Modifiers

ISPA Parser Modifiers Concept

Modifiers in ISPA Parser Generator are annotations applied to grammar elements (rules, expressions, or tokens) using square-bracket syntax [...]. They alter the parsing behavior, AST generation flow, or lexer mechanics for specific definitions without polluting the core language logic.

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🚀 Overview

Modifiers provide fine-grained control over how individual nodes and tokens are processed by the ISPA compiler engine:

• AST Value Optimization: Inlining nodes to simplify the resulting AST hierarchy.
• Lexer Directives: Stripping out non-semantic tokens (like whitespace or comments) early in the pipeline.
• Parser Controls: Fine-tuning error recovery, associativity, or visibility.

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💡 Syntax & Usage

Modifiers are specified directly preceding the definition or rule block using [modifier_name] or parameter-driven [modifier_name(arg)] syntax.

1. The [inline] Modifier

The [inline] directive instructs the compiler to collapse or flatten an AST node directly into its parent. This prevents unnecessary single-child node nesting in the generated tree.

Example: Inlining Single Expressions

[inline] // Collapses the AST node into the caller's AST level
operator:
    @ PLUS | MINUS
    {@}
;

Behavior:

• Without [inline]: Produce operator rule, containing either PLUS or MINUS token as value
• With [inline]: Directly evaluate PLUS or MINUS token to the value calling operator rule

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2. The [skip] Modifier

The [skip] modifier is applied to lexer tokens to indicate that matched sequences should be discarded by the lexer and omitted from token streams passed to the parser stage.

Example: Ignoring Whitespace

[skip] // Automatically skips this token whenever encountered
WHITESPACE: [ ]+;

Common Use Cases:

• Ignoring spaces, tabs, and newlines.
• Stripping single-line and block comments (// ..., /* ... */).