Formatter not idempotent

[cdietrich]

given the grammar

grammar HelloWorld

entry Model:
    (content+=ModelContent)*;

ModelContent:
    Entity;

Entity:
    'entity' name=ID '{'
        (content+=EntityContent)*
    '}';

EntityContent:
    Property | Description;

Description:
    'description' ':' description=STRING;

Property:
    'property' name=ID (type=Type) ('{'  '}')?;

Type: name=ID;

hidden terminal WS: /\s+/;
terminal ID: /[_a-zA-Z][\w_]*/;
terminal STRING: /"(\\.|[^"\\])*"|'(\\.|[^'\\])*'/;

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

and formatter

import { AstNode } from "langium";
import { AbstractFormatter, Formatting, NodeFormatter } from "langium/lsp";
import {
  ModelContent,
  isEntity,
  isModel,
  isProperty,
} from "./generated/ast.js";
const FormatingOptions = {
  lineBreak: Formatting.newLine({ allowMore: true }),
  emptyLine: Formatting.newLines(2, { allowMore: true }),
  oneSpace: Formatting.spaces(1),
  noSpace: Formatting.noSpace(),
  indent: Formatting.indent({ allowMore: true }),
  noIndent: Formatting.noIndent({ allowMore: true }),
};

function formatBody(_: AstNode, formatter: NodeFormatter<AstNode>) {
  const bracesOpen = formatter.keyword("{");
  const bracesClose = formatter.keyword("}");
  const interior = formatter.interior(bracesOpen, bracesClose);
  interior.prepend(FormatingOptions.indent);
  bracesClose.prepend(FormatingOptions.lineBreak);
}

export class HelloWorldFormatter extends AbstractFormatter {
  protected override format(node: AstNode): void {
    const formatter = this.getNodeFormatter(node);
    if (isProperty(node)) {
      formatter.keyword("property").prepend(FormatingOptions.emptyLine);
      formatter.property("name").surround(FormatingOptions.oneSpace);
      formatter.property("type").surround(FormatingOptions.oneSpace);
      if (formatter.keyword("{").nodes.length > 0) {
        formatBody(node, formatter);
      }
    } else if (isModel(node)) {
      const c = node.content as ModelContent[] as AstNode[];
      formatter.nodes(...c).prepend(FormatingOptions.noIndent);
    } else if (isEntity(node)) {
      formatter.keyword("entity").prepend(FormatingOptions.emptyLine);
      formatter.property("name").surround(FormatingOptions.oneSpace);
      formatBody(node, formatter);
    }
  }
}

formatting the following file will toggle the document formatted state

entity TypesContainer {
    description: "Xxxxx"

    property json JSON

    property name Name

    property userEmail UserEmail

    property instant Instant {
    }
}

the problematic part seems to be

formatter.property("type").surround(FormatingOptions.oneSpace);

replacing it with

formatter.property("type").prepend(FormatingOptions.oneSpace)
  formatter.keyword("{").prepend(FormatingOptions.oneSpace)

seems to solve the problem.

is this intentional behaviour?

[msujew]

is this intentional behaviour?

It's not, we have an issue for this: #1362. I'm aware of the root cause, but it's a minorly breaking change that I'm planning to address this issue.

[cdietrich]

ok thx

[cdietrich]

workaround

  protected override isNecessary(_edit: TextEdit, _document: TextDocument): boolean {
    return true
  }