Pipe steps

Every option is also a pipe step that gives a new form, made from the same input with that option added to:

const AuthorForm = Form.make('Author', NewAuthor) // as a library might hand it over

const Finished = AuthorForm.pipe(
  Form.inputs({ bio: Input.multiline() }),
  Form.checks({ name: name => isNameTaken(name) }),
  Form.messages({ required: field => `${field.label} fehlt` }),
)

Form.inputs, Form.checks, Form.messages, Form.nested and Form.debounce. Each adds to what the form already has, so a form can be made in one place and finished in another. A step's keys are checked against the form it is piped into, and Form.checks adds what its checks need to what the form needs. The first form is left as it was.