🔥 Pure & PureC

ChangeLog.md

@@ -68,6 +68,10 @@
 
 - Removes `resetFresh`, as it was unsafe. Greater safety _and_ control over the generation of fresh values can be obtained by use of `runFresh`. ([#267](https://github.com/fused-effects/fused-effects/pull/267))
 
+- Removes `PureC`; `Data.Functor.Identity.Identity` should be used instead. Note that `run` is still provided as a convenient synonym for `runIdentity`. ([#307](https://github.com/fused-effects/fused-effects/pull/307))
+
+- Removes the `Pure` effect. It’s unlikely that this will require changes, as `Pure` had no operations, but `Lift Identity` should be used instead. ([#307](https://github.com/fused-effects/fused-effects/pull/307))
+
 # v0.5.0.1
 
 - Adds support for ghc 8.8.1.

README.md

@@ -137,7 +137,7 @@ example3 = run . runReader "hello" . runState 0 $ do
   put (length (list :: String))
 ```
 
-`run` is itself actually an effect handler for the `Pure` effect, which has no operations and thus can only represent a final result value.
+`run` is itself actually an effect handler for the `Lift Identity` effect, whose only operation is to lift a result value into a computation.
 
 Alternatively, arbitrary `Monad`s can be embedded into effectful computations using the `Lift` effect. In this case, the underlying `Monad`ic computation can be extracted using `runM`. Here, we use the `MonadIO` instance for the `LiftC` carrier to lift `putStrLn` into the middle of our computation:
 

benchmark/NonDet.hs

@@ -3,8 +3,8 @@ module NonDet
 ( benchmark
 ) where
 
+import Control.Algebra
 import qualified Control.Carrier.NonDet.Church as NonDet.Church
-import           Control.Carrier.Pure
 import           Gauge hiding (benchmark)
 import qualified NonDet.NQueens as NQueens
 

fused-effects.cabal

@@ -56,7 +56,6 @@ library
     Control.Carrier.Interpret
     Control.Carrier.Lift
     Control.Carrier.NonDet.Church
-    Control.Carrier.Pure
     Control.Carrier.Reader
     Control.Carrier.State.Lazy
     Control.Carrier.State.Strict
@@ -77,7 +76,6 @@ library
     Control.Effect.Fresh
     Control.Effect.Lift
     Control.Effect.NonDet
-    Control.Effect.Pure
     Control.Effect.Reader
     Control.Effect.State
     Control.Effect.Sum

src/Control/Algebra.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE ConstraintKinds, DeriveFunctor, EmptyCase, FlexibleInstances, FunctionalDependencies, RankNTypes, TypeOperators, UndecidableInstances #-}
+{-# LANGUAGE ConstraintKinds, DeriveFunctor, FlexibleInstances, FunctionalDependencies, RankNTypes, TypeOperators, UndecidableInstances #-}
 
 {- | The 'Algebra' class is the mechanism with which effects are interpreted.
 
@@ -8,25 +8,23 @@ An instance of the 'Algebra' class defines an interpretation of an effect signat
 -}
 module Control.Algebra
 ( Algebra(..)
+, run
 , Has
 , send
 , handleIdentity
 , handleCoercible
   -- * Re-exports
 , (:+:) (..)
-, run
 , module Control.Effect.Class
 ) where
 
-import Control.Carrier.Pure (PureC, run)
 import Control.Effect.Catch.Internal
 import Control.Effect.Choose.Internal
 import Control.Effect.Class
 import Control.Effect.Empty.Internal
 import Control.Effect.Error.Internal
 import Control.Effect.Lift.Internal
 import Control.Effect.NonDet.Internal
-import Control.Effect.Pure
 import Control.Effect.Reader.Internal
 import Control.Effect.State.Internal
 import Control.Effect.Sum ((:+:)(..), Member(..), Members)
