Values that may be missing

There is no null or undefined. A type followed by ? may hold none, and reading it means handling both cases, usually with match.

Writing another language's form is an error with an edit that writes the Jadpo form: null, nil, undefined and None are each type.foreign_value, and ?? and ?. are syntax.null_coalescing and syntax.optional_chaining, whose edits write the match.

type Listing = Object {    name: ProductName    subtitle: ProductName?}function headline(listing: Listing) -> ProductName {    return match listing.subtitle {        some(subtitle) -> subtitle        none -> listing.name    }}

A field that may be none must still be given: a Listing must say subtitle: none explicitly. Being allowed to leave a field out is a separate idea, and only an Input has it:

type ListProducts = Input {    name: ProductName {        optional: true    }    page_size: PageSize {        default: 25    }}type RenameProduct = Input {    name: ProductName    subtitle: ProductName? {        default: none    }}

• optional: true lets the request leave the field out. Reading it gives T?, and a field left out reads as none. Only an update's patch: tells "left out" apart from "sent as none" (Reads and writes).
• default: 25 fills in a value when the request leaves the field out, checked against the field's rules while compiling.
• default: none fills in none for a field that may be none.

A value that may be none widens as the value itself does: a Customer.email? can be used where an Email? is expected.