lookahead primitive ​

Signature ​

function lookahead<T>(parser: Parser<T>): Parser<T>

Description ​

lookahead combinator applies parser without consuming any input. If parser fails and consumes some input, so does lookahead.

Usage ​

The example is rather contrived, but it clearly illustrates how the combinator works, allowing one, for example, collect ambiguous results for further processing.

const Parser = sequence(
  takeLeft(string('hello'), whitespace()),
  lookahead(string('let')),
  string('lettuce')
)

run(Parser).with('hello lettuce')

{
  isOk: true,
  span: [ 0, 13 ],
  pos: 13,
  value: [ 'hello', 'let', 'lettuce' ]
}

run(Parser).with('hello let')

{
  isOk: false,
  span: [ 6, 9 ],
  pos: 9,
  expected: 'lettuce'
}

run(Parser).with('hello something')

{
  isOk: false,
  span: [ 6, 9 ],
  pos: 9,
  expected: 'let'
}