RE: something between try-completion and test-completion?

["Drew Adams" <drew.adams@oracle.com>]

> > I'd be interested in a function similar to both `try-completion' and
> > `test-completion', but which would just test whether its 
> > STRING arg can be completed against its COLLECTION arg,
> > respecting its PREDICATE arg.
> 
> > It would be like `test-completion', in that as soon as some 
> > match is found it would return non-nil, not bothering to test
> > the other completions and calculate the common prefix.
> 
> > It would be like `try-completion', in that it would test 
> > whether the STRING is a prefix of some COLLECTION element,
> > not whether STRING is itself one of the COLLECTION elements.
> 
> > The idea is to have a quick version of `try-completion' for 
> > situations where the common prefix of all matches is not needed,
> > and all you want is an indication of whether the STRING could be
> > completed.
> 
> > Any other interest in this? Any chance this will become available?
> 
> Could you give us some sample situation where there'd be an actual
> benefit (as in measurable performance difference) between 
> try-completion and the function you're looking for?

No. I can't compare performance for a non-existent implementation. ;-)

If there are lots of completions for STRING, and there is a common prefix,
then `try-completion' will do extra work to find all the completions and
calculate that common prefix.

The point is that that extra work is, well, extra - not useful in this
context. What the performance difference would be by avoiding that
computation I can't predict.

But you can see that that work is not necessary, and you can imagine that,
in the case of many, many completions with a common prefix, that wasted time
could be important. How many completions would make it noticeable? And
noticeable in what contexts? I don't know (or care).

> PS: Maybe you can hack it up by hand:
> 
>   (defun try-completion-p (string collection &optional predicate)
>     (lexical-let ((predicate predicate))
>       (catch 'tcp-found
>         (try-completion string collection
>                         (lambda (x)
>                           (if (funcall predicate)
>                               (throw 'tcp-found t))))
>         nil)))

That does nothing in the (more typical) case where predicate is nil.

But that's the idea - have a function `try-completion-p' (or
`any-completions-p') that would stop as soon as it found one match (and not
just with PREDICATE).