fourth

fourth(p: Parser, m?: string): Parser

Applies a parser and returns the fourth element of the resulting array.

This parser works only if p returns an array, and it returns the fourth element of that array. It's most useful with parsers like seq and many that always return arrays, though it will work with parsers like map and always if they are programmed to return arrays.

fourth(p) is an optimized implementation of chain(p, x => always(x[3])).

Example

const parser = fourth(many1(any()))

const s = parse(parser, '12345')
console.log(status(s))  // "ok"
console.log(success(s)) // "4"

const f = parse(parser, '')
console.log(status(f))  // "fail"
console.log(failure(f)) // Parse error at (line 1, column 1):
                        //
                        // 
                        // ^
                        // Expected any character
                        // Note: failure occurred at the end of input

Parameters

p: The parser to apply. This parser should return an array.
m: The optional expected error message that will take the place of the default error message.

Success

Succeeds if p succeeds. Returns the fourth element of the array that p returns. If there are not at least four elements in that array, it will return undefined in the same manner that undefined is always returned when indexing a non-existent element of an array.

Failure

Fails if p fails.

Fatal Failure

Fails fatally if p fails fatally.

Throws

Throws an error if p is not a parser.
Throws an error if m exists and is not a string.
Throws an error if p succeeds but does not return an array.