Canonicalise `MonadPlus` instances

This refactoring exploits the fact that since AMP, in most cases,
`instance MonadPlus` can be automatically derived from the respective
`Alternative` instance.  This is because `MonadPlus`'s default method
implementations are fully defined in terms of `Alternative(empty, (<>))`.

5 files changed, 10 insertions, 22 deletions

diff --git a/libraries/base/Control/Arrow.hs b/libraries/base/Control/Arrow.hs
index 3417f30cc1..9fc2ee5c90 100644
--- a/libraries/base/Control/Arrow.hs
+++ b/libraries/base/Control/Arrow.hs
@@ -321,9 +321,7 @@ instance ArrowPlus a => Alternative (ArrowMonad a) where
    empty = ArrowMonad zeroArrow
    ArrowMonad x <|> ArrowMonad y = ArrowMonad (x <+> y)
 
-instance (ArrowApply a, ArrowPlus a) => MonadPlus (ArrowMonad a) where
-   mzero = ArrowMonad zeroArrow
-   ArrowMonad x `mplus` ArrowMonad y = ArrowMonad (x <+> y)
+instance (ArrowApply a, ArrowPlus a) => MonadPlus (ArrowMonad a)
 
 -- | Any instance of 'ArrowApply' can be made into an instance of
 --   'ArrowChoice' by defining 'left' = 'leftApp'.
diff --git a/libraries/base/Data/Semigroup.hs b/libraries/base/Data/Semigroup.hs
index 6fa0cd85d6..518e215661 100644
--- a/libraries/base/Data/Semigroup.hs
+++ b/libraries/base/Data/Semigroup.hs
@@ -592,9 +592,7 @@ instance Alternative Option where
   Option Nothing <|> b = b
   a <|> _ = a
 
-instance MonadPlus Option where
-  mzero = Option Nothing
-  mplus = (<|>)
+instance MonadPlus Option
 
 instance MonadFix Option where
   mfix f = Option (mfix (getOption . f))
diff --git a/libraries/base/GHC/Conc/Sync.hs b/libraries/base/GHC/Conc/Sync.hs
index e1d894a8c1..2a5164b798 100644
--- a/libraries/base/GHC/Conc/Sync.hs
+++ b/libraries/base/GHC/Conc/Sync.hs
@@ -659,9 +659,7 @@ instance Alternative STM where
   empty = retry
   (<|>) = orElse
 
-instance MonadPlus STM where
-  mzero = empty
-  mplus = (<|>)
+instance MonadPlus STM
 
 -- | Unsafely performs IO in the STM monad.  Beware: this is a highly
 -- dangerous thing to do.
diff --git a/libraries/base/Text/ParserCombinators/ReadP.hs b/libraries/base/Text/ParserCombinators/ReadP.hs
index 6c340e4597..8b84acf24e 100644
--- a/libraries/base/Text/ParserCombinators/ReadP.hs
+++ b/libraries/base/Text/ParserCombinators/ReadP.hs
@@ -108,9 +108,7 @@ instance Applicative P where
   pure x = Result x Fail
   (<*>) = ap
 
-instance MonadPlus P where
-  mzero = empty
-  mplus = (<|>)
+instance MonadPlus P
 
 instance Monad P where
   (Get f)      >>= k = Get (\c -> f c >>= k)
@@ -175,12 +173,10 @@ instance MonadFail ReadP where
   fail _    = R (\_ -> Fail)
 
 instance Alternative ReadP where
-    empty = mzero
-    (<|>) = mplus
+  empty = pfail
+  (<|>) = (+++)
 
-instance MonadPlus ReadP where
-  mzero = pfail
-  mplus = (+++)
+instance MonadPlus ReadP
 
 -- ---------------------------------------------------------------------------
 -- Operations over P
diff --git a/libraries/base/Text/ParserCombinators/ReadPrec.hs b/libraries/base/Text/ParserCombinators/ReadPrec.hs
index 136b8aed00..2a9c1d0fe8 100644
--- a/libraries/base/Text/ParserCombinators/ReadPrec.hs
+++ b/libraries/base/Text/ParserCombinators/ReadPrec.hs
@@ -87,13 +87,11 @@ instance Monad ReadPrec where
 instance MonadFail.MonadFail ReadPrec where
   fail s    = P (\_ -> fail s)
 
-instance MonadPlus ReadPrec where
-  mzero = pfail
-  mplus = (+++)
+instance MonadPlus ReadPrec
 
 instance Alternative ReadPrec where
-    empty = mzero
-    (<|>) = mplus
+  empty = pfail
+  (<|>) = (+++)
 
 -- precedences
 type Prec = Int