followup for #6530

lib/phoenix/controller.ex

@@ -1891,16 +1891,27 @@ defmodule Phoenix.Controller do
 
   @doc """
   Assigns multiple key-value pairs to the connection.
+  Accepts a keyword list, a map, or a single-argument function.
 
   This function accepts a map or keyword list of assigns and merges them into
   the connection's assigns. It is equivalent to calling `Plug.Conn.assign/3`
   multiple times.
 
+  If a function is given, it takes the current assigns as an argument and its return
+  value will be merged into the current assigns.
+
   ## Examples
 
       assign(conn, name: "Alice", role: :admin)
       assign(conn, %{name: "Alice", role: :admin})
+      assign(conn, fn %{name: name, logo: logo} -> %{title: Enum.join([name, logo], " | ")} end)
   """
+  def assign(conn, keyword_or_map_or_fun)
+
+  def assign(conn, fun) when is_function(fun, 1) do
+    assign(conn, fun.(conn.assigns))
+  end
+
   defdelegate assign(conn, assigns), to: Plug.Conn, as: :merge_assigns
 
   @doc false

test/phoenix/controller/controller_test.exs

@@ -994,4 +994,24 @@ defmodule Phoenix.Controller.ControllerTest do
       assert current_url(conn, %{three: 3}) == "https://www.example.com/foo?three=3"
     end
   end
+
+  describe "assign/2" do
+    test "merges assigns" do
+      conn = conn(:get, "/")
+
+      refute conn.assigns[:foo]
+
+      conn = assign(conn, %{foo: :bar})
+      assert conn.assigns.foo == :bar
+
+      conn = assign(conn, bar: :baz)
+      assert conn.assigns.foo == :bar
+      assert conn.assigns.bar == :baz
+
+      conn = assign(conn, fn %{foo: :bar} -> [baz: :quux] end)
+      assert conn.assigns.foo == :bar
+      assert conn.assigns.bar == :baz
+      assert conn.assigns.baz == :quux
+    end
+  end
 end