Taking Zach’s answer into consideration, I did want to ask how you are using these accessors and why they are deemded necessary, but very hard without more context (and especially since my Ash knowledge is unforunately limited). At first I was thinking that it’s always literally :union_field, but now based on your answer (and the bit of docs I read) assuming that is dynamic while :value is literally :value. Assuming this, you can make a 2-arity accessor:

def get_value(resource, union_field) do
  case resource do
    %{^union_field => %{value: value}} ->
      value

    %{^union_field => value} ->
      value
  end
end

Then:

get_value(%MainResource{some_field: %{foo: :bar}}, :some_field)
# %{foo: :bar}
get_value(%MainResource{some_other_field: %{value: %{foo: :bar}}}, :some_other_field)
# %{foo: :bar}

Again, not super sure this is what you’re after but this is my next best attempt

I don’t totally follow what you mean by two functions, like two differently named functions? You can certainly make this work with one function with either two heads or a case statememtn. I’m not sure of the exact API you want but something like this:

def get_value(%MainResource{union_field: %{value: value}}), do: value
def get_value(%MainResource{union_field: value}), do: value

Then you can do:

get_value(%MainResource{union_field: %{foo: bar}})
# %{foo: :bar}
get_value(%MainResource{union_field: %{value: %{foo: bar}}}
# %{foo: :bar}

Is that what you mean or are you talking about the . syntax?