lookahead

lookahead combinator applies parser without consuming any input on success. On failure the failure is returned as is, with pos pointing to the deepest position reached, which yields more precise errors. On success everything parser recovered is discarded, along with the input it consumed.

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(
  first(string('hello'), whitespace()),
  lookahead(string('let')),
  string('lettuce')
)

Success

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

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

Failure

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

{
  isOk: false,
  start: 6,
  end: 9,
  pos: 6,
  expected: 'lettuce'
}

---

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

{
  isOk: false,
  start: 6,
  end: 9,
  pos: 6,
  expected: 'let'
}