Implement signature help request

[mnxn]

Partially completes #312

Depends on rgrinberg/merlin#8

Currently supports showing signature help when an expression is detected as having a function type or is being applied. So it will show signature help for these two scenarios:

let f x = x
let _  = f |

f : 'a -> 'a

let f x = x
let _  = f 1|

f : int -> int

The second one will underline the specific parameter that is currently being applied. Operators also show signature help.

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Notes:

• I haven't figured what the best way to provide function/parameter documentation is. The function could just return positions for function documentation, but I feel like there should be a better way to do it.

• Showing signature help for operators or after entering a space after a function-type value might be annoying to users. Perhaps the functionality should be configurable? I couldn't find a built-in VSCode setting to disable signature help.

• The current implementation seems to depend on having correctly typed arguments. This doesn't show any signature help:

let _ = string_of_int true|

• Providing too many arguments just shows the function type without any parameters highlighted, despite the type error.

let _ = string_of_int 1 2|

string_of_int : int -> string

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Images:
Space after function-typed value:

Providing a function argument:

Anonymous function:

Operator:

[mnxn]

The first non-labelled parameter is now underlined right after a function-typed value:

Typing a ~ will underline the f:('a -> 'b) instead.

[rgrinberg]

Looks good. I'll merge both this and the merlin PR they are both ready.

[rgrinberg]

Worth adding a CHANGES entry as well.

[rgrinberg]

Awesome work. Merged manually.

[mnxn]

Awesome work. Merged manually.

Thanks, I was having trouble figuring out why Github was detecting conflicts...

[tmattio]

Thanks for the great work on this @mnxn, the feature is amazing 🎉

I noticed two things when testing:

• The highlighted argument seems to always be the first one

(I added a space after the first 0)

• The highlighted argument does not switch to the first labeled with only ~, it needs the name of the labeled argument as well:

then

[mnxn]

The highlighted argument seems to always be the first one

The highlighted argument won't always be the first, but it won't detect that you've moved onto the next parameter until you start a new expression. I'm not sure it's possible to detect you've moved on to the next function argument with the expression tree that Merlin provides. It's harder to do in a function-application-by-juxtaposition language like OCaml since it lacks the comma argument delimiters that C-like languages have.

The highlighted argument does not switch to the first labeled with only ~, it needs the name of the labeled argument as well:

The language server actually isn't providing an activeParameter index in this example, but VSCode is defaulting to the first one. We could fix this by always returning an activeParameter based on the triggerCharacter (~, ?, or other).

[mnxn]

• The highlighted argument does not switch to the first labeled with only ~, it needs the name of the labeled argument as well:

@tmattio: your second issue should be fixed in #328