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Re: [External] : Re: Testing whether a list contains at least one non-ni
From: |
Emanuel Berg |
Subject: |
Re: [External] : Re: Testing whether a list contains at least one non-nil element |
Date: |
Sun, 30 Oct 2022 13:48:34 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) |
Stefan Monnier via Users list for the GNU Emacs text editor wrote:
>> You don't want to implement such functions in Emacs with
>> recursion because you'll easily hit `max-lisp-eval-depth'
>> when they are called.
>
> Indeed (and it's significantly slower than the corresponding
> loop without function calls).
Recursion is slower because of the continual function calls
and those will also blow the stack eventually ...
>> Sorry to tell you, but a loop is the preferable way
>> in Elisp.
>
> Luckily, since Emacs-28 you can have your cake and eat it too:
>
> (defun has-non-nil (lst)
> (named-let loop ((lst lst))
> (cond
> ((null lst) nil)
> ((consp lst) (or (not (null (car lst))) (loop (cdr lst))))
> (t (error "Not a proper list! You cheater!")))))
`named-let' is from a "[l]ooping construct taken from Scheme."
I love it that Elisp is like a "Lisp dump" area where
everything from the Lisp world worth having seems to end up
eventually ...
> Admittedly, here the `named-let` construct gives you only
> the elimination of tail-recursion, but in many other
> circumstances it also leads to quite elegant code.
Here tail refers to not the `cdr' of the list but the tail of
the function, "a tail call is a subroutine call performed as
the final action of a procedure. If the target of a tail is
the same subroutine, the subroutine is said to be tail
recursive, which is a special case of direct recursion" [1]
I don't know why that is such an important distinction, I also
remember recursion over trees that recursed both ways from the
top and middle nodes if it/they had several child nodes, so
I guess that recursion was both tail and direct LOL.
Elisp direct recursion example anyone?
And what was this continuation-passing style?
CPS 1975 continuation-passing style, coined in "AI Memo 349"
I used it once, but don't remember what file that was ...
> (defalias 'has-non-nil
> #'(lambda (lst) [...]
TIL: In Scheme-inspired Elisp, quoting lambdas is OK ...
[1] https://en.wikipedia.org/wiki/Tail_call
--
underground experts united
https://dataswamp.org/~incal
- RE: [External] : Re: Testing whether a list contains at least one non-nil element, (continued)
- RE: [External] : Re: Testing whether a list contains at least one non-nil element, Drew Adams, 2022/10/27
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Jean Louis, 2022/10/27
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, tomas, 2022/10/28
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Jean Louis, 2022/10/28
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Michael Heerdegen, 2022/10/28
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Stefan Monnier, 2022/10/28
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Michael Heerdegen, 2022/10/29
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Stefan Monnier, 2022/10/29
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Emanuel Berg, 2022/10/30
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, tomas, 2022/10/29
- Re: [External] : Re: Testing whether a list contains at least one non-nil element,
Emanuel Berg <=
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Emanuel Berg, 2022/10/29
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Emanuel Berg, 2022/10/29
- Re: [External] : Re: Testing whether a list contains at least one non-nil element, Emanuel Berg, 2022/10/28
- Elisp and CL (was: Re: [External] : Re: Testing whether a list contains at least one non-nil element), Emanuel Berg, 2022/10/28
- Re: Testing whether a list contains at least one non-nil element, Emanuel Berg, 2022/10/27
Re: Testing whether a list contains at least one non-nil element, Michael Heerdegen, 2022/10/25