Optional

The Optional rule in RCParsing attempts to match a child rule or token but never fails, even if the child does not match. It’s useful for parsing optional components in a grammar, such as optional semicolons or parameters.

Overview

An Optional rule wraps a single child rule or token. If the child matches, its value and text are captured; if it fails, the rule succeeds with an empty children array and no consumed text. This makes it ideal for parts of the grammar that may or may not appear.

Example

Here’s an example of an Optional rule for parsing an optional semicolon at the end of a statement:

var builder = new ParserBuilder();

builder.CreateRule("statement")
    .Identifier()
    .Optional(b => b.Literal(";"));

var parser = builder.Build();
var result1 = parser.ParseRule("statement", "x;"); // Matches with semicolon
var result2 = parser.ParseRule("statement", "x");  // Matches without semicolon

Use Cases

• Optional Terminators: Parse optional semicolons or commas in languages like JavaScript.
• Optional Parameters: Handle optional function parameters or modifiers.
• Flexible Syntax: Allow optional keywords or tokens in a grammar.

Transformation

The Optional rule propagates the child’s value if it matches; otherwise, the value is null. You can use a transformation to handle this:

builder.CreateRule("statement")
    .Identifier()
    .Optional(b => b.Literal(";"))
    .Transform(v => new Statement
    {
        Name = v.Children[0].Text,
        HasSemicolon = v.Children[1].Children.Count == 1
    });

var parser = builder.Build();
var result = parser.ParseRule("statement", "x;");
var statement = result.Value as Statement; // Name: "x", HasSemicolon: true

Notes

• Never Fails: The Optional rule always succeeds, making it safe for optional grammar components.
• Empty Children: If the child does not match, the Children array is empty, and the Value is null unless transformed.
• Single Child: Only one child rule or token can be specified.

Related Tutorials

• Sequence Rule
• Choice Rule
• Repeat Rule