{- | Module : Control.Monad.Identity Copyright : (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology 2001, (c) Jeff Newbern 2003-2006, (c) Andriy Palamarchuk 2006 License : BSD-style (see the file libraries/base/LICENSE) Maintainer : libraries@haskell.org Stability : experimental Portability : portable [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 non-transformer version of that monad. Inspired by the paper /Functional Programming with Overloading and Higher-Order Polymorphism/, Mark P Jones (<http://web.cecs.pdx.edu/~mpj/>) Advanced School of Functional Programming, 1995. -} module Control.Monad.Identity ( Identity(..), module Control.Monad, module Control.Monad.Fix, ) where import Control.Monad import Control.Monad.Fix {- | 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 the 'Control.Monad.State.State' monad using the 'Control.Monad.State.StateT' monad transformer type 'Control.Monad.State.State' s a = 'Control.Monad.State.StateT' s 'Identity' a @ The @'runIdentity'@ 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 @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 @'Control.Monad.State.State'@ monad. -} newtype Identity a = Identity { runIdentity :: a } -- --------------------------------------------------------------------------- -- Identity instances for Functor and Monad instance Functor Identity where fmap f m = Identity (f (runIdentity m)) instance Monad Identity where return a = Identity a m >>= k = k (runIdentity m) instance MonadFix Identity where mfix f = Identity (fix (runIdentity . f))