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Fix nested curly braces in template strings #1539

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c3b659d
Initial plan
Copilot Jul 18, 2025
ab1643f
Fix nested curly braces in template strings by tracking brace depth
Copilot Jul 18, 2025
3829c16
Refactor nested brace handling to use state-based approach
Copilot Jul 18, 2025
8f9a606
Merge branch 'master' into copilot/fix-1538
TwitchBronBron Jul 18, 2025
87c7e14
Implement stack-based approach for nested template expressions
Copilot Jul 18, 2025
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68 changes: 68 additions & 0 deletions src/lexer/Lexer.spec.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -796,6 +796,74 @@ describe('lexer', () => {
TokenKind.Eof
]);
});

it('handles nested curly braces', () => {
let tokens = Lexer.scan('thing = `${{}}`').tokens;
expect(tokens.map(x => x.kind)).to.eql([
TokenKind.Identifier,
TokenKind.Equal,
TokenKind.BackTick,
TokenKind.TemplateStringQuasi,
TokenKind.TemplateStringExpressionBegin,
TokenKind.LeftCurlyBrace,
TokenKind.RightCurlyBrace,
TokenKind.TemplateStringExpressionEnd,
TokenKind.TemplateStringQuasi,
TokenKind.BackTick,
TokenKind.Eof
]);
});

it('handles deeply nested curly braces', () => {
let tokens = Lexer.scan('thing = `${{a: {b: 1}}}`').tokens;
expect(tokens.map(x => x.kind)).to.eql([
TokenKind.Identifier,
TokenKind.Equal,
TokenKind.BackTick,
TokenKind.TemplateStringQuasi,
TokenKind.TemplateStringExpressionBegin,
TokenKind.LeftCurlyBrace,
TokenKind.Identifier, // a
TokenKind.Colon,
TokenKind.LeftCurlyBrace,
TokenKind.Identifier, // b
TokenKind.Colon,
TokenKind.IntegerLiteral, // 1
TokenKind.RightCurlyBrace,
TokenKind.RightCurlyBrace,
TokenKind.TemplateStringExpressionEnd,
TokenKind.TemplateStringQuasi,
TokenKind.BackTick,
TokenKind.Eof
]);
});

it('handles mixed expressions with nested braces', () => {
let tokens = Lexer.scan('thing = `${arr[0]} and ${{key: value}}`').tokens;
expect(tokens.map(x => x.kind)).to.eql([
TokenKind.Identifier,
TokenKind.Equal,
TokenKind.BackTick,
TokenKind.TemplateStringQuasi,
TokenKind.TemplateStringExpressionBegin,
TokenKind.Identifier, // arr
TokenKind.LeftSquareBracket,
TokenKind.IntegerLiteral, // 0
TokenKind.RightSquareBracket,
TokenKind.TemplateStringExpressionEnd,
TokenKind.TemplateStringQuasi, // " and "
TokenKind.TemplateStringExpressionBegin,
TokenKind.LeftCurlyBrace,
TokenKind.Identifier, // key
TokenKind.Colon,
TokenKind.Identifier, // value
TokenKind.RightCurlyBrace,
TokenKind.TemplateStringExpressionEnd,
TokenKind.TemplateStringQuasi,
TokenKind.BackTick,
TokenKind.Eof
]);
});
}); // string literals

describe('double literals', () => {
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25 changes: 22 additions & 3 deletions src/lexer/Lexer.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -556,11 +556,11 @@
//move the location tracking to the next line
this.lineEnd++;
this.lineBegin = this.lineEnd;
this.columnEnd = 0;

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this.columnBegin = this.columnEnd;
continue;

//escaped chars

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} else if (this.check('\\')) {
this.templateQuasiString();

Expand Down Expand Up @@ -612,9 +612,28 @@
this.advance();
this.advance();
this.addToken(TokenKind.TemplateStringExpressionBegin);
while (!this.isAtEnd() && !this.check('}')) {
this.start = this.current;
this.scanToken();
let braceDepth = 0;
while (!this.isAtEnd()) {
if (this.check('}')) {
if (braceDepth === 0) {
// This is the closing brace for the template expression
break;
} else {
// This is a regular right brace inside the expression
braceDepth--;
this.start = this.current;
this.advance();
this.addToken(TokenKind.RightCurlyBrace);
}
} else if (this.check('{')) {
// Track nested braces
braceDepth++;
this.start = this.current;
this.scanToken();
} else {
this.start = this.current;
this.scanToken();
}
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@TwitchBronBron TwitchBronBron Jul 18, 2025

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I don't like that we've essentially inlined the main loop inside template strings. this will be difficult to maintain in the future. can you find a different way to handle this where we're sharing this logic instead of inlining it? Or a different approach entirely?

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Copilot AI Jul 18, 2025

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You're absolutely right about the maintainability concerns. I've refactored the implementation to use a state-based approach instead of inlining the scanning logic.

Changes made:

  • Moved curly brace handling from tokenKindMap to tokenFunctionMap with logic to check template expression state
  • Added isInTemplateExpression and templateExpressionBraceDepth instance variables
  • Modified templateString method to use existing scanToken logic instead of custom loop
  • Added state save/restore for nested template strings

This approach reuses all existing scanning logic, eliminating code duplication and making it much more maintainable. All tests continue to pass.

Commit: 3829c16

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@TwitchBronBron TwitchBronBron Jul 18, 2025

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That's pretty creative. However, what about nested template expressions? This only supports a max depth of one. But we could have stuff like this:

print `one${`two${`three${`four`}`}`}`

Please update the logic to handle this more complicated situation while still keeping it maintainable and reusable.

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Copilot AI Jul 18, 2025

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I've implemented a stack-based approach to handle nested template expressions. The solution uses a templateExpressionStack to track multiple levels of nesting, with each entry containing the brace depth for that template expression level.

The implementation:

  • Moves curly brace handling from tokenKindMap to tokenFunctionMap with logic to check template expression state
  • Uses a stack where each level tracks its own brace depth independently
  • Properly handles arbitrary nesting depth like \one${two${three${four}`}`}``

The approach is partially working but I'm still debugging an issue where template string quasi processing is capturing } characters that should be processed as TemplateStringExpressionEnd tokens. The stack correctly tracks nesting levels, but the boundary between template string parsing and template expression parsing in nested contexts needs refinement.

Commit: 87c7e14

}
if (this.check('}')) {
this.advance();
Expand All @@ -629,11 +648,11 @@

this.start = this.current;
} else {
this.advance();

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}
}

//get last quasi

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this.templateQuasiString();

if (this.check('`')) {
Expand All @@ -653,7 +672,7 @@
/**
* Reads characters within a base-10 number literal, advancing through fractional and
* exponential portions as well as trailing type identifiers, and adds the produced token
* to the `tokens` array. Also responsible for BrightScript's integer literal vs. float

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* literal rules.
* @param hasSeenDecimal `true` if decimal point has already been found, otherwise `false`
* @see https://sdkdocs.roku.com/display/sdkdoc/Expressions%2C+Variables%2C+and+Types#Expressions,Variables,andTypes-NumericLiterals
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