A form over a variant Entity

An Entity declared with Entity.defineVariants has a row that is one of its cases. Its input names _tag, which is the case picker, and declares each field only some cases have with Schema.optionalKey, so a row of another case leaves it out:

const Domain = Entity.defineVariants('Domain', {
  Available: { id: Schema.String, name: Schema.String, price: Schema.Finite },
  Unconfirmed: { id: Schema.String, name: Schema.String },
})
const Listing = Form.make(
  'Listing',
  Entity.input(
    Domain,
    Schema.Struct({
      _tag: Domain.fields._tag.schema,
      name: Schema.String,
      price: Schema.optionalKey(Schema.Finite),
    }),
  ),
)

Listing.chosen(Listing.initial) // Option.none(): no case chosen yet
Listing.shows(Listing.initial, 'price') // false until Available is chosen

• _tag's control is a choice of the case names, like any key of literals.
• A key of some cases is asked only once one of them is chosen: a view draws the keys shows says, and a submit validates and sends those alone. In its case it is required as the case holds it, though the input declares it optional. What was typed in it under another case stays in its draft and is not sent.
• Such a key that the input does not declare optional is refused when the form is made, since a row of another case could never leave it out.
• The value is the input's Struct, holding the chosen case's keys: an operation taking that input takes it as it is.