 Computation type:
 Computations which read values from a shared environment.
 Binding strategy:
 Monad values are functions from the environment to a value.
The bound function is applied to the bound value, and both have access
to the shared environment.
 Useful for:
 Maintaining variable bindings, or other shared environment.
 Zero and plus:
 None.
 Example type:
 Reader [(String,Value)] a
The Reader monad (also called the Environment monad).
Represents a computation, which can read values from
a shared environment, pass values from function to function,
and execute subcomputations in a modified environment.
Using Reader monad for such computations is often clearer and easier
than using the State monad.
Inspired by the paper
Functional Programming with Overloading and
HigherOrder Polymorphism,
Mark P Jones (http://www.cse.ogi.edu/~mpj/)
Advanced School of Functional Programming, 1995.
