Accept function in Phoenix.Socket.assign/2

[timfjord]

This PR adds assign_lazy/3 to Phoenix.Socket (if you like the idea, I will add a similar change to the Phoenix.Component module in the phoenix_live_view).

It is quite often that you need to update the socket based on the value that it is already in assigns.
For more complicated things, this could be handled with private functions, but this assign_lazy/3 is useful when you need something very simple.

It is similar to the assign_new/3, but it assigns the value even if the given key exists.

socket
|> assign_strcut1()
|> assign_struct2()
|> assign_lazy(:title, %{struct1: struct1, struct2: struct2} -> "#{strcut1.name} #{strcut2.name}") end)

[josevalim]

Thanks for the PR! I am not sure if _lazy is the correct name, as the assignment always happens. _lazy in Elixir only happens if there is no entry.

I feel this is one of the cases where it is probably better to break the pipeline apart (or, as you said, use custom functions).

[timfjord]

Thanks for the feedback, @josevalim.
What if we handle this a bit differently?
We could add another clause for the assign/2 and accept a function as the second argument

def assign(%Socket{} = socket, fun) when is_function(fun, 1) do
  Map.update!(socket, :assigns, &Map.merge(&1, Map.new(fun.(&1))))
end

This way, the example from the description will look like

socket
|> assign_strcut1()
|> assign_struct2()
|> assign(%{struct1: struct1, struct2: struct2} -> [title: "#{strcut1.name} #{strcut2.name}")] end)

[josevalim]

That would work, yes!

[timfjord]

@josevalim Done ✅

I've slightly adjusted the implementation, but it has a potential issue.
Because I use recursion (which is great because I don't need to duplicate the assigns merging logic), if the callback returns a single arity function there would be an infinite loop instead of the ArgumentError (if I went with this implementation Map.update!(socket, :assigns, &Map.merge(&1, Map.new(fun.(&1)))))

But it looks shorter and cleaner.

[SteffenDE]

🙌🏻