Function optional

If the decoder fails, the given value is returned instead.

Example

const decoder = Decode.optional(42, Decode.number);

decode(decoder, 100) === 100
decode(decoder, -10) === -10

decode(decoder, '3') === 42
decode(decoder, NaN) === 42
decode(decoder, null) === 42
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Remarks

This is implemented by using Decode.oneOf:

function optional(value, child) {
    return Decode.oneOf([ child, Decode.succeed(value) ]);
}
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