Playground displays misleading linker errors

[Rophuine]

When using the Langium Playground, you're really only editing a grammar and seeing how it parses a sample input into an AST - the linking stage doesn't seem relevant. But linker errors are displayed, which as someone experimenting with Langium for the first time was very confusing until I realised what was happening.

I spent over an hour trying to understand why my grammar and content were producing errors, including diving into the XText docs. I nearly gave up on Langium as it just seemed broken, even in very simple scenarios. It turns out these errors just aren't resolvable in the Playground, because you're only working on the grammar and not doing anything related to resolution or scope.

The Playground is very likely to be used by people trying out Langium for the first time, and displaying these errors is very likely to lead to frustration and people giving up.

Steps To Reproduce

1. Load the Playground at https://langium.org/playground/
2. Put the following into the Grammar pane:

   
   grammar MyLang

entry Model: body+=Statement+;

Statement: 'const' assignment=ConstantAssignment ';'; ConstantAssignment: name=ID '=' value=Expression; Expression: value=NUMBER | reference=[ConstantAssignment];

hidden terminal WS: /\s+/; terminal ID: /[a-zA-Z][\w]/; terminal NUMBER returns number: /[0-9]+(.[0-9])?/;

3. Put the following into the Content pane:

const n=5; const y=n;

## The current behavior

![image](https://github.com/eclipse-langium/langium/assets/4100713/089e997b-0f1b-42fb-ba60-153e3588b1de)

## The expected behavior

No error should be displayed - the grammar is valid and has been successfully parsed into an AST. The reference hasn't been resolved, but we don't expect it to be - you can't write linker code in the playground, so many valid examples will result in errors.

[msujew]

Thank you for the discussion @Rophuine, though I'm not sure I can agree with this. You're completely right in the regard that the inability to write custom scoping code will lead to unintended error messages about unresolved cross references and is actually something we're investigating how to address. Maybe @Lotes can chime in on that. However, IMO at this point you'd be past using the playground as a means to develop/iterate your language. I think it's fairly valuable to have the full cross reference support in there, including error messages in case the linking failed.

@spoenemann Do you have an opinion on this?

[spoenemann]

Idea: enable to switch linking errors on/off. They're useful even in the playground sometimes to demonstrate how the default scoping/linking works and e.g. how renaming a symbol without the automatic rename feature breaks the reference. But I'd be ok with turning that switch off by default.

[Lotes]

The playground will get an update soon (waiting for some depending PRs). The perfect opportunity for me to add such a switch.

[Lotes]

If we turn linking errors entirely off, any identifier can be put in as a cross-reference and nothing complains. This looks more broken to me than having a linking error in red. I would like to propose a third 'middle' way: Since we will never implement a custom scope provider by user, the user is not able to remove the error. It would be ok to raise warnings instead, that explains that a custom scope provider needs to be injected and that this is out of scope for the playground. I would vote against a switch because it does not change the possible actions for the user. A switch on off just ignores linking, which is not so helpful for testing Langium out. Langium is more than just a parser.

WDYT? @Rophuine @spoenemann

Here is an alternative view (preview):

Linking failed. Please inject a custom scope provider. This is a limitation of the playground. To overcome this issue, please study the learning section in the Langium documentation ...

I am open for other wordings if we want to go this route :)

[Rophuine]

A warning sounds perfect, especially with some explanation. I wasted quite a lot of time in the playground thinking that Langium just wasn't processing a perfectly valid grammar before I realised that it was actually a scope error - I nearly assumed the parsing was just broken and moved on to other options. I'm glad I persevered because Langium has been great!

Having a warning that pointed me to a custom scope provider as a possible solution would have been a much better experience as a beginner!