Carp/core/Opaque.carp

(doc Opaque "The opaque type is an uninhabited type with no constructors.
Opaque can be used to force some abstract type to range over a type constructor
without concerning oneself with the inhabitant of the constructor. It may be
used to enable a type to work for all inhabitants and can facillitate generic
programming.

Consider an example:

```
;; The type of indicies over containers of a single type argument
(deftype (Index (f Opaque) b) [at b])

(definterface tabulate (Fn [(Ref (Fn [(Index (f Opaque) b)] c))] (f c)))
(definterface positions (f (Index (f Opaque) b)))

(implements tabulate tabulate)
(defn tabulate [h]
  (fmap h @&positions))

(deftype (Tuple a) [x a y a])

(defmodule Tuple
  (sig positions (Tuple (Index (Tuple Opaque) Bool)))
  (def positions (Tuple.init (Index.init true) (Index.init false)))
)
```

In the above example, the Opaque type allows us to define tabulate generically
over Tuples without having to ensure their inhabitants match. The result of
tabulate is fully determined by the Index passed to it, which determines the
constructor that will be used, and the return type of its function argument.
Without Opaque, Index would contain a generic type which would be unreseolved
upon the call to `tabulate` and Carp would have no means of determining which
implementation of `positions` to use (unless we used a `the` declaration or a
similar mechanism). Opaque allows us to ensure the `positions` we call are the
positions of the correct constructor type `f` wihtout having to restrict
ourselves to only calling `Indexes` over an `f` with a  specific inhabitant
type (e.g. `(f a)`)--in other words, it allows us to constrain functions by
constructor types only.

Thanks to Opaque, tabulate can generate an `(Array Int)`, `(Array Bool)`,
`(Array String)`, `(Maybe Bool)`, etc. all solely dependent on the return type
of `h` and the `f` in the given `Index`.
")
(deftype Opaque)