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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Moduleghc-internal-9.1003.0Haskell2010

GHC.Internal.Data.Functor.Identity

The identity functor and monad.

This trivial type constructor serves two purposes:

  • It can be used with functions parameterized by functor or monad classes.

  • It can be used as a base monad to which a series of monad transformers may be applied to construct a composite monad. Most monad transformer modules include the special case of applying the transformer to Identity. For example, State s is an abbreviation for StateT s Identity.

  • 1 type
newtypenewtype Identity a
#

Identity functor and monad. (a non-strict monad)

Examples
Example1 expression
fmap (+1) (Identity 0)Identity 1
Example1 expression
Identity [1, 2, 3] <> Identity [4, 5, 6]Identity [1,2,3,4,5,6]
>>> do
      x <- Identity 10
      y <- Identity (x + 5)
      pure (x + y)
Identity 25

Constructors

Instances31Monad, Functor, MonadFix, Applicative, Foldable, Traversable, …