class for types that can be converted to a hash value

 It defines a class, Hashable, for types that can be converted to a
 hash value. This class exists for the benefit of hashing-based data
 structures. The package provides instances for basic types and a way
 to combine hash values.
 .
 This package provides a library for the Haskell programming language.
 See http://www.haskell.org/ for more information on Haskell.