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bug#16334: 24.3.50; company-capf eats the first char in IELM filename co

From: Dmitry Gutov
Subject: bug#16334: 24.3.50; company-capf eats the first char in IELM filename completions
Date: Tue, 07 Jan 2014 06:52:11 +0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0

On 06.01.2014 19:23, Stefan Monnier wrote:
`completion-file-name-table' is more of an exception, I think.  But if
it was only passed the segment of STRING after the last path
separator, it could still look behind it in the buffer and see the
full path.
But the completion may actually want to *change* the text before
the boundary.  E.g. completion of /u/s/d to /usr/share/doc.

Maybe I didn't ask the right question. If the completion may want to change the text outside of its defined boundaries, doesn't that subvert the meaning of the "boundaries"? Or do you mean that, depending on the completion style, `completion-file-name-table' may return different boundaries, with the same prefix and suffix? So that modifications will still remain within the boundaries?

In that case, "/usr/share/doc" is the completion candidate, not "doc",

Not sure what you mean by "completion candidate":

Elements returned by the completion table?
Strings of text, one of which is likely to replace the text between completion boundaries.

try-completion and all-completions will both return nil because there's
no "/u/s" directory.  Assuming we use partial-completion style,
completion-try-completion should return "/usr/share/doc" and
completion-all-completions should return ("usr/share/doc"), i.e. without
the leading "/".  If we had started from "/usr/s/d" the results would
have been the same except completion-all-completions would return

Sounds quite convoluted.

To be clear, I'm not convinced that the notion of "sub-fields" is
useful. Defining limits to the text that can be affected by completion only
looks good to me from the presentation point of view: if the candidate
strings can be shorter, we can show more of them in the *Candidates* buffer,
whereas it's less useful for popup-style UIs where the candidates are
displayed vertically anyway.

Then just have company-capf check completion-boundaries and concat the
missing prefix to every element returned by all-completions.

Already done: https://github.com/company-mode/company-mode/compare/b70540b5fcd062c4670dea7004453de326ff4f70...8ecec3594931ae8e2329fec4b793ad4ba392e4ef

IOW, if I were to add a `boundaries' action to company-backends API, it
would only be used for presentation: the popup will cut off that many
characters from the candidate strings, and it will be rendered that many
columns to the right.

If you want to let Company provide completion styles like
partial-completion you'll need some additional info about "subfields".
But as long as you limit yourself to prefix or substring completion you
don't need that.

I wonder if this information has to be reflected in the backend API. For example, some external service might want to pick the completion styles itself, and we can just give the backend prefix, get back a list of completion candidates, and eventually replace the prefix with one of the candidates. Same with suffix whenever we get around to supporting that.

From where I stand, knowing the completion style in advance only gives us benefits in visualization (know which characters in candidate strings match) and, in some cases, performance (no need to do concatenation even if the strings we receive from some subsystem have some offset in the prefix).

On the other hand, the backend is free to try all completion styles it knows/is allowed to use, sort and return together, like in this feature request: https://github.com/company-mode/company-mode/issues/45#issuecomment-31564029, saving several network road-trips and possibly further amount of CPU time.

And as far as highlighting the matching characters in candidates goes, we could just skip that for non-prefix matches (checking each on the fly), or do it sloppily, without regard for the backend logic.

Come to think of it, though, this new action may be incompatible with the
notion of merged backends. If we have candidates that come from backends
that return the same prefix but different boundaries, there's no way to
reflect the boundaries in the popup.

Yup.  Just like you have a problem when the start/end of the
completion text is not identical.  E.g. you could have a "word" backend
and a "varname" backend, and you type "my_fanc" and now the "word"
backend wants to complete "fanc" whereas the varname backend wants to
complete "my_fanc".

We currently deal with that by only using the backends that respond with the same prefix as the first one that returned non-nil. Guess we could add the boundaries to the comparison list, too...

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