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Re: how useful are collecting lists?



On 3/14/13, David A. Wheeler <dwheeler@xxxxxxxxxxxx> wrote:
> I said:
>> > Hmm. If that's a *problem*, one solution without significantly changing
>> > the existing semantics might be to allow <*...*> after ".".
>
> Alan Manuel Gloria <almkglor@xxxxxxxxx> wrote:
>
>> Ara ara, I thought this was *already* allowed...
>
> Actually, they weren't.  I was trying to be picky about what's allowed after
> "." (e.g., I don't think "$" and "\\" are sensible),

EEeeeeeh??

Well, \\ isn't sensible because it ends the current expression so it
comes out as something like (. ), which doesn't make sense), but why
not "$"?

Say:

foo $ a b
===>
(foo (a b))

foo . $ a b
===>
(foo . (a b))
===>
(foo a b)

....hmmm.

Okay, so there's really no use for $ after . LOL.  I can't think of a
use case for when you'd rather have a ". $" instead of just writing
the next datum directly.

> and obviously was *too*
> picky.  You're absolutely right, that should be permitted.
>
> So this is an important tip, we need to allow this case. I've changed the
> BNF and Scheme implementation so that a collecting list is legal after the
> dot in "rest".  Thus, this is now legal:
> define x . <*
> ! define y 5
>
> ! define z 6
> *>
>
> I haven't modified "head", so if "." is the first atom on a line, a
> collecting list currently can't follow.  I can't figure out why you'd want
> to do that, I'd expect a user to just use a collecting list in that case.
>
> So we already have an important comment (from David Vanderson) that's
> pointed out a problem, and I've posted a possible solution (one that I hope
> others will agree is sensible).  Excellent!  Seems to me that's why we have
> a SRFI process...
>
> --- David A. Wheeler
>
>