Re: [Chicken-users] Weird scope of variable defined inside cond

[Vítor De Araújo]

Thanks for the replies! I found it strange because I was under the
impression that in Chicken you generally can define variables in places
not permitted by R5RS. For instance:

  (define (f)
    (print 23)
    (define x 42))  ; definition after expressions,
                    ; which I think is non-standard

  (f)               ; prints 23
  (print x)         ; unbound variable

Anyway, for the program I had this problem with I redefined 'cond' to
wrap the clause body in a (let () ...) and everything seems to work.

(By the way, I didn't know about "-debug 2", which is something I
wanted before. :D)

On 12/05/2015 08:12 -0400, Mario Domenech Goulart wrote:

> Hi Vitor,
> 
> On Tue, 12 May 2015 02:44:30 -0300 Vítor De Araújo <address@hidden>
> wrote:
> 
> > I'm seeing some weird behavior regarding scope of a variable defined
> > with 'define' inside a cond clause. For instance:
> >
> >   (define (foo x)
> >     (define bar 23)
> >     (cond [(number? x)
> >            (define baz 42)
> >            (list baz)]
> >           [else 'whatever]))
> >
> >   (foo 3)
> >   (print baz)     ; prints 42!
> >   (print bar)     ; unbound variable, as expected
> >
> > Additionally, if I type the above definition of 'foo' directly in
> > csi, I get a message:
> >
> >   Note: the following toplevel variables are referenced but unbound:
> >
> >     baz (in foo)
> >
> > Is this a bug, or I'm missing something? I'm running Chicken
> > 4.9.0.1 on Debian GNU/Linux x86-64. I also tried it with the
> > version from the git repository and got the same result.
> 
> That's a little can of worms. :-)
> 
> The definition of `baz' is not valid in that context according to
> R5RS, since `define' forms are not expressions (they are
> definitions), and `cond' clauses should contain expressions:
> http://www.schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-7.html#%_sec_4.2.1
> 
> CHICKEN implements `define' with `set!', and `set!' in CHICKEN can be
> applied to unbound variables, in that case creating toplevel ones:
> http://wiki.call-cc.org/man/4/Extensions%20to%20the%20standard#set
> 
> So, that code ends up creating a toplevel binding for `baz':
> 
> $ csc -debug 2 foo.scm
> [canonicalized]
> (##core#callunit "library")
> 
> (##core#callunit "eval")
> 
> (##core#callunit "chicken_2dsyntax")
> 
> (##core#undefined)
> 
> (set! foo
>   (##core#lambda
>     (x1)
>     (let ((bar6 (##core#undefined)))
>       (let ((t9 (set! bar6 '23)))
>         (if (number? x1) (let ((t8 (set! baz '42))) (list baz))
> 'whatever)))))
> 
> (foo '3)
> 
> (print baz)
> 
> ((##sys#implicit-exit-handler))
> 
> (##core#undefined)
> 
> 
> Best wishes.
> Mario
> -- 
> http://parenteses.org/mario
> 
> 


-- 
Vítor De Araújo
http://inf.ufrgs.br/~vbuaraujo