Re-export `<$>` from Prelude (#10113)

Whether to re-export the `<$>` non-method operator from `Prelude` wasn't
explicitly covered in the original AMP proposal[1], but it turns out that
not doing so forces most code that makes use of applicatives to import
`Data.Functor` or `Control.Applicative` just to get that operator into
scope.  To this end, it was proposed to add `<$>` to Prelude as well[2].

The down-side is that this increases the amount of redundant-import
warnings triggered, as well as the relatively minor issue of stealing
the `<$>` operator from the default namespace for good (although at this
point `<$>` is supposed to be ubiquitous anyway due to `Applicative`
being implicitly required into the next Haskell Report)

 [1]: https://wiki.haskell.org/Functor-Applicative-Monad_Proposal
 [2]: http://thread.gmane.org/gmane.comp.lang.haskell.libraries/24161

Reviewed By: austin, ekmett

Differential Revision: https://phabricator.haskell.org/D680

1 files changed, 2 insertions, 1 deletions

diff --git a/libraries/base/Prelude.hs b/libraries/base/Prelude.hs
index d1dc1a442a..8e275aa77f 100644
--- a/libraries/base/Prelude.hs
+++ b/libraries/base/Prelude.hs
@@ -70,7 +70,7 @@ module Prelude (
     Monoid(mempty, mappend, mconcat),
 
     -- ** Monads and functors
-    Functor(fmap),
+    Functor(fmap), (<$>),
     Applicative(pure, (<*>), (*>), (<*)),
     Monad((>>=), (>>), return, fail),
     mapM_, sequence_, (=<<),
@@ -158,6 +158,7 @@ import System.IO.Error
 import Data.List
 import Data.Either
 import Data.Foldable    ( Foldable(..) )
+import Data.Functor     ( (<$>) )
 import Data.Maybe
 import Data.Traversable ( Traversable(..) )
 import Data.Tuple