Documentation

A Uniq is a value that can only be constructed under controlled conditions (in IO or ST, basically), and once constructed can only be compared to Uniq values created under the same conditions (in the same monad). Upon comparison, a Uniq is ONLY ever equal to itself. Beyond that, no promises regarding ordering are made except that once constructed the order is deterministic and a proper ordering relation (eg, > is transitive and irreflexive, etc.)

Construct a new Uniq that is equal to itself, unequal to every other Uniq constructed in the same monad, and incomparable to every Uniq constructed in any other monad.

RealWorld is deeply magical. It is primitive, but it is not unlifted (hence ptrArg). We never manipulate values of type RealWorld; it's only used in the type system, to parameterise State#.