Load struct given its name from string

Questions / Help
1

Hi!

I have a couple of structs and I want to build one of them depending on some STRING as input:

defmodule Mystruct1 do ... end
defmodule Mystruct2 do ... end
...
defmodule Mystructn do ... end

def get_struct(name) do
    name
    |> <transformations_here>
    |> struct()
end

the_struct = get_struct("Mystruct5")

Naively I’m trying something like

name |> String.capitalize |> String.to_atom |> struct

but I’m getting this error

** (UndefinedFunctionError) function :Mystruct5.__struct__/0 is undefined (module :Mystruct5 is not available)
    :Mystruct5.__struct__()
    (elixir) lib/kernel.ex:2161: Kernel.struct/3

Besides whether this can be done or not, is this a good idea in order to allow the app to scale by merely adding new structs instead of use pattern matching for every possible input?

Thanks

2

Module name in elixir are atoms with the :"Elixir." prefix, so :"Elixir.Mystruct5" == Mystruct5.

1 Like
3

And i think it’s better to use String.to_existing_atom/1, just in case

4

Works perfectly.

Thanks! @LostKobrakai @dmitrykleymenov

5

Maybe you should use Module.safe_concat([your_string]) for this? It will fail with ArgumentError if the module doesn’t exist.

7 Likes
6

Consider writing this out explicitly, unless there are a LOT of Mystructs:

def name_to_module(name) do
  case name do
    "Mystruct1" -> Mystruct1
    "Mysturct3" -> Mystruct3 # NOTE: client sends this one spelled wrong and won't change it
  end
end

Advantages:

  • very clear exactly which modules this could return
  • easy to add aliases / typo-corrections

Downsides:

  • more typing when adding a new struct
3 Likes
7

Nice improvement, thanks @Nicd.

@al2o3cr, yes, I also prefer to be explicit. In this case

  • the input is not coming from the user but from a generated code so I expect no mispelling happening here.
  • the idea is to keep the same code working and don’t touch it as that library grows with more structs names, it’s a bit of magic, but I was wondering how useful this approach would be.