Re: Fwd: [Patch] definitions in when, unless, do as well as in cond- and case-clauses

[Stefan Israelsson Tampe <stefan.itampe@gmail.com>]

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For conditional variables you gave a default value. So then why on earth do
you not have an implicit let ?
There must be a good reason.

On Sat, Feb 5, 2022 at 12:39 AM Maxime Devos <maximedevos@telenet.be> wrote:

> Stefan Israelsson Tampe schreef op vr 04-02-2022 om 22:40 [+0100]:
> > Anyhow conditional defining vars is a common theme in other languages
> > so I think it was kind of natural to implement if as it was done.
>
> AFAIK no Lisp or Scheme except for Guile < 2.0 implements conditionally
> defining local variables (but then I usually only consider Guile Scheme
> and the RnRS, so this doesn't mean much).  In my experience, I have
> never seen a need for conditionally defining a local variable in Scheme
> code (if you have a real-world example, please share).
>
> It also seems impossible to implement this w.r.t. the macro system ---
> what should, say, bound-identifier=? do when one of its identifiers is
> only conditionally bound?  Or for another example:
>
> If I do
>
> (define foo 'bar)
> (define-syntax foobar
>   (syntax-rules (foo)
>     ((_ foo)
>      (begin (pk "it's a foo!") foo))
>     ((_ goo)
>      (begin (pk "it's not a foo ...") goo))))
>
> (define (zebra stripes)
>   (if stripes
>       (define foo 'quux))
>   (foobar foo)) ;; <--- ***
>
> then sometimes the 'foo' in '***' refers to the global variable 'foo'
> and hence 'foobar' expands to the "it's a foo'.'  Sometimes the 'foo'
> in '***' refers to the local variable 'foo' (!= the global foo) hence
> 'foobar' expands to the "it's not a foo ...".
>
> However, it's impossible for a macro to expand to multiple things at
> once!
>
> Greetings,
> Maxime.
>

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