Improve docs of mapKeys.

The problem found out when improving tests.
roconnor · Nov 23, 2011 · 820efd4 · 820efd4
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diff --git a/Data/Map/Base.hs b/Data/Map/Base.hs
@@ -1598,8 +1598,8 @@ mapAccumRWithKey f a (Bin sx kx x l r) =
 -- @'mapKeys' f s@ is the map obtained by applying @f@ to each key of @s@.
 -- 
 -- The size of the result may be smaller if @f@ maps two or more distinct
--- keys to the same new key.  In this case the value at the smallest of
--- these keys is retained.
+-- keys to the same new key.  In this case the value at the greatest of the
+-- original keys is retained.
 --
 -- > mapKeys (+ 1) (fromList [(5,"a"), (3,"b")])                        == fromList [(4, "b"), (6, "a")]
 -- > mapKeys (\ _ -> 1) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")]) == singleton 1 "c"

diff --git a/Data/Map/Strict.hs b/Data/Map/Strict.hs
@@ -978,8 +978,8 @@ mapAccumRWithKey f a (Bin sx kx x l r) =
 -- @'mapKeys' f s@ is the map obtained by applying @f@ to each key of @s@.
 --
 -- The size of the result may be smaller if @f@ maps two or more distinct
--- keys to the same new key.  In this case the value at the smallest of
--- these keys is retained.
+-- keys to the same new key.  In this case the value at the greatest of
+-- the original keys is retained.
 --
 -- > mapKeys (+ 1) (fromList [(5,"a"), (3,"b")])                        == fromList [(4, "b"), (6, "a")]
 -- > mapKeys (\ _ -> 1) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")]) == singleton 1 "c"