Sebb
Load json into struct in module-attribute
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).
Marked As Solved
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.
Also Liked
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. 
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.
Popular in Questions
Other popular topics
Latest on Elixir Forum
Sponsor Spotlight
Real-time error tracking, performance insights, and observability for devs.
Categories:
Sub Categories:
Forums
Our Sponsors
Build Elixir applications with speed and confidence.
Supporting innovation across the BEAM ecosystem.
We build reliable cloud platforms for business-critical systems.
Catch errors, track performance, monitor hosts and more.
Real-time error tracking, performance insights, and observability for devs.
Practical resources that improve the lives of professional developers.
Producing high quality Elixir screencasts since 2017.
Enabling companies to succeed by building software people love.
Courses that'll move you from confusion to "Aha, now I get it!"