fail

fail always fails with the given expected message, without consuming any input. It's the counterpart of nothing, and is mostly useful inside when callbacks to reject input based on an already parsed value.

Usage

const Parser = when(
  integer(),
  ({ value }) => value % 2 === 0 ? mapTo(nothing(), value) : fail('even number')
)

Success

run(Parser).with('42')

{
  isOk: true,
  start: 0,
  end: 2,
  pos: 2,
  value: 42
}

Failure

run(Parser).with('43')

{
  isOk: false,
  start: 2,
  end: 2,
  pos: 2,
  expected: 'even number'
}