{-# LANGUAGE DeriveFunctor, ExistentialQuantification, RankNTypes, StandaloneDeriving #-} {- | An effect allowing writes to an accumulated quantity alongside a computed value. A 'Writer' @w@ effect keeps track of a monoidal datum of type @w@ and strictly appends to that monoidal value with the 'tell' effect. Writes to that value can be detected and intercepted with the 'listen' and 'censor' effects. Predefined carriers: * "Control.Carrier.Writer.Strict". (A lazy carrier is not provided due to the inherent space leaks associated with lazy writer monads.) * "Control.Monad.Trans.RWS.Lazy" * "Control.Monad.Trans.RWS.Strict" * "Control.Monad.Trans.Writer.Lazy" * "Control.Monad.Trans.Writer.Strict" * If 'Writer' @w@ is the last effect in a stack, it can be interpreted to a tuple @(w, a)@ given some result type @a@ and the presence of a 'Monoid' instance for @w@. @since 0.1.0.0 -} module Control.Effect.Writer ( -- * Writer effect Writer(..) , tell , listen , listens , censor -- * Re-exports , Algebra , Has , run ) where import Control.Algebra import Control.Effect.Writer.Internal (Writer(..)) -- | Write a value to the log. -- -- @ -- runWriter ('tell' w '>>' m) = 'Data.Bifunctor.first' ('mappend' w) '<$>' runWriter m -- @ -- -- @since 0.1.0.0 tell :: Has (Writer w) sig m => w -> m () tell w = send (Tell w (pure ())) {-# INLINE tell #-} -- | Run a computation, returning the pair of its output and its result. -- -- @ -- runWriter ('listen' m) = 'fmap' ('fst' 'Control.Arrow.&&&' 'id') (runWriter m) -- @ -- -- @since 0.2.0.0 listen :: Has (Writer w) sig m => m a -> m (w, a) listen m = send (Listen m (curry pure)) {-# INLINE listen #-} -- | Run a computation, applying a function to its output and returning the pair of the modified output and its result. -- -- @ -- 'listens' f m = 'fmap' ('Data.Bifunctor.first' f) ('listen' m) -- @ -- -- @since 0.2.0.0 listens :: Has (Writer w) sig m => (w -> b) -> m a -> m (b, a) listens f m = send (Listen m (curry pure . f)) {-# INLINE listens #-} -- | Run a computation, modifying its output with the passed function. -- -- @ -- runWriter ('censor' f m) = 'fmap' ('Data.Bifunctor.first' f) (runWriter m) -- @ -- -- @since 0.2.0.0 censor :: Has (Writer w) sig m => (w -> w) -> m a -> m a censor f m = send (Censor f m pure) {-# INLINE censor #-}