Support nullable type.

[OzairP]

interface User {
    user_id: number,
    nickname: string | null
}

export const UserDecoder: Decoder<User> = object({
    user_id: number(),
    nickname: nullable(string())
})

[mulias]

@OzairP I realize that I've provided optional but not nullable as a core decoder, which is inconsistent. Fortunately nullable is a pretty simple decoder to build from the existing combinators:

const nullabe = <T>(decoder: Decoder<T>): Decoder<T | null> =>
  union(decoder, constant(null));

My hope is that this library can act as a toolbox for you to make whatever decoders you need for your project. Do you think there's some part of the documentation that I could improve on to make examples like this more evident?

[mulias]

I originally assumed that you were looking for something equivalent to the Elm nullable decoder, but I happened to think of a slightly different use case that might be worth mentioning. I tend not to see explicit null values in JSON, but I do sometimes use field: FieldType | null instead of field?: FieldType in order to be more explicit. Here's how you could implement a decoder that returns null when a value isn't present in the input:

/**
 * Returns a `placeholder` value if the input is not found -- for example when a field is not
 * present in an object, or an array index is out of bounds. Unlike `withDefault`, the placeholder
 * is not returned if the input is found and fails to decode.
 */
const whenAbsent = <T, Q>(placeholder: T, decoder: Decoder<Q>): Decoder<T | Q> =>
  union(decoder, constant(undefined)).map((val) => val === undefined ? placeholder : val)

/**
 * Validates the given `decoder` or `null`. If the input is not found then decode to `null`.
 */
const nullable = <T>(decoder: Decoder<T>): Decoder<T | null> =>
  whenAbsent(null, union(decoder, constant(null)));

[mulias]

This issue is still under specified, #37 is a more productive line of conversation.