Make the completion trigger character conditional

[jdneo]

This is a feature request to make the completion trigger character take effect conditionally.

Considering the following Java code:

String s = new |

When user enter the space, it would be helpful to show a completion list. This behavior can be observed in other IDEs:

But simply adding space to the trigger characters could be a problem because not every space needs to follow a completion action.

Is it possible to make it conditional? For example, only taking effect when there is a "new" before space.

[rcjsuen]

@jdneo I do not believe this is possible. This question/request may be more appropriate for the microsoft/language-server-protocol repository.

[dbaeumer]

@jrieken has this come up in VS Code ?

[hrsh7th]

The lua-language-server achives the similar functionality.
That LS specified the as triggerCharacter and textDocument/completion returns context-aware completion response.

[jdneo]

Share one more information. I observed the ms-dotnettools.csharp also implemented this feature.

Currently in Java, we are planning to add " " as a trigger character and looking backward to see if it's "new " at where the completion is triggered.

[puremourning]

In ycmd triggers were always regex based. LSP using a single character has always seemed limiting (think, even c -> and c++ ::). Adding space as a trigger character is a bad idea imo as semantic completion requests can be quite slow, particularly for ‘global scope’ in many languages. And this is going to lead to a lot of pointless requests.

I think a protocol supported regex trigger would be better. Or just press a manual trigger.

[jrieken]

@jrieken has this come up in VS Code ?

Yes, it has come up in the past but not in recent years. I believe extension use the last character of their words like > of -> and check the prefix themselves. We can consider adding support for longer prefix strings but I am unsure about regex because it comes at a certain cost.

[hrsh7th]

I'm against regular expressions. I think there are many dialects and some clients cannot handle it.

The advantage of a single character is that it can be handled by the editor's keypress handler.
However, if we make the space a trigger string, we may consider that the completion request runs indented...

[puremourning]

FWIW, these are the regex triggers that ycmd uses by default. We add the 'trigger characters' provided by LSP servers to these lists dynamically: https://github.com/ycm-core/ycmd/blob/master/ycmd/completers/completer_utils.py#L180-L211 (for some languages we actually ignore them, as they are inferior in practice to our hard-coded ones).

The LSP already includes a regular expression syntax, defined to be some subset of javascript's (https://microsoft.github.io/language-server-protocol/specifications/specification-current/#regExp). Anyway, I do tend to agree that is somewhat unfortunate and in practice (as you see above) most expression are just "multiple characters" rather than actually complex expressions.

Perhaps the 'trigger characters' could be augmented with alternative "trigger prefixes" which is to say "Trigger if the prefix at the current cursor position matches these characters". The challenge there is that the "whitespace" trigger that prompted this request no longer works if you enter multiple spaces as in X x = new <space><space><space><here> (whereas it's trivial with a regex)