Choice

10/13/25Less than 1 minute

Choice

The Choice token in RCParsing attempts to match one of several child tokens, trying them in order until one succeeds or all fail. It’s useful for defining alternative token patterns, such as different literals or patterns.

Overview

A Choice token tries each child token in the specified order, succeeding with the first match and propagating its intermediate value. If all children fail, the token fails.

Example

Here’s an example of a Choice token for matching different keywords:

var builder = new ParserBuilder();

builder.CreateToken("keyword")
    .Choice(
        b => b.Literal("if"),
        b => b.Literal("else"),
        b => b.Literal("while")
    );

var parser = builder.Build();
var result = parser.MatchToken("keyword", "if");
var value = result.GetIntermediateValue<string>(); // "if"

LongestChoice and ShortestChoice

The LongestChoice and ShortestChoice variants control which match is selected when multiple children could match:

LongestChoice: Selects the first child that consumes the most input.
ShortestChoice: Selects the first child that consumes the least input.

Example

var builder = new ParserBuilder();

builder.CreateToken("longest_keyword")
    .LongestChoice(
        b => b.Literal("pre"),
        b => b.Literal("prefix")
    );

builder.CreateToken("shortest_keyword")
    .ShortestChoice(
        b => b.Literal("prefix"),
        b => b.Literal("pre")
    );

var parser = builder.Build();
var result1 = parser.MatchToken("longest_keyword", "prefixx"); // Matches "prefix"
var result2 = parser.MatchToken("shortest_keyword", "prefixx"); // Matches "pre"

Use Cases

Keyword Matching: Match one of several keywords.
Pattern Alternatives: Parse different numeric formats or literals.
Ambiguity Resolution: Use LongestChoice or ShortestChoice for specific match behavior.

Intermediate Values

The Choice token propagates the matched child’s intermediate value:

builder.CreateToken("value")
    .Choice(
        b => b.Number<int>(),
        b => b.Literal("true")
    );

var parser = builder.Build();
var result = parser.MatchToken("value", "123");
var value = result.GetIntermediateValue<int>(); // 123

Notes

Order Matters: Place specific patterns first to avoid premature matches.
Longest/Shortest: Use for disambiguating overlapping patterns.
No Rules: Tokens cannot reference rules, only other tokens.