Crash on invoking function signature.

[hifall]

monaco-editor version: 0.14.3
Browser: Chrome 67.0.3396.99
OS: macOS 10.12

Okay, it seems like a VS Code issue from where the crash happens, but since I am using Monaco, I am filing it here.

I am using Monaco editor connected with a backend language server. When trying to invoke displaying the signature of a function, I am seeing a crash with the following call stack in the browser console:

errors.js:58 Uncaught Error: Cannot read property 'length' of undefined

TypeError: Cannot read property 'length' of undefined
    at ParameterHintsWidget.render (parameterHintsWidget.js:272)
    at parameterHintsWidget.js:182
    at Emitter.fire (event.js:105)
    at parameterHintsWidget.js:104
    at ParameterHintsWidget.render (parameterHintsWidget.js:272)
    at parameterHintsWidget.js:182
    at Emitter.fire (event.js:105)
    at parameterHintsWidget.js:104
    at errors.js:58

The Monaco client sends a request like this:

{"jsonrpc":"2.0","id":6,"method":"textDocument/signatureHelp","params":{"textDocument":{"uri":"/home/model.r"},"position":{"line":36,"character":6}}}

And the language server responds with:

{"jsonrpc":"2.0","id":6,"result":{"signatures":[{"label":"paste(..., sep = \" \", collapse = NULL)"}],"activeSignature":0}}

This happens because of the following line inside ParameterHintsWidget's render function:

var hasParameters = signature.parameters.length > 0;

This line assumes that parameters field always present inside signature, while in fact parameters is absent from the server response above. However, the LSP specification says SignatureInformation's parameters is optional. Therefore, the assumption doesn't seem correct.

[rcjsuen]

@hifall How are you hooking up the language server to your Monaco editor? Are you using TypeFox's monaco-languageclient?

[hifall]

@rcjsuen, yes.

[rcjsuen]

https://github.com/Microsoft/monaco-editor/blob/935bd5275a4e9170db586c514051936c0749237b/monaco.d.ts#L4763-L4783

@hifall As the API does not state that it is optional, I believe this is a bug in monaco-languageclient. See TypeFox/monaco-languageclient#112.

Of course, if the VS Code team wants to loosen up their API requirement, they are welcome to do so. Irregardless of their decision, we should fix monaco-languageclient in the meantime...

[hifall]

The official spec says it's optional: LSP spec, search for SignatureInformation.

[rcjsuen]

@hifall That's the LSP specification and not the Monaco/VS Code API. They may look like they are 1-to-1 but they do not have to be 1-to-1.

It's up to bridges like the TypeFox/monaco-languageclient and Microsoft/vscode-languageserver-node projects to alter LSP information into a way that is consumable by the client editor.