Get field names of a struct

alexcastano · May 13, 2020, 1:01pm

Hello,

Is there a way to get the list of fields with only the struct module?

iex> something(Date)
[:calendar, :year, :day, :month]

I tried this, but it doesn’t work if @enforce_keys are defined:

iex(4)> Map.keys(%Date{})
** (ArgumentError) the following keys must also be given when building struct Date: [:year, :month, :day]
    (elixir) expanding struct: Date.__struct__/1
    iex:4: (file)

At first glance it does not seem that any function is created for this either:

github.com

elixir-lang/elixir/blob/v1.10.3/lib/elixir/lib/kernel.ex#L4333


`%User{}`.

The types of the struct fields that are not included in `%User{}` default to
`term()` (see `t:term/0`).

Structs whose internal structure is private to the local module (pattern
matching them or directly accessing their fields should not be allowed) should
use the `@opaque` attribute. Structs whose internal structure is public should
use `@type`.
"""
defmacro defstruct(fields) do
  builder =
    case bootstrapped?(Enum) do
      true ->
        quote do
          case @enforce_keys do
            [] ->
              def __struct__(kv) do
                Enum.reduce(kv, @struct, fn {key, val}, map ->
                  Map.replace!(map, key, val)
                end)

LostKobrakai · May 13, 2020, 1:05pm

This seems to work:

iex(2)> Date.__struct__() |> Map.keys()
[:__struct__, :calendar, :day, :month, :year]

krisleech · June 25, 2024, 2:30pm

This seems to work for any struct:

struct(Foo, %{}) |> Map.from_struct() |> Map.keys()