Signatures for (non-monadic) mapping.
The main signatures are S0 and S1. We also define container forms of the above, which include the Core container interfaces, and Extensions1, a signature of extensions to arity-1 mappable containers.
As with Traversable, we define the signature of mappable structures in an arity-generic way, then specialise it for arity-0 and arity-1 types.
module type Generic = sig ... endGeneric describes mapping on either an arity-0 or arity-1 type.
The basic signatures are S0, which defines mapping across an arity-0 type t (with a fixed, associated element type elt), and S1, which defines mapping across an arity-1 type 'a t (with element type 'a).
module type S0 = sig ... endS0 is the signature of an arity-0 mappable type.
module type S1 = sig ... endS1 is the signature of an arity-1 mappable type.
Unlike with Traversable's map_m, we can't actually implement the Core container signatures over map alone. We still define versions of the Mappable interfaces that include their respective container signatures, both for symmetry and to allow for extensions.
module type S0_container = sig ... endS0_container is the signature of an arity-0 mappable container.
module type S1_container = sig ... endS1_container is the signature of an arity-1 mappable container.
The signatures below describe various functions we can derive from mappable types and mappable containers. To apply them to existing types, use the functors in Mappable.
module type Extensions1 = sig ... endExtensions1 describes various extensions of arity-1 mappable containers.