maxime Devos
Subject: Re: [PATCH] Print backtraces for syntax errors too.
Date: Sat, 25 Feb 2023 00:38:50 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.0

On 24-02-2023 16:48, Ludovic Courtès wrote:
Maxime Devos <> skribis:

;; Before:
;;  unknown file:#f:#f: syntax-stuff-twice: bad in subform
     #<syntax:reproducer.scm:15:27 "imagine this syntax is very hard to
     follow"> of #<syntax:reproducer.scm:15:27 "imagine this syntax is
     very hard to follow">
;; After:
;;  [the same thing]
;; Looks like another patch is needed ...

What backtrace are you trying to get?

Getting a backtrace showing which macros are being expanded (similar to
what GCC does) would be great, but it’s much more work; changing this
one line in libguile won’t achieve that.

Just a regular backtrace like Guile already makes for exceptions unrelated to syntax, not some kind of expansion backtrace that tracks macro expansion.

For example, let's say you have a macro that during expansion throws an exception in some cases. For non-'syntax-error' exception types, a backtrace is printed:

;; a.scm
(define-module (a) #:export (whatever))
(define (syntax-negate s)
  (syntax-case s ()
    (#false #true)
    (#true #false)
    (_ (error "bogus!")))) ; <--- line 6!
(define (syntax-identity s) ; identity
  (syntax-case s ()
    (#false #'#false)
    (#true #'#true)
    (_ (error "bogus!"))))
(define-syntax whatever
  (lambda (s)
    (syntax-case s ()
      ((_ x) #`(#,(syntax-negate #'x) #,(syntax-identity #'x))))))
;; b.scm
(use-modules (a))
(whatever 0)
;; Shell commands
guild compile a.scm
guild compile -L . b.scm
;; Output: a backtrace that mentions on which line of a.scm things went wrong:
[Lots of lines]
In ice-9/psyntax.scm:
[More lines]
In a.scm:
      6:7  1 (_ _)
In ice-9/boot-9.scm:
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:

However, suppose I removed the (_ (error "bogus!")) lines and hence the code produces syntax-error exceptions:

;; c.scm
(define-module (c) #:export (whatever))
(define (syntax-negate s)
  (syntax-case s () ; L3
    (#false #true)
    (#true #false)))
(define (syntax-identity s) ; identity
  (syntax-case s () ; L7
    (#false #'#false)
    (#true #'#true)))
(define-syntax whatever
  (lambda (s)
    (syntax-case s ()
      ((_ x) #`(#,(syntax-negate #'x) #,(syntax-identity #'x))))))
;; d.scm
(use-modules (c))
(whatever 0)
;; Shell commands
guild compile c.scm
guild compile -L . d.scm
;; Output: no backtrace at all!
ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Syntax error:
d.scm:2:10: source expression failed to match any pattern in form 0
;; In case of complicated macros, it would be nice if it said _which_
;; pattern matcher failed: L3, or L7, like with a.scm+b.scm.

Summarised, I want the relatively nice backtrace that happens for non-'syntax-error' exceptions from a.scm+b.scm (*) (it's verbose, has lots of irrelevant stuff, but ultimately it provides an useful piece of information: the line number on which a pattern matcher failed).

(*) Ideally you would have both the backtrace _and_ the line number in b.scm/d.scm.


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