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.

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/GHC/Utils/Monad.hs b/compiler/GHC/Utils/Monad.hs
index 3bf8737990..f9fc02b338 100644
--- a/compiler/GHC/Utils/Monad.hs
+++ b/compiler/GHC/Utils/Monad.hs
@@ -27,7 +27,7 @@ module GHC.Utils.Monad
 -- Imports
 -------------------------------------------------------------------------------
 
-import GHC.Prelude
+import GHC.Prelude hiding (Applicative(..))
 
 import Control.Applicative
 import Control.Monad