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, (<>))`.

1 files changed, 5 insertions, 9 deletions

diff --git a/compiler/utils/IOEnv.hs b/compiler/utils/IOEnv.hs
index 4470420a64..6c081ea3d0 100644
--- a/compiler/utils/IOEnv.hs
+++ b/compiler/utils/IOEnv.hs
@@ -1,4 +1,4 @@
-{-# LANGUAGE DeriveDataTypeable, UndecidableInstances #-}
+{-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE CPP #-}
 
 --
@@ -177,15 +177,11 @@ uninterruptibleMaskM_ (IOEnv m) = IOEnv (\ env -> uninterruptibleMask_ (m env))
 -- Alternative/MonadPlus
 ----------------------------------------------------------------------
 
-instance MonadPlus IO => Alternative (IOEnv env) where
-      empty = mzero
-      (<|>) = mplus
+instance Alternative (IOEnv env) where
+    empty   = IOEnv (const empty)
+    m <|> n = IOEnv (\env -> unIOEnv m env <|> unIOEnv n env)
 
--- For use if the user has imported Control.Monad.Error from MTL
--- Requires UndecidableInstances
-instance MonadPlus IO => MonadPlus (IOEnv env) where
-    mzero = IOEnv (const mzero)
-    m `mplus` n = IOEnv (\env -> unIOEnv m env `mplus` unIOEnv n env)
+instance MonadPlus (IOEnv env)
 
 ----------------------------------------------------------------------
 -- Accessing input/output