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()

beanlanda · August 16, 2025, 2:40am

I’d like to know if there is any way to introspect struct fields obtaining the order they were defined in defstruct? For background I’m doing some macros with structs to implement various protocols, and would like to default the ordering but map.keys is of course unordered.