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Re: Why is there no `until' in elisp?

From: Stefan Monnier
Subject: Re: Why is there no `until' in elisp?
Date: Fri, 19 Oct 2018 11:34:01 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

> think `until' should refer to the version I gave because this is
> probably the one which is going to be implemented naively by most
> people,

BTW, this is only true if those people have to implement it on top of
the pre-existing `while`.  Once you go down to implementing it as
a sequence of byte-codes, then the `until` you proposed is no simpler
than the one I proposed.

So reversing your argument, if Elisp had my `until` and you wanted to
define `while` would you define it to return nil or to return t?
IOW would you define it as

    (defmacro while (test &rest body)
      `(until (not ,test) ,@body))
    (defmacro while (test &rest body)
      `(progn (until (not ,test) ,@body) nil))



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