Re: [PATCH] Source properties on arbitrary non-immediate values

[Ludovic Courtès]

Neil Jerram <address@hidden> writes:

> Kevin Ryde <address@hidden> writes:

>> A symbol used in two places is the same object, so file/line attached
>> by both occurrances will overwrite, or whatever.  Ditto other uniques
>> like keywords, and immediates like fixnums and chars.

Symbols and strings are different from immediates.  What you say is true
for symbols, not for strings:

  guile> (define x 'sym)
  guile> (define y 'sym)
  guile> (set-source-properties! x '((hello . world)))
  ((hello . world))
  guile> (source-properties y)
  ((hello . world))

  guile> (define x "str")
  guile> (define y "str")
  guile> (set-source-properties! x '((hello . world)))
  ((hello . world))
  guile> (source-properties y)
  ()

This is because (1) `source-properties' looks up objects using
`hashq-ref' (and, therefore, `eq?'), and (2) because `eq?' behaves
differently on symbols than on strings (quoting R5RS):

   `Eq?' and `eqv?' are guaranteed to have the same behavior on
   symbols, booleans, the empty list, pairs, procedures, and non-empty
   strings and vectors.

In particular, `(eq? "a" "a")' returns `#f' with Guile.

> Indeed.  What was your motivation for wanting to attach source
> properties to non-pairs, though?

Well, the manual doesn't mention this restriction and, rather than
fixing the manual, I wanted to understand the rationale behind this
restriction.

I see no reason not to allow source properties to be attached to
arbitrary non-immediates.  The fact that this "won't work" for symbols
is not, IMO, a sufficiently good reason to the current restriction.  For
instance, one might want to have the following definition of `eval':

  (let ((real-eval eval))
    (set! eval
          (lambda (expr env)
            (let ((props (source-properties expr))
                  (result (real-eval expr env)))
              (if (or (vector? result) (record? result) (pair? result))
                  (set-source-properties! result props))
              result))))

This allows to keep source information further at run-time.

Thanks,
Ludovic.