Summary:
A comparer `('a, 'compare_a) t` for type `'a` is a "compare" function
of type `'a -> 'a -> int` tagged with a phantom type `'compare_a`
acting as a singleton type denoting an individual compare function.
The point of these is to enable writing type definitions of containers
that depend on a compare function prior to applying a functor. For
example, a type of sorted lists could be exposed as:
```
type elt
type (elt, 'compare_elt) t = private elt list
```
and the operations manipulating sorted lists would be defined by a
functor that accepts a `Comparer.S` and implements the operations
using
```
let compare = (comparer :> elt -> elt -> int)
```
Reviewed By: ngorogiannis
Differential Revision: D26250528
fbshipit-source-id: ea61844ec
Josh Berdine