bug#50583: 27.2; test-completion should return non-nil

bug#50583: 27.2; test-completion should return non-nil

[Kevin Vigouroux via Bug reports for GNU Emacs, the Swiss army knife of text editors]

In the following code, I assume that `test-completion' should return a
non-‘nil’ value because "foo" is a valid completion alternative.

(test-completion
 "foo"
 '("foobar" "bar" "barfoo"))

=> nil

(try-completion
 "foo"
 '("foobar" "bar" "barfoo"))

=> "foobar"

#+begin_quote 
-- Function: test-completion string collection &optional predicate
     This function returns non-‘nil’ if STRING is a valid completion
     alternative specified by COLLECTION and PREDICATE.  The arguments
     are the same as in ‘try-completion’.  For instance, if COLLECTION
     is a list of strings, this is true if STRING appears in the list
     and PREDICATE is satisfied.

     This function uses ‘completion-regexp-list’ in the same way that
     ‘try-completion’ does.

     If PREDICATE is non-‘nil’ and if COLLECTION contains several
     strings that are equal to each other, as determined by
     ‘compare-strings’ according to ‘completion-ignore-case’, then
     PREDICATE should accept either all or none of them.  Otherwise, the
     return value of ‘test-completion’ is essentially unpredictable.

     If COLLECTION is a function, it is called with three arguments, the
     values STRING, PREDICATE and ‘lambda’; whatever it returns,
     ‘test-completion’ returns in turn.
#+end_quote


In GNU Emacs 27.2 (build 1, aarch64-unknown-linux-gnu, GTK+ Version 3.24.27, cairo version 1.17.4)
 of 2021-03-26 built on leming
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Manjaro ARM

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Quit
Mark set [2 times]
Making completion list...
Quit
nil
"foobar"
nil
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bug#50583: 27.2; test-completion should return non-nil

[Dmitry Gutov]

On 14.09.2021 16:25, Kevin Vigouroux via Bug reports for GNU Emacs, the 
Swiss army knife of text editors wrote:
> In the following code, I assume that `test-completion' should return a
> non-‘nil’ value because "foo" is a valid completion alternative.

"foo" is not a member of the list '("foobar" "bar" "barfoo"), so that's 
not true.

Perhaps you want to use 'try-completion' instead?

bug#50583: 27.2; test-completion should return non-nil

[Lars Ingebrigtsen]

Dmitry Gutov <dgutov@yandex.ru> writes:

> On 14.09.2021 16:25, Kevin Vigouroux via Bug reports for GNU Emacs,
> the Swiss army knife of text editors wrote:
>> In the following code, I assume that `test-completion' should return a
>> non-‘nil’ value because "foo" is a valid completion alternative.
>
> "foo" is not a member of the list '("foobar" "bar" "barfoo"), so
> that's not true.

The doc string here is vague, though:

Return non-nil if STRING is a valid completion.

So perhaps it could be clarified.  I think it wants to say something
like "if STRING is in COMPLETIONS", but since COMPLETIONS can be a
function, that's not quite right.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#50583: 27.2; test-completion should return non-nil

[Dmitry Gutov]

On 15.09.2021 11:13, Lars Ingebrigtsen wrote:
> So perhaps it could be clarified.  I think it wants to say something
> like "if STRING is in COMPLETIONS", but since COMPLETIONS can be a
> function, that's not quite right.

"Valid completion" should be described somewhere where the completion 
tables are described, I guess.

bug#50583: 27.2; test-completion should return non-nil

[Kevin Vigouroux via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Indeed, the documentation is not very clear. It is not known that the
STRING must be an exact match in COMPLETION nor the context of use of
the `test-completion' function.

bug#50583: 27.2; test-completion should return non-nil

[Lars Ingebrigtsen]

Kevin Vigouroux <ke.vigouroux@laposte.net> writes:

> Indeed, the documentation is not very clear. It is not known that the
> STRING must be an exact match in COMPLETION nor the context of use of
> the `test-completion' function.

I've now clarified the doc string by copying some text from the manual.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no