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when primitive ​

Signature ​

ts
function when<T, R extends Parser<unknown>>(
  context: Parser<T>,
  parser: (ctx: Context<T>) => R
): ToParser<R>

Description ​

when combinator allows to create chained, context-aware parsers, that may depend on the output of the context parser. Returns a parser produced by the parser callback, which is called only if the context parser succeeds, i.e. if it fails, then when fails as well.

Usage ​

ts
const Parser = when(takeLeft(letters(), whitespace()), ({ value }) => {
  switch (value) {
    case 'integer': return integer()
    case 'string': return letters()
    case 'bracketed': return takeMid(string('('), letters(), string(')'))
    default: return rest()
  }
})

Success

ts
run(Parser).with('integer 42')

{
  isOk: true,
  span: [ 8, 10 ],
  pos: 10,
  value: 42
}
ts
run(Parser).with('string Something')

{
  isOk: true,
  span: [ 7, 16 ],
  pos: 16,
  value: 'Something'
}
ts
run(Parser).with('bracketed (Something)')

{
  isOk: true,
  span: [ 10, 21 ],
  pos: 21,
  value: 'Something'
}
ts
run(Parser).with('some input')

{
  isOk: true,
  span: [ 5, 10 ],
  pos: 10,
  value: 'input'
}

Failure

ts
run(Parser).with('0x42')

{
  isOk: false,
  span: [ 0, 0 ],
  pos: 0,
  expected: 'letters'
}