error

error combinator allows to replace parser's error message with expected.

A committed failure keeps its own message, so a top-level error can't flatten the precise messages a grammar commits to. To label one, commit on the outside: commit(error(parser, expected), label).

Usage

const Parser = error(
  choice(
    string('true'),
    string('false')
  ),
  `expecting either 'true' or 'false'`
)

Success

run(Parser).with('true')

{
  isOk: true,
  start: 0,
  end: 4,
  pos: 4,
  value: 'true'
}

Failure

run(Parser).with('maybe')

{
  isOk: false,
  start: 0,
  end: 4,
  pos: 0,
  expected: "expecting either 'true' or 'false'"
}