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/Compile/GeneratePMCFG.hs

@@ -201,11 +201,11 @@ instance Fail.MonadFail CnvMonad where
   fail = bug
 
 instance Applicative CnvMonad where
-  pure = return
+  pure a = CM (\gr c s -> c a s)
   (<*>) = ap
 
 instance Monad CnvMonad where
-    return a   = CM (\gr c s -> c a s)
+    return     = pure
     CM m >>= k = CM (\gr c s -> m gr (\a s -> unCM (k a) gr c s) s)
 
 instance MonadState ([ProtoFCat],[Symbol]) CnvMonad where

src/compiler/GF/Compile/TypeCheck/ConcreteNew.hs

@@ -644,7 +644,7 @@ data TcResult a
 newtype TcM a = TcM {unTcM :: MetaStore -> [Message] -> TcResult a}
 
 instance Monad TcM where
-  return x = TcM (\ms msgs -> TcOk x ms msgs)
+  return = pure
   f >>= g  = TcM (\ms msgs -> case unTcM f ms msgs of
                                 TcOk x ms msgs -> unTcM (g x) ms msgs
                                 TcFail    msgs -> TcFail msgs)
@@ -659,7 +659,7 @@ instance Fail.MonadFail TcM where
 
 
 instance Applicative TcM where
-  pure = return
+  pure x = TcM (\ms msgs -> TcOk x ms msgs)
   (<*>) = ap
 
 instance Functor TcM where