optional

optional combinator tries to apply parser. Returns the result of parser or null. Never fails on its own, but a committed failure from parser propagates instead of resolving to null.

Usage

const Parser = sequence(
  optional(string('-')),
  whole()
)

Success

run(Parser).with('-2')

{
  isOk: true,
  start: 0,
  end: 2,
  pos: 2,
  value: [ '-', 2 ]
}

---

run(Parser).with('2')

{
  isOk: true,
  start: 0,
  end: 1,
  pos: 1,
  value: [ null, 2 ]
}

Failure

run(Parser).with('~2')

{
  isOk: false,
  start: 0,
  end: 0,
  pos: 0,
  expected: 'whole number'
}