Deprecate map after reduce

Closes #109.

lib/flow.ex

@@ -1171,9 +1171,9 @@ defmodule Flow do
 
   ## Examples
 
-      iex> flow = Flow.from_enumerable([foo: 1, foo: 2, bar: 3, foo: 4, bar: 5], stages: 1)
-      iex> flow |> Flow.group_by_key() |> Flow.map_values(&Enum.sort/1) |> Enum.sort()
-      [bar: [3, 5], foo: [1, 2, 4]]
+      iex> flow = Flow.from_enumerable([a: 1, b: 2, c: 3, d: 4, e: 5], stages: 1)
+      iex> flow |> Flow.map_values(& &1 * 2) |> Enum.sort()
+      [a: 2, b: 4, c: 6, d: 8, e: 10]
 
   """
   def map_values(flow, value_fun) when is_function(value_fun) do
@@ -1912,6 +1912,11 @@ defmodule Flow do
               "since events have already been emitted"
     end
 
+    if has_any_reduce?(flow) do
+      IO.warn("Using a mapper operation, such as map/filter/reject, after reduce/3 is deprecated. " <>
+              "Use Flow.on_trigger/2 instead")
+    end
+
     add_operation(flow, {:mapper, name, args})
   end
 

test/flow_test.exs

@@ -727,34 +727,6 @@ defmodule FlowTest do
       assert Enum.sort(flow) == [6, 10, 14, 18, 22]
     end
 
-    test "keeps ordering after reduce" do
-      flow =
-        @flow
-        |> Flow.reduce(fn -> [] end, &[&1 | &2])
-        |> Flow.filter(&(rem(&1, 2) == 0))
-        |> Flow.map(fn x -> x + 1 end)
-        |> Flow.map(fn x -> x * 2 end)
-
-      assert Enum.sort(flow) == [6, 10, 14, 18, 22]
-    end
-
-    test "keeps ordering after reduce + map_state" do
-      flow =
-        @flow
-        |> Flow.reduce(fn -> [] end, &[&1 | &2])
-        |> Flow.filter(&(rem(&1, 2) == 0))
-        |> Flow.map(fn x -> x + 1 end)
-        |> Flow.map(fn x -> x * 2 end)
-        |> Flow.on_trigger(&{[{&2, Enum.sort(&1)}], &1})
-
-      assert Enum.sort(flow) == [
-               {{0, 4}, [6, 14, 18]},
-               {{1, 4}, [22]},
-               {{2, 4}, []},
-               {{3, 4}, [10]}
-             ]
-    end
-
     test "start_link/2" do
       parent = self()