First

The First token in RCParsing matches a sequence of two child tokens and propagates the intermediate value of the first token. It’s optimized for patterns where only the first part of a sequence is needed.

Overview

The First token is a specialized sequence that matches two tokens but only calculates and propagates the first token’s value. This is useful for patterns where a prefix is the primary focus.

Example

Here’s an example of parsing a prefixed identifier:

var builder = new ParserBuilder();

builder.CreateToken("prefixed_identifier")
    .First(
        b => b.Literal("id_"),
        b => b.Identifier()
    );

var parser = builder.Build();
var result = parser.MatchToken("prefixed_identifier", "id_variable");
var value = result.GetIntermediateValue<string>(); // "id_"

Use Cases

• Prefixed Tokens: Match prefixes like id_ or const_.
• Structured Patterns: Extract the first part of a two-token sequence.
• Simplified Parsing: Avoid manual value propagation.

Intermediate Values

The First token propagates the first child’s intermediate value, while the second token’s value is ignored and not calculated:

builder.CreateToken("prefixed_number")
    .First(
        b => b.Literal("+"),
        b => b.Number<int>() // 'First' token says the number token that it should not calculate intermediate value
    );

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

Notes

• Optimized: More efficient than a Sequence with .Pass(0).
• Two Children: Requires exactly two child tokens.
• No Rules: Tokens cannot reference rules, only other tokens.

Related Tutorials

• Between Token
• Second Token
• Sequence Token