Signature help on function applications

[amiralies]

https://microsoft.github.io/language-server-protocol/specification#textDocument_signatureHelp

[cristianoc]

What's the idea -- different than hover?

[amiralies]

It automatically opens when typing function applications (via ( for example) and also users can config a hotkey to open the signature help while writing.
Another feature i see in ts extension is it highlights current parameter you're applying in signature help box.

[amiralies]

recording.mp4

This is an exmaple from ts extension

[cristianoc]

How's that triggered in the example? By ( ?
And any ideas of how the mechanism is different from normal auto-completion? Because it has a notion of n-th argument?

[cristianoc]

This feature is quite compelling.

[amiralies]

I'm not sure currently.
they are two plugins i'm aware which are implementing this feature.
tsserver (bundled with vscode)
metals (scala language server https://github.com/scalameta/metals-vscode)

[cristianoc]

This looks roughly like:

• ( triggers signature help
• , re-triggers (for the n+1-th argument)

Needs a list of labels (the function parameters), and an index (of the current argument).

So:

let add = (x,y) => x+y

Then given:
add(3,

would need to return:

• [int, int] (the list of parameters, perhaps named with conventional names, perhaps one can retrieve "x" and "y" from the ast)
• 1 (index of the second parameter)

[cristianoc]

As usual some care is required about context:
foo(3, (2, should not trigger anything as it's about an inner tuple, not an argument to foo.

[cristianoc]

For perf reasons, context scanning should happen before doing any other work that scans project structure etc. As context scanning is pure string parsing.

[cristianoc]

In fact, the same perf comment apply to completion: #255

[oblitum]

The server publishes the characters that trigger this in Server Capability SignatureHelpOptions.triggerCharacters.

[oblitum]

This is must have, most servers provide this. Just having bare function names completed is a bit lacking. Also, a good chunk of servers also provide snippet expansion on top, meaning, not only is signatureHelp available, you can also expand the function call with all arguments filled with placeholders you can jump to.

[zth]

Unless @cristianoc disagrees, this sounds like a good first issue for a contributor with some OCaml skills. Essentially, we should have all we need in the analysis bin now that we've integrated the full parser.