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Towards Qualified Do #30

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2048337
Try to drop redundant import
snowleopard Aug 1, 2020
29eca7a
Make branch one of the methods
snowleopard Aug 1, 2020
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34 changes: 16 additions & 18 deletions src/Control/Selective.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,7 @@
-----------------------------------------------------------------------------
module Control.Selective (
-- * Type class
Selective (..), (<*?), branch, selectA, apS, selectM,
Selective (..), (<*?), selectA, apS, selectM,

-- * Conditional combinators
ifS, whenS, fromMaybeS, orElse, andAlso, untilRight, whileS, (<||>), (<&&>),
Expand Down Expand Up @@ -47,9 +47,12 @@ import Data.Functor.Identity
import Data.Functor.Product
import Data.List.NonEmpty
import Data.Proxy
import Data.Semigroup (Semigroup (..))
import GHC.Conc (STM)

#if !MIN_VERSION_base(4,11,0)
import Data.Semigroup (Semigroup (..))
#endif

import qualified Control.Monad.Trans.RWS.Strict as S
import qualified Control.Monad.Trans.State.Strict as S
import qualified Control.Monad.Trans.Writer.Strict as S
Expand Down Expand Up @@ -136,8 +139,19 @@ import qualified Control.Monad.Trans.Writer.Strict as S
--
-- If f is also a 'Monad', we require that 'select' = 'selectM', from which one
-- can prove '<*>' @=@ 'apS'.
--
-- The 'branch' method is a natural generalisation of 'select': instead of
-- skipping an unnecessary effect, it chooses which of the two given effectful
-- functions to apply to a given argument; the other effect is unnecessary. It
-- is possible to implement 'branch' in terms of 'select', which is a good
-- puzzle (give it a try!), and 'select' via 'branch' (which is much easier).
class Applicative f => Selective f where
{-# MINIMAL select | branch #-}
select :: f (Either a b) -> f (a -> b) -> f b
select x y = branch x y (pure id)

branch :: f (Either a b) -> f (a -> c) -> f (b -> c) -> f c
branch x l r = fmap (fmap Left) x <*? fmap (fmap Right) l <*? r

-- | An operator alias for 'select', which is sometimes convenient. It tries to
-- follow the notational convention for 'Applicative' operators. The angle
Expand All @@ -148,22 +162,6 @@ class Applicative f => Selective f where

infixl 4 <*?

-- | The 'branch' function is a natural generalisation of 'select': instead of
-- skipping an unnecessary effect, it chooses which of the two given effectful
-- functions to apply to a given argument; the other effect is unnecessary. It
-- is possible to implement 'branch' in terms of 'select', which is a good
-- puzzle (give it a try!).
--
-- We can also implement 'select' via 'branch':
--
-- @
-- selectB :: Selective f => f (Either a b) -> f (a -> b) -> f b
-- selectB x y = branch x y (pure id)
-- @
--
branch :: Selective f => f (Either a b) -> f (a -> c) -> f (b -> c) -> f c
branch x l r = fmap (fmap Left) x <*? fmap (fmap Right) l <*? r

-- | We can write a function with the type signature of 'select' using the
-- 'Applicative' type class, but it will always execute the effects associated
-- with the second argument, hence being potentially less efficient.
Expand Down