The last example should be
(when (and (list? l) (pair? l))
(let ([x (cdr l)])
(list? x) ;==> #t
))
Gustavo
Technical note: The optimizer wants to be sure that (cdr l) is safe before
asserting that x is a list?. It should be ok to mark x as a list?, because
if there is an error the inner code will not be run anyway. I'll try to
remember why I made this, and send a PR if necessary.
On Wed, Nov 28, 2018 at 6:36 PM Gustavo Massaccesi <gust...@oma.org.ar>
wrote:
> Some additional comments about this subject.
>
> A big difference is that in Chez Scheme the cons are mutable, and that
> makes it almost impossible to make any optimization with the lists at the
> Chez Scheme level.
>
> With the current Racket implementation there are a few reductions at
> compile time for the lists, for example
>
> (car (list 1 2 3)) ; ==> 1
>
> (when (list? l)
> (let ([x (cdr l)])
> (list? x) ;==> #t
> ))
>
> The optimizer "understands" lists, but it doesn't understand custom
> structs that have are list like nesting.
>
> Also, there was an attempt to implement the mcons using a struct instead
> of a special construction, but some programs were too slow with the
> modification, so it was never merged.
> https://github.com/racket/racket/pull/1316
>
> Gustavo
>
>
>
>
>
> On Wed, Nov 28, 2018 at 3:26 PM Matthew Flatt <mfl...@cs.utah.edu> wrote:
>
>> Right. An alternative would be to set decide eagerly, but `list?` tests
>> are infrequent relative to pair constructions.
>>
>> Then again, a function like `list` can and does set the "is a list" bit
>> eagerly and cheaply on newly allocated pairs.
>>
>> At Wed, 28 Nov 2018 18:16:31 +0100, Robby Findler wrote:
>> > Also "don't know yet".
>> >
>> > Robby
>> >
>> > On Wed, Nov 28, 2018 at 6:15 PM Alexis King <lexi.lam...@gmail.com>
>> wrote:
>> >
>> > > > On Nov 28, 2018, at 07:15, Matthew Flatt <mfl...@cs.utah.edu>
>> wrote:
>> > > >
>> > > > Yes, that's special handling for pairs in the sense that the
>> > > > traditional Racket implementation takes advantage of leftover bits
>> in a
>> > > > pair object, and it uses two of them for "is a list" and "not a
>> list".
>> > > >
>> > > > Racket-on-Chez doesn't have the extra bits to work with, so "is a
>> list"
>> > > > and "not a list" information is recorded separately in an
>> `eq?`-based
>> > > > hash table.
>> > >
>> > > Why does keeping track of “is a list” and “not a list” require two
>> bits?
>> > > It seems like a pair either is or is not a list, so one bit of
>> information
>> > > would be sufficient. Are there situations where the system can
>> neither be
>> > > sure that something is or is not a list? Circular lists, or something
>> like
>> > > that?
>> > >
>> > > (This is not really important, of course, I’m just curious.)
>> > >
>> > > Alexis
>> > >
>> > > --
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