Add convenience functions (#332)

* Re-export (<&>) from Data.Functor

* Add `reduce` function

src/Turtle.hs

@@ -87,6 +87,7 @@ module Turtle (
     , ExitCode(..)
     , IsString(..)
     , (&)
+    , (<&>)
     ) where
 
 import Turtle.Format
@@ -163,3 +164,32 @@ infixl 1 &
 (&) :: a -> (a -> b) -> b
 x & f = f x
 #endif
+
+#if __GLASGOW_HASKELL__ >= 821
+import Data.Functor ((<&>))
+#else
+-- | Flipped version of '<$>'.
+--
+-- @
+-- ('<&>') = 'flip' 'fmap'
+-- @
+--
+-- @since 4.11.0.0
+--
+-- ==== __Examples__
+-- Apply @(+1)@ to a list, a 'Data.Maybe.Just' and a 'Data.Either.Right':
+--
+-- >>> Just 2 <&> (+1)
+-- Just 3
+--
+-- >>> [1,2,3] <&> (+1)
+-- [2,3,4]
+--
+-- >>> Right 3 <&> (+1)
+-- Right 4
+--
+(<&>) :: Functor f => f a -> (a -> b) -> f b
+as <&> f = f <$> as
+
+infixl 1 <&>
+#endif

src/Turtle/Shell.hs

@@ -66,6 +66,7 @@ module Turtle.Shell (
     , foldIO
     , foldShell
     , fold
+    , reduce
     , sh
     , view
 
@@ -143,6 +144,16 @@ foldShell s f = liftIO (_foldShell s f)
 fold :: MonadIO io => Shell a -> Fold a b -> io b
 fold s f = foldIO s (Foldl.generalize f)
 
+-- | Flipped version of 'fold'. Useful for reducing a stream of data
+--
+-- ==== __Example__
+-- Sum a `Shell` of numbers:
+--
+-- >>> select [1, 2, 3] & reduce Fold.sum
+-- 6
+reduce :: MonadIO io => Fold a b -> Shell a -> io b
+reduce = flip fold
+
 -- | Run a `Shell` to completion, discarding any unused values
 sh :: MonadIO io => Shell a -> io ()
 sh s = fold s (pure ())