Lookahead

The Lookahead token in RCParsing ensures that a specified child token does not match at the current position without consuming input. It’s a zero-width assertion for enforcing constraints.

Overview

A NegativeLookahead token checks if the input does not match the specified child token. If the child would match, the token fails; if it would not match, the token succeeds without consuming input.
A PositiveLookahead token checks if the input matches the specified child token. If the child matches, the token succeeds without consuming input; if it does not match, the token fails.

Example

Here’s an example of ensuring an identifier is not a keyword:

var builder = new ParserBuilder();

builder.CreateToken("non_keyword_identifier")
    .NegativeLookahead(b => b.KeywordChoice("if", "else"))
    .Identifier();

var parser = builder.Build();
var result1 = parser.MatchToken("non_keyword_identifier", "variable"); // Succeeds
var result2 = parser.MatchToken("non_keyword_identifier", "if"); // Fails

Use Cases

• Keyword Avoidance: Prevent matching reserved keywords.
• Pattern Constraints: Ensure specific tokens are not present.
• Context-Sensitive Tokens: Enforce rules based on what follows.

Intermediate Values

The NegativeLookahead token does not produce an intermediate value since its child needs to be failed, but PositiveLookahead propagates intermediate value from its child:

builder.CreateToken("non_keyword_identifier")
    .NegativeLookahead(b => b.KeywordChoice("if", "else"))
    .CaptureText(b => b.Identifier())
    .Pass(v => v[1]);

builder.CreateToken("number_with_leading_one")
    .PositiveLookahead(b => b.Literal("1"))
    .Number<int>()
    .Pass(v => (string)v[0] + ((int)v[1]).ToString());

var parser = builder.Build();

var result1 = parser.MatchToken("non_keyword_identifier", "variable");
var value1 = result1.GetIntermediateValue<string>(); // "variable"

var result2 = parser.MatchToken("number_with_leading_one", "123");
var value2 = result2.GetIntermediateValue<string>(); // "1123" string

Notes

• Zero-Width: Does not consume input.
• Failure on Match: Fails if the child token would match.
• Parser Guidance: Avoid incorrect parsing paths.
• Context Checking: Ensure specific tokens follow without consuming them.
• No Rules: Tokens cannot reference rules, only other tokens.

Related Tutorials

• Identifier Token
• Choice Token