Add support for semantic predicates #763
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This PR adds support for semantic predicates to Lrama, allowing conditional enabling of grammar rules based on runtime conditions.
Syntax:
```yacc
rule : {expression}? TOKEN { action }
| TOKEN { action }
;
```
The predicate {expression}? is evaluated at parse time. If it returns true (non-zero), the alternative is enabled.
Example:
```yacc
widget
: {new_syntax}? WIDGET ID NEW_ARG
{ printf("New syntax\n"); }
| {!new_syntax}? WIDGET ID OLD_ARG
{ printf("Old syntax\n"); }
;
```
Motivation / Background
Semantic predicates enable context-sensitive parsing, which is useful for:
- Version-dependent syntax (e.g., supporting both old and new language features)
- Context-sensitive keywords (e.g., async in JavaScript that behaves differently based on context)
- Conditional grammar rules based on parser state
This feature is similar to ANTLR4's semantic predicates and fills a gap in Lrama's capabilities for handling context-dependent grammars. Leading predicates (at the start of a rule) affect prediction, while trailing predicates act as validation.
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Overview
This PR adds support for semantic predicates to Lrama, allowing conditional enabling of grammar rules based on runtime conditions.
Syntax:
rule : {expression}? TOKEN { action } | TOKEN { action } ;The predicate {expression}? is evaluated at parse time. If it returns true (non-zero), the alternative is enabled.
Example:
widget : {new_syntax}? WIDGET ID NEW_ARG { printf("New syntax\n"); } | {!new_syntax}? WIDGET ID OLD_ARG { printf("Old syntax\n"); } ;Motivation / Background
Semantic predicates enable context-sensitive parsing, which is useful for:
This feature is similar to ANTLR4's semantic predicates and fills a gap in Lrama's capabilities for handling context-dependent grammars. Leading predicates (at the start of a rule) affect prediction, while trailing predicates act as validation.