Get map value from Code.eval string in Elixir

Question

Im trying to get the value of a map by running Code.eval_string/1 and it fails with error:

warning: variable "data" does not exist and is being expanded to "data()", please use parentheses to remove the ambiguity or change the variable name
  nofile:1

** (CompileError) nofile:1: undefined function data/0
    (elixir 1.10.2) lib/code.ex:332: Code.eval_string_with_error_handling/3
    main.exs:65: RulesEngine.evaluate_object_by_condition/2
    (elixir 1.10.2) lib/enum.ex:1396: Enum."-map/2-lists^map/1-0-"/2
    main.exs:17: RulesEngine.evaluate_object_by_condition/2

Code is:

  @doc "Evaluate condition"
  @default_op "="
  def evaluate_object_by_condition(data, condition) do
    IO.puts("Evaluando una regla en particular:")
    IO.inspect(condition)
    IO.inspect(data)
    attr = condition.attr
    value = condition.value
    IO.inspect(attr)
    eval_data = Code.eval_string(attr)
    op = Map.get(condition, "op", @default_op)
    IO.puts("DATA to EVAL")
    IO.inspect(eval_data)

    # value_type = Map.get(condition, "type")
    # Hacer type checking y agregar a value_type
    ## Falta obtener el valor del objeto
    # res = evaluate("=", value, obj.value)
    true
  end

then I run:

obj = %{
  value: 1,
  tiene_beca: 1,
  tiene_credito: 1
}

condition= %{
          attr: "data.tiene_beca",
          value: 1
        }   


RulesEngine.evaluate_object_by_condition(obj, rules_or)

So I'm trying to get the value of data.tiene_beca, and getting that variable name from a string, which would be the correct way of doing that in elixir?

legoscia · Accepted Answer · 2020-04-16 17:39:06Z

2

First of all, consider whether you actually need all this flexibility. Code.eval_string runs the code without any checks or restrictions whatsoever, so this potentially opens a security hole in the code. I would do something like this instead:

["data", field_name] = String.split(attr, ".")
field_name = String.to_existing_atom(field_name)
eval_data = data[field_name]

That said, the reason your code isn't working is that Code.eval_string doesn't have access to local variables in the calling function, so you'd need to pass the variable as a binding explicitly:

    eval_data = Code.eval_string(attr, [data: data])

answered Apr 16, 2020 at 17:39

legoscia

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1 Comment

paltaa Over a year ago

Thanks a lot for both the correct way of doing so in elixir and why it was failing!

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