Load json into struct in module-attribute

Sebb

I want to load a json (list of objects) into list of structs in a module attribute.
Seems like I need three modules for that.

json

[
    {"foo": 1, "bar": 2},
    {"foo": 11, "bar": 22}
]

struct

defmodule Foo do
  defstruct [:foo, :bar]
end

load into struct, note that function in reality is too complex to be added to Foos/@foos - pipe

defmodule JsonLoader do
  def load_foos(foos) do
    Enum.map(foos, &struct(Foo, &1))
  end
end

module that stores the structs in a module attribute.

defmodule Foos do
  @foos File.read!("lib/foos.json") |> Jason.decode!(keys: :atoms) |> JsonLoader.load_foos()

  def get_foo(foo) do
    Enum.find(@foos, fn f -> f.foo == foo end)
  end
end

Is there way to bring all that into one module (Foo).

5 comments

#compile #struct #defstruct

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2021-10-14 14:26:32 UTC

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LostKobrakai

There is not. You can neither execute functions of a module within its own module body nor create structs of the to be compiled module at compile time. Both the struct definition as well as the function will only be available once the module is fully compiled. At the time you provide the module attribute value the module is still being compiled.

The best you can do is 2 modules. One for the struct and one with the module attribute, by inlining the code of load_foos.

Post #1

Sebb

OK thank you. I expected that, just wanted to make sure. Often there is some magic hidden I do not know.

Post #2

aziz

I’d like to challenge that claim.

defmodule Foo do
  defstruct [:foo, :bar]

  @foos File.read!("lib/foos.json")
        |> Jason.decode!(keys: :atoms)
        |> Enum.map(&Map.put(&1, :__struct__, __MODULE__))

  def list do
    @foos
  end

  def equal? do
    @foos == [%Foo{bar: 2, foo: 1}, %Foo{bar: 22, foo: 11}]
  end
end

Foo.list() |> IO.inspect(label: "list()")
Foo.equal?() |> IO.inspect(label: "equal?()")

So you can’t use the struct function but that’s a small price to pay if you really want/need to do this.

Post #3

Sebb

nice. So you can use __MODULE__, but not %__MODULE__{}

@foos File.read!("lib/foos.json")
        |> Jason.decode!(keys: :atoms)
        |> Enum.map(fn foo -> %__MODULE__{foo: foo.foo, bar: foo.bar} end)

** (CompileError) lib/foos.ex:7: cannot access struct Foo, the struct was not yet defined or the struct is being accessed in the same context that defines it

Still, I’ll stay with the 3 Module approach, &Map.put(&1, :__struct__, __MODULE__) - looks a little cryptic and hacky. I also can’t inline the function that creates the struct because there is too much going on. (I could still preprocess the json, but I don’t want to do that).

Post #4

LostKobrakai

The first is just a dump alias for the current module name. The second is creating a struct, which does do things like checking keys, and such, which does fail without the struct being defined.

&Map.put(&1, :__struct__, __MODULE__) just uses knowledge about implementation details of structs to construct them without doing all the consistency checks structs have when being constructed otherwise.

Post #5

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