Add instances for GHC.Tuple.Solo

The `Applicative` instance is the most important one (for
array/vector/sequence indexing purposes), but it deserves
all the usual ones.

T12545 does silly 1% wibbles both ways, it seems, maybe depending
on architecture.

Metric Increase:
    T12545

Metric Decrease:
    T12545

2 files changed, 12 insertions, 0 deletions

diff --git a/libraries/base/Control/Monad/Fix.hs b/libraries/base/Control/Monad/Fix.hs
index 96133777be..b54ee66531 100644
--- a/libraries/base/Control/Monad/Fix.hs
+++ b/libraries/base/Control/Monad/Fix.hs
@@ -33,6 +33,7 @@ import Data.Ord ( Down(..) )
 import GHC.Base ( Monad, NonEmpty(..), errorWithoutStackTrace, (.) )
 import GHC.Generics
 import GHC.List ( head, tail )
+import GHC.Tuple (Solo (..))
 import Control.Monad.ST.Imp
 import System.IO
 
@@ -63,6 +64,11 @@ class (Monad m) => MonadFix m where
 
 -- Instances of MonadFix for Prelude monads
 
+-- | @since 4.15
+instance MonadFix Solo where
+    mfix f = let a = f (unSolo a) in a
+             where unSolo (Solo x) = x
+
 -- | @since 2.01
 instance MonadFix Maybe where
     mfix f = let a = f (unJust a) in a
diff --git a/libraries/base/Control/Monad/Zip.hs b/libraries/base/Control/Monad/Zip.hs
index dc192bfc81..36ebeb9985 100644
--- a/libraries/base/Control/Monad/Zip.hs
+++ b/libraries/base/Control/Monad/Zip.hs
@@ -25,6 +25,7 @@ import Data.Ord ( Down(..) )
 import Data.Proxy
 import qualified Data.List.NonEmpty as NE
 import GHC.Generics
+import GHC.Tuple (Solo (..))
 
 -- | Instances should satisfy the laws:
 --
@@ -71,6 +72,11 @@ instance MonadZip Identity where
     mzipWith                 = liftM2
     munzip (Identity (a, b)) = (Identity a, Identity b)
 
+-- | @since 4.15.0.0
+instance MonadZip Solo where
+    mzipWith = liftM2
+    munzip (Solo (a, b)) = (Solo a, Solo b)
+
 -- | @since 4.8.0.0
 instance MonadZip Dual where
     -- Cannot use coerce, it's unsafe