abstraction for things that work like IORef; profiling libraries

 A collection of type-classes generalizing the read/write/modify
 operations for stateful variables provided by things like IORef,
 TVar, &c. Note that The interface has changed a bit from the 0.2.*
 version. "*Ref" functions are now called "*Reference" and new "*Ref"
 function exist with simpler signatures. The new Ref existential type
 provides a convenient monad-indexed reference type, and the HasRef
 class indicates monads for which there is a default reference type
 for every referent.
 .
 This package provides a library for the Haskell programming language, compiled
 for profiling. See http://www.haskell.org/ for more information on Haskell.