Export liftA2 from Prelude

Changes:
In order to be warning free and compatible, we hide Applicative(..)
from Prelude in a few places and instead import it directly from
Control.Applicative.
Please see the migration guide at
https://github.com/haskell/core-libraries-committee/blob/main/guides/export-lifta2-prelude.md
for more details.

This means that Applicative is now exported in its entirety from
Prelude.

Motivation:

This change is motivated by a few things:
* liftA2 is an often used function, even more so than (<*>) for some
  people.
* When implementing Applicative, the compiler will prompt you for either
  an implementation of (<*>) or of liftA2, but trying to use the latter
  ends with an error, without further imports. This could be confusing
  for newbies.
* For teaching, it is often times easier to introduce liftA2 first,
  as it is a natural generalisation of fmap.
* This change seems to have been unanimously and enthusiastically
  accepted by the CLC members, possibly indicating a lot of love for it.
* This change causes very limited breakage, see the linked issue below
  for an investigation on this.

See https://github.com/haskell/core-libraries-committee/issues/50
for the surrounding discussion and more details.

7 files changed, 10 insertions, 9 deletions

diff --git a/libraries/base/Data/Complex.hs b/libraries/base/Data/Complex.hs
index c9b8040f92..734b2fabf4 100644
--- a/libraries/base/Data/Complex.hs
+++ b/libraries/base/Data/Complex.hs
@@ -35,6 +35,7 @@ module Data.Complex
 
         )  where
 
+import Prelude hiding (Applicative(..))
 import GHC.Base (Applicative (..))
 import GHC.Generics (Generic, Generic1)
 import GHC.Float (Floating(..))
diff --git a/libraries/base/Data/List/NonEmpty.hs b/libraries/base/Data/List/NonEmpty.hs
index 03f631a2a7..6a31569bbb 100644
--- a/libraries/base/Data/List/NonEmpty.hs
+++ b/libraries/base/Data/List/NonEmpty.hs
@@ -102,7 +102,7 @@ import           Prelude             hiding (break, cycle, drop, dropWhile,
                                       last, length, map, repeat, reverse,
                                       scanl, scanl1, scanr, scanr1, span,
                                       splitAt, tail, take, takeWhile,
-                                      unzip, zip, zipWith, (!!))
+                                      unzip, zip, zipWith, (!!), Applicative(..))
 import qualified Prelude
 
 import           Control.Applicative (Applicative (..), Alternative (many))
diff --git a/libraries/base/Data/Semigroup.hs b/libraries/base/Data/Semigroup.hs
index 08040dfea8..256f2761fd 100644
--- a/libraries/base/Data/Semigroup.hs
+++ b/libraries/base/Data/Semigroup.hs
@@ -98,7 +98,7 @@ module Data.Semigroup (
   , ArgMax
   ) where
 
-import           Prelude             hiding (foldr1)
+import           Prelude             hiding (foldr1, Applicative(..))
 
 import GHC.Base (Semigroup(..))
 
diff --git a/libraries/base/Prelude.hs b/libraries/base/Prelude.hs
index 2e21888abc..1309c0969d 100644
--- a/libraries/base/Prelude.hs
+++ b/libraries/base/Prelude.hs
@@ -73,7 +73,7 @@ module Prelude (
 
     -- ** Monads and functors
     Functor(fmap, (<$)), (<$>),
-    Applicative(pure, (<*>), (*>), (<*)),
+    Applicative(pure, (<*>), (*>), (<*), liftA2),
     Monad((>>=), (>>), return),
     MonadFail(fail),
     mapM_, sequence_, (=<<),
diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib.hs b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
index 8e8e41df2f..e052818c0b 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib.hs
@@ -178,10 +178,10 @@ import Language.Haskell.TH.Lib.Internal hiding
 import qualified Language.Haskell.TH.Lib.Internal as Internal
 import Language.Haskell.TH.Syntax
 
-import Control.Applicative ( liftA2 )
+import Control.Applicative (Applicative(..))
 import Foreign.ForeignPtr
 import Data.Word
-import Prelude
+import Prelude hiding (Applicative(..))
 
 -- All definitions below represent the "old" API, since their definitions are
 -- different in Language.Haskell.TH.Lib.Internal. Please think carefully before
diff --git a/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs b/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
index 5d2a32d761..e95a449e1d 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Lib/Internal.hs
@@ -20,12 +20,12 @@ module Language.Haskell.TH.Lib.Internal where
 
 import Language.Haskell.TH.Syntax hiding (Role, InjectivityAnn)
 import qualified Language.Haskell.TH.Syntax as TH
-import Control.Applicative(liftA, liftA2)
+import Control.Applicative(liftA, Applicative(..))
 import qualified Data.Kind as Kind (Type)
 import Data.Word( Word8 )
 import Data.List.NonEmpty ( NonEmpty(..) )
 import GHC.Exts (TYPE)
-import Prelude
+import Prelude hiding (Applicative(..))
 
 ----------------------------------------------------------
 -- * Type synonyms
diff --git a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
index 48db744f72..c719541074 100644
--- a/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
+++ b/libraries/template-haskell/Language/Haskell/TH/Syntax.hs
@@ -39,7 +39,7 @@ import GHC.IO.Unsafe    ( unsafeDupableInterleaveIO )
 import Control.Monad (liftM)
 import Control.Monad.IO.Class (MonadIO (..))
 import Control.Monad.Fix (MonadFix (..))
-import Control.Applicative (liftA2)
+import Control.Applicative (Applicative(..))
 import Control.Exception (BlockedIndefinitelyOnMVar (..), catch, throwIO)
 import Control.Exception.Base (FixIOException (..))
 import Control.Concurrent.MVar (newEmptyMVar, readMVar, putMVar)
@@ -60,7 +60,7 @@ import GHC.Lexeme       ( startsVarSym, startsVarId )
 import GHC.ForeignSrcLang.Type
 import Language.Haskell.TH.LanguageExtensions
 import Numeric.Natural
-import Prelude
+import Prelude hiding (Applicative(..))
 import Foreign.ForeignPtr
 import Foreign.C.String
 import Foreign.C.Types