define return in terms of pure, >> as *>, mappend as <>

In preparation for deprecation, see https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/semigroup-monoid and https://gitlab.haskell.org/ghc/ghc/-/wikis/proposal/monad-of-no-return

src/compiler/GF/Data/BacktrackM.hs

@@ -64,11 +64,11 @@ finalStates :: BacktrackM s () -> s -> [s]
 finalStates bm = map fst . runBM bm
 
 instance Applicative (BacktrackM s) where
-    pure = return
+    pure a = BM (\c s b -> c a s b)
     (<*>) = ap
 
 instance Monad (BacktrackM s) where
-    return a   = BM (\c s b -> c a s b)
+    return     = pure
     BM m >>= k = BM (\c s b -> m (\a s b -> unBM (k a) c s b) s b)
         where unBM (BM m) = m
 

src/compiler/GF/Data/ErrM.hs

@@ -34,7 +34,7 @@ fromErr :: a -> Err a -> a
 fromErr a = err (const a) id
 
 instance Monad Err where
-  return      = Ok
+  return      = pure
   Ok a  >>= f = f a
   Bad s >>= f = Bad s
 
@@ -54,7 +54,7 @@ instance Functor Err where
   fmap f (Bad s) = Bad s
 
 instance Applicative Err where
-  pure = return
+  pure = Ok
   (<*>) = ap
 
 -- | added by KJ