Vex validate w anon. function: cannot inject attribute @vex_validations into function/macro because cannot escape #Function

chemist

Hi, Im having a bit of trouble with anon. functions using the Vex validator.

defmodule Test do
  @enforce_keys [:wallet_id, :withdrawal, :currency, :funds_balance]

  defstruct [
    :wallet_id,
    :withdrawal,
    :currency,
    :funds_balance
  ]

  use ExConstructor
  use Vex.Struct
  alias __MODULE__

  validates(:withdrawal, presence: [message: "expected"], 
  by: [function: &Test.test(&1, Map.get(&2, :funds_balance)), 
  message: "invalid withdrawal."])

  def test(withdrawal, funds_balance) do
    # some logic here
  end
end

The above has two capture operators, &1 which is the field being validated: withdrawal and &2 which is the entire Test struct, %Test{}

Basically the above snippet results in this error:
** (ArgumentError) cannot inject attribute @vex_validations into function/macro because cannot escape function

I need a nested function that can effectively capture the second field, &2. I am aware that it is possible to move this function to the function, test but will like to learn if it is possible to do it as a nested function as part of an anonymous function

Many thanks.

20 comments

#anonymous-functions #vex

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2022-02-26 05:35:53 UTC

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03juan

Providing a stacktrace of your error would help in debugging.

Post #1

chemist

Many thanks. Not too sure if it helps but here goes.

== Compilation error in file test.ex ==
** (ArgumentError) cannot inject attribute @vex_validations into function/macro because
 cannot escape #Function<0.45330970/2 in :elixir_compiler_5.__MODULE__/1>. The 
 supported values are: lists, tuples, maps, atoms, numbers, bitstrings, PIDs and remote 
 functions in the format &Mod.fun/arity
    (elixir 1.13.2) lib/kernel.ex:3509: Kernel.do_at/5
    (elixir 1.13.2) expanding macro: Kernel.@/1
    (test 0.1.0) test.ex:1: Test.__vex_validations__/0
    (vex 0.9.0) d:/test.ex:1: Vex.Struct.__before_compile__/1

Post #2

kartheek

function is supposed to be capture like &Test.test/1 or &Test.test/2.

Post #3

chemist

Hi kartheek, apologies but if i understood you correctly, you are saying change the statement to:

  validates(:withdrawal, presence: [message: "expected"], 
  by: [function: &Test.test/2(&1, Map.get(&2, :funds_balance)), 
  message: "invalid withdrawal."])

Adding the arity operator to the function &Test.test does not help.

Post #4

kartheek

Its supposed to be

 by: [function: &Test.test/2,

github.com/CargoSense/vex

lib/vex/validators/by.ex

715c644c9


      
            def validate(value, context, options) when is_list(options) do
              unless_skipping(value, options) do
                function = Keyword.get(options, :function)
          
                case call_function(function, value, context) do
                  {:error, reason} ->
                    {:error, reason}
          
                  falsy when falsy === false or falsy === nil ->
                    {:error, message(options, "must be valid", value: value)}
          
                  _ ->
                    :ok
                end
              end
            end
          
            defp call_function(f, value, _context) when is_function(f, 1), do: f.(value)
            defp call_function(f, value, context) when is_function(f, 2), do: f.(value, context)
          end

It accepts function of arity 1 or 2 - if it is 2 it is passing context as second param.

You might have to relook at your test function

Post #5

03juan

Not always true as the examples show passing an anonymous function definition

Vex.Validators.By.validate(
  [], [function: fn (v) when is_list(v) -> :ok
                    (v) -> {:error, {:not_list, v}} end])
:ok

The docs say

Options

None, a function with arity 1 must be provided.

:function: The function to check. Should have an arity of 1 and return true/false.

But the code you posted does seem to accept arity 2.

I assumed validates can be called with 3 arguments, but the code only provides VexStruct.validates/2

Post #6

kartheek

OP is not using anonymous function - wants to invoke Test.test ?

Post #7

chemist

  validates(:withdrawal, presence: [message: "expected"], 
  by: [function: &Test.test(&1, Map.get(&2, :funds_balance)), 
  message: "invalid withdrawal."])

Map.get(&2, :funds_balance) will evaluate to the value held in funds_balance and @enforce_keys guarantee a non nil value. And i have tested it with a non-nil value.

The function Test.test has an arity of 2 and accepts two values &1: withdrawal (which is being evaluated) and the outcome of evaluating Map.get&2: funds balance resulting in an evaluation which is either false or true

So as a result, the function for the validator above will return true or false.

What I believe is the anon function utilizing the Vex.validator does not seem to like having a nested function using a capture operator. So im checking my understanding.

Post #8

03juan

Officially you can only have a function with arity 1. There seems to be some support within the code for arity 2 but the Vex.Struct implementation does not use it.

If you follow the Vex.Struct and Vex.Extract.Struct code you may be able to re-implement them to provide your own validates/3 that takes a context, but that would be a more advance solution at the moment.

I respectfully disagree. The Kernel.SpecialForms.&/1 docs say

Capture operator. Captures or creates an anonymous function.

So in this case OP is trying to create an anonymous 2-arity function that calls test/2, but the parsing of @validators is failing because it does seem to expect the form &function/arity

Post #9