feat/pattern matching

[xav-db]

Description

Integrate Pattern matching into HQL that allow user to match on enum variants within queries. e.g.

// schema
N::User {}
N::Admin {}

Enum::UserType {
     Normal(User)
     Admin(Admin)
}

// query
QUERY patternMatch() => 
    N<User>::MATCH|_| {
        UserType::Normal(u) => ...,
        UserType::Admin(u) => ...,
    }
    RETURN NONE

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Other (please describe):

Related Issues

Closes #

Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

[Copilot]

Pull Request Overview

This PR integrates pattern matching and variable binding into HQL by extending the AST, parser, analyzer, and grammar.

• Introduces VariableDeclaration, MatchStep, and related enums/structs in the parser AST.
• Updates GraphStepType and StartNode to carry optional variable declarations.
• Extends the grammar (grammar.pest) with enum_def and match_step constructs.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
helixdb/src/helixc/parser/helix_parser.rs	Expanded StartNode and GraphStepType, added VariableDeclaration and pattern-matching logic.
helixdb/src/helixc/analyzer/analyzer.rs	Updated analyzer pattern matches to use the new GraphStepType variants and variable metadata.
helixdb/src/grammar.pest	Added enum_def, match_step, and variable_declaration rules to support pattern matching.

Comments suppressed due to low confidence (1)

helixdb/src/helixc/analyzer/analyzer.rs:2523

• This error message appears in the InE branch but refers to an "outgoing" edge. Update it to mention "incoming" edge to accurately describe the scenario.

format!("Edge of type `{}` exists but it is not a valid outgoing edge type for node of type `{}`", edge_type, node_label),

[Copilot]

The variable (and similarly loc) binding in the OutE { edge_type, variable, loc } match arm is never used. Prefix unused bindings with an underscore to avoid compiler warnings (e.g., _variable).

Suggested change

	variable,
	loc,
	_variable,
	_loc,

[Copilot]

Calling .unwrap() after .ok_or_else(...) will panic on error; consider using the ? operator to propagate the ParserError instead of panicking.

Suggested change

	identifier: p.into_inner().next().unwrap().as_str().to_string(),
	identifier: p.into_inner().next()
	.ok_or_else(|| ParserError::new("Expected identifier"))?
	.as_str()
	.to_string(),