@@ -55,9 +53,13 @@ class (Effect sig, Monad m) => Algebra sig m | m -> sig where
   -- | Construct a value in the carrier for an effect signature (typically a sum of a handled effect and any remaining effects).
   alg :: sig m a -> m a
 
-instance Algebra Pure PureC where
-  alg v = case v of {}
-  {-# INLINE alg #-}
+
+-- | Run an action exhausted of effects to produce its final result value.
+--
+-- @since 1.0.0.0
+run :: Identity a -> a
+run = runIdentity
+{-# INLINE run #-}
 
 
 -- | @m@ is a carrier for @sig@ containing @eff@.
@@ -103,8 +105,8 @@ handleCoercible = handleIdentity coerce
 instance Algebra (Lift IO) IO where
   alg = join . unLift
 
-instance Algebra Pure Identity where
-  alg v = case v of {}
+instance Algebra (Lift Identity) Identity where
+  alg = join . unLift
 
 instance Algebra Choose NonEmpty where
   alg (Choose m) = m True S.<> m False

src/Control/Effect/Lift.hs

@@ -1,11 +1,12 @@
 {- | Provides a mechanism to kick off the evaluation of an effect stack that takes place in a monadic context.
 
-'Lift' effects are always the last effect in a given effect stack. These stacks are invoked with 'Control.Effect.Lift.runM'. The 'Control.Effect.Pure.Pure' effect is equivalent to @Lift Identity@.
+'Lift' effects are always the last effect in a given effect stack. These stacks are invoked with 'Control.Carrier.Lift.runM' or 'Control.Algebra.run'.
 
 Predefined carriers:
 
 * "Control.Carrier.Lift"
 * 'IO'
+* 'Data.Functor.Identity.Identity'
 
 @since 0.1.0.0
 -}

test/Gen.hs

@@ -1,8 +1,7 @@
 {-# LANGUAGE DataKinds, DeriveFunctor, DeriveGeneric, FlexibleInstances, FunctionalDependencies, GADTs, GeneralizedNewtypeDeriving, KindSignatures, LambdaCase, PatternSynonyms, RankNTypes, ScopedTypeVariables, StandaloneDeriving, TypeApplications, TypeOperators, UndecidableInstances, ViewPatterns #-}
 {-# OPTIONS_GHC -Wno-identities #-}
 module Gen
-( module Control.Carrier.Pure
-, Identity(..)
+( module Data.Functor.Identity
   -- * Polymorphic generation & instantiation
 , m
 , GenM
@@ -59,7 +58,6 @@ module Gen
 ) where
 
 import Control.Applicative
-import Control.Carrier.Pure
 import Control.Monad.Trans.Class
 import Control.Monad.Trans.Writer
 import Data.Foldable (traverse_)
@@ -183,18 +181,18 @@ subtermM2 t1 t2 f = Comp1 (Hedgehog.subtermM2 (unComp1 t1) (unComp1 t2) (fmap un
 
 
 -- | This captures the shape of the handler function passed to the "Monad" & "MonadFix" tests.
-newtype Run f g m = Run (forall a . f (m a) -> PureC (g a))
+newtype Run f g m = Run (forall a . f (m a) -> Identity (g a))
 
 -- | Handlers with output state, but no input state (e.g. 'Control.Carrier.Error.Either.ErrorC').
-runL :: (forall a . m a -> PureC (f a)) -> Run Identity f m
+runL :: (forall a . m a -> Identity (f a)) -> Run Identity f m
 runL run = Run (run . runIdentity)
 
 -- | Handlers with input state, but no output state (e.g. 'Control.Carrier.Reader.ReaderC').
-runR :: (forall a . f (m a) -> PureC a) -> Run f Identity m
+runR :: (forall a . f (m a) -> Identity a) -> Run f Identity m
 runR run = Run (fmap Identity . run)
 
 -- | Handlers with curried input state (e.g. 'Control.Carrier.Reader.ReaderC', 'Control.Carrier.State.Strict.StateC').
-runC :: (forall a . s -> m a -> PureC (f a)) -> Run ((,) s) f m
+runC :: (forall a . s -> m a -> Identity (f a)) -> Run ((,) s) f m
 runC run = Run (uncurry run)