Portability  portable 

Stability  experimental 
Maintainer  libraries@haskell.org 
 Computation type:
 Simple function application.
 Binding strategy:
 The bound function is applied to the input value.
Identity
x >>= f ==Identity
(f x)  Useful for:
 Monads can be derived from monad transformers applied to the
Identity
monad.  Zero and plus:
 None.
 Example type:

Identity
a
The Identity
monad is a monad that does not embody any computational strategy.
It simply applies the bound function to its input without any modification.
Computationally, there is no reason to use the Identity
monad
instead of the much simpler act of simply applying functions to their arguments.
The purpose of the Identity
monad is its fundamental role in the theory
of monad transformers.
Any monad transformer applied to the Identity
monad yields a nontransformer
version of that monad.
Inspired by the paper /Functional Programming with Overloading and HigherOrder Polymorphism/, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.
 newtype Identity a = Identity {
 runIdentity :: a
Documentation
Identity wrapper. Abstraction for wrapping up a object. If you have an monadic function, say:
example :: Int > Identity Int example x = return (x*x)
you can "run" it, using
Main> runIdentity (example 42) 1764 :: Int
A typical use of the Identity monad is to derive a monad from a monad transformer.
 derive theControl.Monad.State.State
monad using theControl.Monad.State.StateT
monad transformer typeControl.Monad.State.State
s a =Control.Monad.State.StateT
sIdentity
a
The
label is used in the type definition because it follows
a style of monad definition that explicitly represents monad values as
computations. In this style, a monadic computation is built up using the monadic
operators and then the value of the computation is extracted
using the runIdentity
run******
function.
Because the Identity
monad does not do any computation, its definition
is trivial.
For a better example of this style of monad,
see the
monad.
Control.Monad.State.State
Identity  
