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Label

As explained in Functional Programming in Scala, Second Edition

A parser operation that replaces a parse-error stack with a contextual message when the enclosed parser fails.

label assigns a meaningful message to a parser failure. When the enclosed parser returns a failure, label replaces the existing ParseError message while preserving the most recent failure location. section 9.5.1 section 9.6.3

Use label when a low-level parser error needs a clearer description:

  • Implement it by mapping over the failing Result and applying ParseError.label. section 9.6.3
  • Prefer scope when nested context matters, because scope adds a message instead of discarding inner details. section 9.5.2 section 9.6

Naming a nonnegative integer failure

Wrap a numeric parser with nonNegativeInt.label("non-negative integer"). A failed conversion then reports the simpler domain-level message rather than exposing the parser's previous error details; scope would preserve those details instead. section 9.8

How Functional Programming in Scala, Second Edition covers it

How deep it goes
Defines label, specifies its failure property, and implements stack trimming through mapError; it stops short of full error formatting.
Examples
Uses magic-word parsing and a nonnegative-integer parser to contrast concise labels with preserved scoped errors.
What it assumes
Assumes parsers return Result values containing ParseError and introduces error stacks, locations, and failure mapping through concrete implementations.
What you can do after
Apply it via a parser-combinator API to produce clearer domain-level errors and choose between replacement and nesting.
How it connects
Connects label to Parser, ParseError, Location, mapError, scope, and parser combinators.
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