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.

3 files changed, 6 insertions, 6 deletions

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