Missing Proxy instances, make U1 instance more Proxy-like

This accomplishes three things:

* Adds missing `Alternative`, `MonadPlus`, and `MonadZip` instances for
  `Proxy`
* Adds a missing `MonadPlus` instance for `U1`
* Changes several existing `U1` instances to use lazy pattern-matching,
  exactly how `Proxy` does it (in case we ever replace `U1` with
  `Proxy`). This is technically a breaking change (albeit an extremely
  minor one).

Test Plan: ./validate

Reviewers: austin, ekmett, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1962

GHC Trac Issues: #11650

1 files changed, 8 insertions, 0 deletions

diff --git a/libraries/base/Data/Proxy.hs b/libraries/base/Data/Proxy.hs
index 9f602ea0c8..f0760e855d 100644
--- a/libraries/base/Data/Proxy.hs
+++ b/libraries/base/Data/Proxy.hs
@@ -89,10 +89,18 @@ instance Applicative Proxy where
     _ <*> _ = Proxy
     {-# INLINE (<*>) #-}
 
+instance Alternative Proxy where
+    empty = Proxy
+    {-# INLINE empty #-}
+    _ <|> _ = Proxy
+    {-# INLINE (<|>) #-}
+
 instance Monad Proxy where
     _ >>= _ = Proxy
     {-# INLINE (>>=) #-}
 
+instance MonadPlus Proxy
+
 -- | 'asProxyTypeOf' is a type-restricted version of 'const'.
 -- It is usually used as an infix operator, and its typing forces its first
 -- argument (which is usually overloaded) to have the same type as the tag