withDefault()

function withDefault<T, F>(parser, fallbackValue): InfallibleParser<T | F>

Provide a default value to fall back to when parsing fails. Since the default value is static, the parser will always succeed.

See

oneOf for a more flexible alternative.

Type Parameters

Type Parameter
T
F

Parameters

Parameter	Type	Description
parser	Parser<T>
fallbackValue	F

Returns

InfallibleParser<T | F>

Examples

Parse a number with a default value:

const parseNum = withDefault(parseNumber, 0)
parseNum(1) // -> ParseSuccess<number>
parseNum(null) // -> ParseSuccess<0>

Parse an array of objects, but replace the objects with a default value if the parsing fails:

const parseContent = array(
  withDefault(
    object({
      tag: equals('text'),
      value: parseString,
    }),
    {
      tag: 'unknown',
    },
  ),
)

const res = parseContent([{
  tag: 'text',
  value: 'hello'
}, {
  tag: 'number'
  value: 123
}])

where res becomes:

[
  { tag: 'text', value: 'hello'},
  { tag: 'unknown' }
]

Calling withDefault is almost the same as:

oneOf(parser, () => success(fallbackValue))

The only difference is that the return type of the parser will always be a success.