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Re: Changes to completing-read

From: Stefan Monnier
Subject: Re: Changes to completing-read
Date: Mon, 20 Oct 2008 09:31:41 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.60 (gnu/linux)

>> The `lambda' is when the function is called by `test-completion'.
>> The `boundaries' is a new functionality, still undocumented.

> Thanks. I guess this will be documented before the release?

Of course.

> Could someone explain the meaning of `boundaries' please?

There's an attempt in the comment at the top of lisp/minibuffer.el.
And here's another attempt:

Consider a completion table such as the one used for file names, where
the string to complete is conceptually divided into several subparts.
In such a case the *Completions* buffer may only want to show the
relevant affected subparts rather than complete copies.  I.e. if you
complete /usr/s, all-completions may prefer to return '("sbin" "share" "src")
rather than ("/usr/sbin" "/usr/share" "/usr/src").

In order for the completion functions to be able to know how to
correctly interpret this abbreviated list (e.g. such that when you
middle-click in the *Completions* buffer, the "share" string only
replaces the "s" rather than the whole "/usr/s"), there needs to be
a way to know the position of the boundary between the prefix that's
elided and the subpart whose possible completions are returned by
`all-completions'.  Calling the completion table with (boundaries
. SUFFIX) should return that boundary info.

Now "what's this SUFFIX thingy", you'll say.  Here's the deal:
all-completions will return all the relevant completions for the
relevant subpart, but the relevant subpart may not be the end of
the completion.  E.g. all-completions on "/usr/s" will only return
"share" and not "share/zoneinfo/Canada/Eastern".  So calling the
file completion table with (boundary . "e/zoneinfo") will indicate that
the returned values only cover the "e" part of the SUFFIX.


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