Returns the key of the tag attribute that distinguishes structures of this
Form; returns undefined if this Form has no distinguishing tag
attribute. Used to accelerate distrcrimination of polymorphic structural
types with nominal type hints.
Returns a version of this Form that requires a head [Attr] with the
given tag name.
Returns a default–possibly undefined–value of type T. Used as the
fallback return value when [Item.coerce coercing] an invalid structural
value.
Returns a version of this Form with the given unit value.
Transformation between a structurally typed [Item] and a nominally typed JavaScript object.