Re: Pattern matching on match-string groups #elisp #question

[Stefan Monnier <monnier@iro.umontreal.ca>]

> I went with dotted lists (a b c . d) because benchmarking showed it to be
> faster than either proper lists or vectors, the generated code is smaller
> than for lists, and the case of a single variable reduces naturally to no
> consing at all.

Nice.

> Looking a bit closer it gets more nuanced: one reason why
>
>   (pcase (list 1 2 3) (`(,a ,b ,c) (+ a b c)))
>
> is faster than
>
>   (pcase (vector 1 2 3) (`[,a ,b ,c] (+ a b c)))
>
> is that the latter contains three general function calls: to `vector`,
> `vectorp`, and `eql` (for checking the length), whereas the list version has
> byte-ops for everything.

Very good point.  The choice of which primitives deserve their own
bytecode was made many many years ago and it likely deserves
a serious reconsideration [ we still have dedicated byte codes for
Btemp_output_buffer_show and Btemp_output_buffer_setup ;-(  ]

Personally, I'd vote to start by making the `eq` byte code return the
value of `Feql` ;-)


        Stefan