Cryptic "undefined function arg0/0" error message in EEx templates

thiagomajesk

Hi! I’m working on this small lib that provides a structure to better organize components on top of Phoenix’s views (influenced by AspNet’s and Ruby’s ViewComponents)…

I was testing some use-cases today and got this compile error that only happens when I use EEx templates. I have some unit tests for function calls that do not break, but when the code goes to an actual template it does break. I have a separate branch with a commit that contains a test case that better shows the problem:

https://github.com/thiagomajesk/viewplex/commit/b67072f3a4e180f5117e9f8145ad27d4240d4e47#diff-9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08

The error I get is:

== Compilation error in file test/support/page_view.ex ==
** (CompileError) test/support/page.html.eex:1: undefined function arg0/0
    (elixir 1.10.4) src/elixir_locals.erl:114: anonymous fn/3 in :elixir_locals.ensure_no_undefined_local/3
    (stdlib 3.13) erl_eval.erl:680: :erl_eval.do_apply/6

Also, while testing the lib in a Phoenix application, I got:

variable "arg0" does not exist and is being expanded to "arg0()", please use parentheses to remove the ambiguity or change the variable name

8 comments

/phoenix #eex #templates #views

1 2655 8

2021-02-05 20:30:12 UTC

Showing Posts 1 to 8

Show Best Posts
Show All (oldest first)
Show All (newest first)

axelson

Scenic Core Team

What version of Elixir are you testing it on? Do you get the same error with multiple different versions?

Post #1

thiagomajesk

Hi @axelson! I’m using Elixir 1.10.04:

Erlang/OTP 23 [erts-11.0.3] [source] [64-bit] [smp:2:2] [ds:2:2:10] [async-threads:1] [hipe]

Elixir 1.10.4 (compiled with Erlang/OTP 22)

Haven’t tested with a different version tbh.

Post #2

thiagomajesk

I’ve tested with more versions and it still doesn’t compile…

1.10.04-otp-22
1.10.04-otp-23
1.11.03-otp-22
1.11.03-otp-23

I have no clue what this could be… Do you have any other suggestions?

Post #4

sanswork

Do you get the same error with

<%= component Slot do %>
  <p>How are you?</p>
<% end %>

in your test?

Post #5

thiagomajesk

Hey, @sanswork! The error seems to happen only when I’m using the slot/2 function inside a component/2 block inside an EEx template… In other words, your example would succeed (at least at compilation time).

Post #6

thiagomajesk

Should I open an issue on Phoenix’s repository perhaps?

Update: Cryptic “undefined function arg0/0” error message in EEx templates · Issue #4200 · phoenixframework/phoenix · GitHub

Post #7

al2o3cr

The AST from that issue pretty-prints to:

[arg0 = case(slot(:greet) do
  {:safe, ["    <strong>Hello!<strong>\n"]}
end) do
  {:safe, data} ->
    data
  bin when is_binary(bin) ->
    Plug.HTML.html_escape_to_iodata(bin)
  other ->
    Phoenix.HTML.Safe.to_iodata(other)
end, arg1 = case(slot(:name) do
  {:safe, ["    <span>John</span>\n"]}
end) do
  {:safe, data} ->
    data
  bin when is_binary(bin) ->
    Plug.HTML.html_escape_to_iodata(bin)
  other ->
    Phoenix.HTML.Safe.to_iodata(other)
end, {:safe, [arg0, arg1, "  <p>How are you?</p>\n"]}]

or distilled to its essence:

[a = 1, b = 2, a + b]

This fragment doesn’t compile either:

** (CompileError) iex:5: undefined function a/0
    (stdlib 3.13.2) lists.erl:1358: :lists.mapfoldl/3

The issue is that a list of AST nodes is not the same as a block.

This version of the simplified example does compile (and returns 3):

(a = 1; b = 2; a + b)

(or you can use newlines instead of ;)

The difference is clear in their respective ASTs:

iex(9)> quote do: [a = 1, b = 2, a + b] 
[
  {:=, [], [{:a, [], Elixir}, 1]},
  {:=, [], [{:b, [], Elixir}, 2]},
  {:+, [context: Elixir, import: Kernel], [{:a, [], Elixir}, {:b, [], Elixir}]}
]

iex(10)> quote do: (a = 1; b = 2; a + b)
{:__block__, [],
 [
   {:=, [], [{:a, [], Elixir}, 1]},
   {:=, [], [{:b, [], Elixir}, 2]},
   {:+, [context: Elixir, import: Kernel], [{:a, [], Elixir}, {:b, [], Elixir}]}
 ]}

Post #8

thiagomajesk

Yes, I noticed something was wrong when I actually printed that AST. I couldn’t quite put my finger on it though. One thing that I noticed is that this compiles:

<%= component Slot do %>
  <% slot :greet do %>
    <strong>Hello!<strong>
  <% end %>
  <% slot :name do %>
    <span>John</span>
  <% end %>
  <p>How are you?</p>
<% end %>

And this does not:

<%= component Slot do %>
  <%= slot :greet do %>
    <strong>Hello!<strong>
  <% end %>
  <%= slot :name do %>
    <span>John</span>
  <% end %>
  <p>How are you?</p>
<% end %>

Notice the <% versus <%=, which generates a completely different AST (much more complicated one) from the previous example. I guess because it involves actually outputting something on Phoenix’s EEx Engine side.

Since I was counting on having specific elements while expanding the block, I didn’t expect that the block would be different (but obviously it is).

Post #9

— All posts loaded —