Add operator HashMap.!? (#175)

Add operator HashMap.!?.

This is equivalent to `lookup` with the arguments flipped. This operator was
added to containers-0.5.9.1, this makes the HashMap interface consistent.

This partially addresses
#172.

Author: m-renaud

CHANGES.md

@@ -2,6 +2,8 @@
 
  * Add `HashMap.findWithDefault` (deprecates `HashMap.lookupDefault`)
 
+ * Add `HashMap.!?`, a flipped version of `lookup`.
+
 ## 0.2.10.0
 
  * Add `HashMap.alterF`.

Data/HashMap/Base.hs

@@ -25,6 +25,7 @@ module Data.HashMap.Base
     , size
     , member
     , lookup
+    , (!?)
     , findWithDefault
     , lookupDefault
     , (!)
@@ -624,6 +625,17 @@ lookupCont absent present !h0 !k0 !m0 = go h0 k0 0 m0
         | otherwise = absent (# #)
 {-# INLINE lookupCont #-}
 
+-- | /O(log n)/ Return the value to which the specified key is mapped,
+-- or 'Nothing' if this map contains no mapping for the key.
+--
+-- This is a flipped version of 'lookup'.
+--
+-- @since 0.2.11
+(!?) :: (Eq k, Hashable k) => HashMap k v -> k -> Maybe v
+(!?) m k = lookup k m
+{-# INLINE (!?) #-}
+
+
 -- | /O(log n)/ Return the value to which the specified key is mapped,
 -- or the default value if this map contains no mapping for the key.
 --

Data/HashMap/Lazy.hs

@@ -38,6 +38,7 @@ module Data.HashMap.Lazy
     , size
     , member
     , lookup
+    , (!?)
     , findWithDefault
     , lookupDefault
     , (!)

Data/HashMap/Strict.hs

@@ -37,6 +37,7 @@ module Data.HashMap.Strict
     , size
     , member
     , lookup
+    , (!?)
     , findWithDefault
     , lookupDefault
     , (!)

Data/HashMap/Strict/Base.hs

@@ -39,6 +39,7 @@ module Data.HashMap.Strict.Base
     , size
     , HM.member
     , HM.lookup
+    , (HM.!?)
     , HM.findWithDefault
     , lookupDefault
     , (!)

tests/HashMapProperties.hs

@@ -122,6 +122,9 @@ pMember k = M.member k `eq` HM.member k
 pLookup :: Key -> [(Key, Int)] -> Bool
 pLookup k = M.lookup k `eq` HM.lookup k
 
+pLookupOperator :: Key -> [(Key, Int)] -> Bool
+pLookupOperator k = M.lookup k `eq` (HM.!? k)
+
 pInsert :: Key -> Int -> [(Key, Int)] -> Bool
 pInsert k v = M.insert k v `eq_` HM.insert k v
 
@@ -363,6 +366,7 @@ tests =
       [ testProperty "size" pSize
       , testProperty "member" pMember
       , testProperty "lookup" pLookup
+      , testProperty "!?" pLookupOperator
       , testProperty "insert" pInsert
       , testProperty "delete" pDelete
       , testProperty "deleteCollision" pDeleteCollision