Passing anonymous function as plug option

notexactlyawe

I can’t seem to pass an anonymous function as an option to a Plug. I am using plug_hmouse which from the documentation takes an option digest that is a function to be called on a string. The example is provided below:

plug PlugHMouse,
  ... # other options
  digest: fn string -> Base.encode16(string) end

However when I try this in my router.ex, I get the following error:

router.ex

plug PlugHMouse,
  validate: {"x-hub-signature", webhook_secret},
  hash_algo: :sha,
  split_digest: true,
  digest: fn string -> Base.encode16(string, :lower) end

error

== Compilation error in file lib/spellcanary_web/router.ex ==
** (ArgumentError) cannot escape #Function<0.85932033 in file:lib/spellcanary_web/router.ex>. The supported values are: lists, tuples, maps, atoms, numbers, bitstrings, PIDs and remote functions in the format &Mod.fun/arity
    (elixir) src/elixir_quote.erl:122: :elixir_quote.argument_error/1
    (elixir) src/elixir_quote.erl:259: :elixir_quote.do_quote/3
    (elixir) src/elixir_quote.erl:421: :elixir_quote.do_quote_splice/5
    (plug) lib/plug/builder.ex:312: Plug.Builder.init_module_plug/4
    (plug) lib/plug/builder.ex:286: anonymous fn/5 in Plug.Builder.compile/3
    (elixir) lib/enum.ex:1940: Enum."-reduce/3-lists^foldl/2-0-"/3
    (plug) lib/plug/builder.ex:284: Plug.Builder.compile/3
    lib/spellcanary_web/router.ex:17: (module)
    (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6
    (elixir) lib/kernel/parallel_compiler.ex:208: anonymous fn/4 in Kernel.ParallelCompiler.spawn_workers/6

I don’t get this error when passing &Base.encode16/1 but I do get it when using an anonymous function or when defining a function in my router and passing that, eg.

def encode16lower(string) do
  Base.encode16(string, :lower)
end

...
plug PlugHMouse,
  validate: {"x-hub-signature", webhook_secret},
  hash_algo: :sha,
  split_digest: true,
  digest: &self().encode16lower/1

I need the anonymous function since I want to add an extra argument to the function I’m calling. How can I get around this error?

9 comments

/phoenix #metaprogramming #plug #anonymous-functions

1 1999 9

2024-10-03 21:41:57 UTC

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NobbZ

Module attributes can not contain anonymous functions. They may only contain a very strict subset of values. Whereas &Mod.fun/arit is one of these.

&self().encode16lower/1

self() does not return a module name, but a PID. Probably you meant __MODULE__.encode16lower/1?

Post #1

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notexactlyawe

Thanks NobbZ, the __MODULE__ tip fixed the error! I didn’t realise that calling plug would create a module attribute. For anyone reading this thread in future, the docs on Module attributes actually have some info on this.

Post #2

al2o3cr

The issue is that Plug pre-compiles the stack of plugs, so the init function is called at compile time and its result is stored in the compiled module.

As the error message states, that place can’t store anonymous functions.

The “standard idiom” for this situation is to use a tuple to specify a function by name (you’ll see the term “MFA tuple” a lot). Something like:

# in the plug
  def call(conn, opts) do
    {mod, fun, args} = Keyword.fetch!(opts, :handler)

    destination = apply(mod, fun, [conn | args])

    Phoenix.Controller.redirect(conn, to: destination)
  end

# in the config
get "/work", Plugs.Redirect, handler: {SomeModule, some_fun, []}

# the function definition
defmodule SomeModule do
  def some_fun(conn) do
    ...etc...
  end
end

If some_fun expects more than one argument, you’d include it in the tuple’s third element.

Post #4

al2o3cr

&SomeModule.other_fun(&1, other_arg) is a shorthand for fn x -> SomeModule.other_fun(x, other_arg) end and will trigger precisely the same problems as any other anonymous function.

Despite the similar notation, &SomeModule.other_fun/1 is represented differently internally.

Post #8