[plt-scheme] Pattern-matching macro chokes on conditional statement which would otherwise work.

What version of DrScheme are you running?  There was a bug in the v371
debugger that would produce error messages like that, but it should
have been fixed in v372 (and in svn as of r7169 I think).

On Jan 19, 2008 10:28 AM, Ran Gutin <manicmessiah at gmail.com> wrote:
> (define (doublequoted-symbol? sym)
>   (and (pair? sym) (eqv? (car sym) 'quote)))
>
> (define-syntax match
>   (syntax-rules ()
>     ((match vars [pfirst prest ...] body ...)
>
>      (cond
>
>         ((and (not (doublequoted-symbol? (quote pfirst)))
>              (symbol? (quote pfirst)))
>         (let ((pfirst (car vars)))
>           (match (cdr vars) [prest ...] body ...)))
>
>        ((eqv? pfirst (car vars))
>         (match (cdr vars) [prest ...] body ...))
>
>        (else #f)))
>
>     ((match vars [] body ...) (begin body ...))))
>
> (match '(1 2 3) (one 2 three)
>   (display three))
>
>
>
> Sorry for not commenting on the code, but here's a basic idea of what it
> SHOULD do:
> (match '(1 2 3) (1 2 3) (display 3)) should match perfectly and display 3 on
> the terminal (it doesn't).
> (match '(1 2 3) (one two three) (display 3)) works perfectly, and prints 3.
> (match '(1 2 3) ('one 'two 'three) (display 'one)) - This should fail to
> match the pattern altogether, as symbols which are already quoted are
> matched for equality, rather than used as identifiers as above. This dies
> miserably without returning #f.
> (match '(1 2 3) (one 2 three) (display three)) - This should immediately
> create an alias for the values 1 and 3 ('one' and 'three) and match the
> second values for equality. Unfortunately, this is not the case.
>
>
>  (match '(1 2 3) (one 2 three)
>    (display three))
> ... gives the following error:
> "let: bad syntax (not an identifier) in: 2"
>
> It seems to be completely ignoring the conditional.
> So I performed a test:
> (and (not (doublequoted-symbol? (quote 2)))
>              (symbol? (quote 2)))
> ... actually returns FALSE. So then why is the conditional statement
> branching off completely wrong?
>
> And then, here's the real kill. I can't debug the damn thing.
>  Doing so sends an alert window flying in my face, and a larger one behind
> it.
>
> "rest: expected argument of type <non-empty list>; given ()
>
>  === context ===
> C:\Program Files\PLT\collects\mzlib\list.ss:295:2: rest
> C:\Program Files\PLT\collects\mztake\debug-tool.ss:804:10: can-step-out?
>
> rest: expected argument of type <non-empty list>; given ()
>
>  === context ===
> C:\Program Files\PLT\collects\mzlib\list.ss:295:2: rest
> C:\Program Files\PLT\collects\mztake\debug-tool.ss:804:10: can-step-out?
>
> rest: expected argument of type <non-empty list>; given ()
>
>  === context ===
> C:\Program Files\PLT\collects\mzlib\list.ss:295:2: rest
> C:\Program Files\PLT\collects\mztake\debug-tool.ss:804:10: can-step-out?
>
> rest: expected argument of type <non-empty list>; given ()
>
>  === context ===
> C:\Program Files\PLT\collects\mzlib\list.ss:295:2: rest
> C:\Program Files\PLT\collects\mztake\debug-tool.ss:804:10: can-step-out?
>
> rest: expected argument of type <non-empty list>; given ()
>
>  === context ===
> C:\Program Files\PLT\collects\mzlib\list.ss:295:2: rest
> C:\Program Files\PLT\collects\mztake\debug-tool.ss:804:10: can-step-out?
>
> rest: expected argument of type <non-empty list>; given ()
>
>  === context ===
> C:\Program Files\PLT\collects\mzlib\list.ss:295:2: rest
> C:\Program Files\PLT\collects\mztake\debug-tool.ss:804:10: can-step-out?
>
> rest: expected argument of type <non-empty list>; given ()
>
>  === context ===
> C:\Program Files\PLT\collects\mzlib\list.ss:295:2: rest
> C:\Program Files\PLT\collects\mztake\debug-tool.ss:804:10: can-step-out?"
>
> ^ Can someone tell me what the hell is going on?
>
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