default values with optional attributes

[Yokozuna59]

Hi, I have a question regarding the default values feature: what is the expected result when having an optional attribute with a default value?

For example, the Langium interface:

interface Node {
  // ...
  shape?: 'default' | 'cloud' = "default";
}

My thought was that it doesn't have to be parsed, but it would be required, not optional, in the TS interface:

interface Node {
  // ...
  shape: 'default' | 'cloud';
}

But it's not the case I guess.

[msujew]

The grammar type system mostly follows the same logic as TypeScript's type system. In TypeScript:

class X {
  readonly value?: string = 'default';
}

The value property still has type string | undefined, even though it is statically assigned. We do the same thing. You should be able to remove the ? in the field declaration though, right?

[Yokozuna59]

How would we be able to know the type (shape) of the node without it? Our plan is to take the AST result, generate DB using the result, then render the diagram.

The approach I thought of is to override the parser, modify each node AST by checking the shape and setting the shape value.

[msujew]

Can you share the part of the grammar that's supposed to do the shape parsing? Maybe you could get away with a well constructed ValueConverter for this.

[Yokozuna59]

Here is it:

fragment MindmapNode returns MindmapNode:
  id=MINDMAP_NODE_ID?
  title=(
    MINDMAP_NODE_SQUARE_TITLE
    | MINDMAP_NODE_CIRCLE_TITLE
    | MINDMAP_NODE_ROUNDED_SQUARE_TITLE
    | MINDMAP_NODE_BANG_TITLE
    | MINDMAP_NODE_CLOUD_TITLE
    | MINDMAP_NODE_HEXAGON_TITLE
    | MINDMAP_NODE_DEFAULT_TITLE
  )
  EOL
  children+=MindmapNode*
;

// it's an override rule: /([^\n\r()[{]+)/;
terminal MINDMAP_NODE_ID: /mindmap-node-id/;
terminal MINDMAP_NODE_SQUARE_TITLE: /\[("[^"]+"|[^\]]+)]/;
terminal MINDMAP_NODE_CIRCLE_TITLE:  /\(\(("[^"]+"|[^)]+)\)\)/;
terminal MINDMAP_NODE_ROUNDED_SQUARE_TITLE: /\(("[^"]+"|[^)]+)\)/;
terminal MINDMAP_NODE_BANG_TITLE: /\)\)("[^"]+"|[^(]+)\(\(/;
terminal MINDMAP_NODE_CLOUD_TITLE: /\)("[^"]+"|[^(]+)\(/;
terminal MINDMAP_NODE_HEXAGON_TITLE: /{{("[^"]+"|[^}]+)}}/;
// it's an override rule: /([^\n\r]+)/;
terminal MINDMAP_NODE_DEFAULT_TITLE: /mindmap-node-default-title/; 

Each node can have an id, a title, or both. If it's a default node, it doesn't have a title:

cool      %% id:cool,            shape:default
cool[wow] %% id:cool, title:wow, shape:square
[wow]     %%          title:wow, shape:square

[msujew]

Oh, I see, so a computed property effectively? That's definitely not supported right now, and I'm not sure whether it's in scope of Langium at all.

[Yokozuna59]

Yes. Well, that's the syntax of the mindmap diagram, which is similar to the flowchart and block diagrams.