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

Modulererebase-1.21.2Haskell2010

Control.Monad.Cont

  • 2 types
  • 1 class
  • 10 values
valuelabel :: MonadCont m => a -> m (a -> m b, a)
#

Introduces a recursive binding to the continuation. Due to the use of callCC, calling the continuation will interrupt execution of the current block creating an effect similar to goto/setjmp in C.

typetype Cont r = ContT r Identity
#

Continuation monad. Cont r a is a CPS ("continuation-passing style") computation that produces an intermediate result of type a within a CPS computation whose final result type is r.

The return function simply creates a continuation which passes the value on.

The >>= operator adds the bound function into the continuation chain.

valuecont :: ((a -> r) -> r) -> Cont r a
#

Construct a continuation-passing computation from a function. (The inverse of runCont)

valueevalCont :: Cont r r -> r
#

The result of running a CPS computation with the identity as the final continuation.

valuemapContT :: (m r -> m r) -> ContT r m a -> ContT r m a
#

Apply a function to transform the result of a continuation-passing computation. This has a more restricted type than the map operations for other monad transformers, because ContT does not define a functor in the category of monads.

valuerunCont
  1. :: Cont r a

    continuation computation (Cont).

  2. -> (a -> r)

    the final continuation, which produces the final result (often id).

  3. -> r
#

The result of running a CPS computation with a given final continuation. (The inverse of cont)

classclass Monad m => MonadCont (m :: Type -> Type) where
#

Methods

  • callCC :: ((a -> m b) -> m a) -> m a

    callCC (call-with-current-continuation) calls a function with the current continuation as its argument. Provides an escape continuation mechanism for use with Continuation monads. Escape continuations allow to abort the current computation and return a value immediately. They achieve a similar effect to throwError and catchError within an Except monad. Advantage of this function over calling return is that it makes the continuation explicit, allowing more flexibility and better control (see examples in Control.Monad.Cont).

    The standard idiom used with callCC is to provide a lambda-expression to name the continuation. Then calling the named continuation anywhere within its scope will escape from the computation, even if it is many layers deep within nested computations.

Instances22MonadCont, …
newtypenewtype ContT (r :: k) (m :: k -> Type) a
#

The continuation monad transformer. Can be used to add continuation handling to any type constructor: the Monad instance and most of the operations do not require m to be a monad.

ContT is not a functor on the category of monads, and many operations cannot be lifted through it.

Constructors

Instances24MonadAccum, MonadReader, MonadState, MonadSelect, MonadBase, MonadFree, …