How to correctly handle assigns in LiveView Sigil without getting Dialyzer warnings?

Phoenix Forum Questions / Help
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1

Hello,

It seems that any LiveView sigil is expected to have in its scope an assigns map.
So not a keyword list or anything else, but a map.

But often I end up with a Dialyzer error (or warning) that The pattern can never match the type

Consider the following simple example for a helper:

defp product_link(product) do
  assigns = %{name: product.name, url: product.url}

  ~L"""
  <a href="<%= @url %>">
    <%= @name %>
  </a>
  """
end

And calling simply like that:

<%= product_link(product) %>

With ElixirLS Dialyzer, I got the following warning/error:

The pattern can never match the type.

Pattern:
%{:__changed__ => _}

Type:
%{:name => _, :url => _}

Now if instead I do the following, I don’t get the error but it’s more verbose on the calling side (particularly bothering when I have a list with more than two keywords pair).

defp product_link(assigns) do
  assigns = Enum.into(assigns, %{})

  ~L"""
  <a href="<%= @url %>">
    <%= @name %>
  </a>
  """
end

And calling like that:

<%= product_link(name: product.name, url: product.url) %>

But in this case I don’t get any warning/error.

So I wanted to know first, what exactly is the meaning of that warning?
Where does it come from?

Is it possible to fix it?
I’m not using any spec, but does defining some specs could prevent this error? (Though, it’s a private function so I’m not sure for that).

If not, how to disable only that warning/error here?

I didn’t defined anything particular to use Dialyzer, but it came for free with ElixrLS in VSCode. So in the meantime to learn more about specs, I rather want to stick with this default usage and try to get no warnings/errors.

And lastly, besides the Dialyzer warning, what is the best way to handle the assigns in LiveView sigils?

Thank you very much folks!

2

Well I found a way to mix both versions, ie. having a simpler call while not having any error.
Simply build the keyword list in the function like so:

defp product_link(product) do
  assigns =
    [name: product.name, url: product.url] 
    |> Enum.into(%{})

  ~L"""
  <a href="<%= @url %>">
    <%= @name %>
  </a>
  """
end

But I don’t know if it’s a valid workaround and if it’s correct idiomatic Elixir.
Also, while doing this I don’t know at all why this is needed…

Any thought are welcome…

2 Likes
3

Hello everybody…
I’m coming back to this topic…
The following is obviously correct and match to:

%{name: "my product"} = [name: "my product"] |> Enum.into(%{})

But why using the left hand side in a liveview template brings the mentioned error while using the right hand side doesn’t bring any error?

I hope I’ll receive some answer this time :wink:

4

You can remove the warning by using…

assigns = %{__changed__: nil, name: product.name, url: product.url}

I guess it needs this key.

2 Likes
5

Indeed, this way the warning disappears.
But it seems weird to add that key out of context…

But for a better understanding…
How people are managing this case?
But maybe it is not a commonly used pattern (though it seems common enough to me) particularly when trying to “componentize” UI parts…

Is it possible to rather disable that very warning?
If so, could that cause some side effect else where?

Thank you very much BTW…

6

Hi, bumping this topic a bit, as I’m running into the same issue after following the suggestion made here in the LiveView docs:

https://hexdocs.pm/phoenix_live_view/Phoenix.LiveComponent.html#module-cost-of-stateful-components

Specifically this code example:

def my_button(text, click) do
  assigns = %{text: text, click: click}

  ~L"""
  <button class="css-framework-class" phx-click="<%= @click %>">
      <%= @text %>
  </button>
  """
end

I read this as a best practice, and ran into the dialyzer error as described by the topic starter:

The pattern can never match the type.

Pattern:
%{:__changed__ => _}

Type:
%{:foo => [any()]}

I’d love to hear from others how they worked around this other that adding a (functionally speaking unneeded) :__changed__ key to the assigns variable inside the function outputting a ~L.

7

Hello! I’m glad I found this thread cause I just bumped into the very same problem. Thanks for the suggested workarounds. Still, it would be nice to understand where the errors is coming from. Maybe somebody has found some valuable insights in the meantime.

8

This may be hard to debug because of macros generate code with generated: true to supress Dialyzer warnings for all the dead code they generate, and in this case probably the root cause is invalid / slightly incompatible spec for one of the functions called from generated code, but the warning pointing to that place is also supressed.

9

The issue in this case is exactly that we were not marking it as generated. I will have fixed it on LiveVIew master.

4 Likes
10

Just found it myself as well, by commenting out all pattern matches on a map with :__changed__ field in live view code one by one, it was the one in Phoenix.LiveView.Engine.

So in this case this is one of those only-slightly annoying dialyzer warnings about dead code being detected. These don’t propagate so it’s good enough to mark as generated