Re: A function to take the regexp-matched subsring directly

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

>> `save-match-data` is costly and extremely rarely needed.
> What makes it so expensive?  The implementation appears to be trivial.

It allocates cons cells (by calling `match-data`) to store the values.
If you put it in such a place it will be wasted 99% of the time.

>> So I'd much rather not save it here.
> If the function is supposed to be side-effect-free, then it ought not to
> sometimes replace the match data and not touch it when optimised away.

The match-data is defined to be something very ephemeral, so it's OK for
pure functions to clobber it.

Admittedly, here it doesn't just clobber it but sets it to a reliable
value, so there's a high risk that someone will rely on that match-data,
so better not mark it as pure, indeed, contrary to what I said earlier.


        Stefan