can't get grammar to work

[trusktr]

Hello! Taking Langium for a spin. I started with the hello-world project template. When I hit F5, it started the extension host. I made a .hello file, and that worked great, checking both person and hello statements.

Next I proceeded to modify my .langium grammar, but I'm not having luck with the new grammar yet.

Here's the new definition, in case I'm missing something (VS Code shows no errors):

grammar LanguageExperiment

entry Code:
    (vars+=LetStatement | fns+=Functionn)*;

LetStatement:
    'let' name=ID '=' value=Functionn;

Functionn:
    'function' name=ID '(' ID ')' '{' '}';

hidden terminal WS: /\s+/;
terminal ID: /[_a-zA-Z][\w_]*/;
terminal INT returns number: /[0-9]+/;
terminal STRING: /"[^"]*"|'[^']*'/;

hidden terminal ML_COMMENT: /\/\*[\s\S]*?\*\//;
hidden terminal SL_COMMENT: /\/\/[^\n\r]*/;

Here's a sample file in the extension host:

let foo = function foo(foo) {};

I see these errors:

In a sibling file I still have

person Joe
Hello John!

person John
Hello Joe!

and that still works.

Why does the hello example still work, although I've changed the grammar? What's wrong with the new grammar (or the sample "code")?

[luan-xiaokun]

Did you run npm run langium:generate and npm run build? The first command will generate TypeScript files for your DSL, and the second command will compile the TypeScript files to build your extension.
https://github.com/langium/langium/blob/4115491e39e3eb2ad36cbe4223a02ce212d0ecd0/packages/generator-langium/langium-template/package.json#L37-L44

[luan-xiaokun]

I'm not familiar with VS Code's launch configuration stuff, I just think f5 as running ./foo to execute the code, and npm run build as gcc demo.c to compile the code. I think it's possible to customize your own running procedure by modifying /.vscode/launch.json, but I don't know how to do it.

[msujew]

You can add a prelaunchTask to the extension debug config and call the Langium generator + build script in there. We might add that to the yeoman generator in the future.

[bnhmn]

It also took us 2 hours to figure out that you need to run npm run langium:generate and npm run build in order to apply the changes to the grammar.
Maybe it makes sense to mention it in the Getting Started page. We looked for it there, but didn't find the information.

After 2-3 days I also figured out that everything is explained in the langium-quickstart.md file.
If we had seen it earlier it would have saved quite a bit of time.

Regarding the prelaunchTask thing: I have tested it but it adds some delay to the startup of the Extension Host.
Ideally, there would be a way to automatically run "npm run watch" in the background when opening a Langium project.
But it's also not that hard to run it manually in the terminal.

[msujew]

@ben5311 Our getting started page specifically mentions to run npm run langium:generate to regenerate the TypeScript files:

Regarding the prelaunchTask thing: I have tested it but it adds some delay to the startup of the Extension Host.

Well yes, it's adds security at the cost of time. That's expected.

Ideally, there would be a way to automatically run "npm run watch" in the background when opening a Langium project.

I'm quite sure that this isn't in the spirit of vscode's security model, given that the watch script could contain any kind of code. In fact, it could even be totally unrelated to Langium.

[bnhmn]

Ideally, there would be a way to automatically run "npm run watch" in the background when opening a Langium project.

Actually VS Code can do this.

Instructions:

1. Make sure concurrently is installed so you can run the two commands as one command: npm i -D concurrently.
2. Add / update this npm scripts entry in package.json

    "scripts": {
        "watch": "concurrently \"langium generate --watch\" \"tsc -b tsconfig.json --watch\""
    }

3. Create the file .vscode/tasks.json inside the project folder.

{
    "version": "2.0.0",
    "tasks": [
        {
        "label": "Run npm watch on startup",
        "type": "shell",
        "command": "npm run watch",
        "presentation": {
            "reveal": "always",
            "panel": "new"
        },
        "runOptions": { "runOn": "folderOpen" }
        }
    ]
}

Works pretty well. Credits to: https://www.roboleary.net/vscode/2020/10/19/vscode-task-onstartup.html.