Lexical let and setq

[Michael Welsh Duggan <mwd@md5i.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

[...]

> I do hope to fix those issues by introducing other byte-codes which will
> let us generate significantly more efficient code for those constructs,
> but in 24.1, the priority was to get lexical-binding to work correctly,
> performance being a secondary concern (tho for most idiomatic Elisp
> code, the performance tends to be competitive).
>
> What people should know is that
>
>    (let (x y z)
>      ...(setq x ...)
>      ...(setq z ...)
>      ...(setq y ...)
>
> is often a bad idea in Elisp, and even more so in lexical-binding code
> (in some cases, if a variable is immutable it can be handled
> significantly more efficiently, so the mere existence of a single `setq'
> on a variable can sometimes slow other chunks of code: in many cases
> `let' is cheaper than `setq').

The primary reason I have seen the (let (foo) (setq foo ...)) idiom is
in looping code.  The way I would normally try to avoid this idiom in
most FP languages would be to use recursion (specifically tail
recursion, if possible).  I know some work was done on implementing
efficient tail-recursion in the byte compiler.  Has any of that made it
onto the trunk yet?

-- 
Michael Welsh Duggan
(md5i@md5i.com)