Generate map pattern matching functions

Questions / Help
,
1

I have a goal to code generate the following module

defmodule C do 
  def first(%{company_id: company_id}, user}, do: do_first(company_id, user)
  def first(%{sales_id: sales_id}, user}, do: do_first(sales_id, user)
  #... etc
end

I’d like to create a macro potentially? Or even a function where I can give it a list of those atoms, and have it delegate those arguments to the second function
i.e

defmodule C do
  use FirstMacro, [:company_id, :sales_id]  #... could be a very long list
end

I keep writing macros but am unable to figure out how to code pattern match on a map macro the quoted ast for a map is something like {:%{}, , [key: value]} but I’m not able grab value like I would in defining a function so I can’t achieve my goal here, can someone point me in the right direction kindly ^^. Thanks for any help!

my attempt on the 100th iteration looks something like this

defmodule C do
  defmacro first_list(list, user) do
    quote bind_quoted: [list: list, user: user] do
      Enum.map(list, fn x ->
        def first(unquote({:%{}, [], []}), user),
          do: do_first(:need_this_variable, user)
      end)
    end
  end
end

But I know its all wrong :frowning:

2

As %{foo: foo} is syntactic sugar for %{:foo => foo} the following should work:

[:company_id, :sales_id]
|> Enum.each(fn key ->
  def first(%{unquote(key) => val}, user), do: do_first(val, user)
end)

But to be honest, how is do_first/2supposed to know what kind of id you are passing in?

2 Likes
3

I think the idea is to filter out the wrong ids using pattern matching, I thought about this but in my elixir noob head I thought that the %{key => value} syntax was sugar for the other %{key: value}

4

Thank you very much sir this is exactly what I needed.

1 Like
5

Whats a wrong ID?

first(%{company_id: 1}, user) #=> do_first(1, user)
first(%{sales_id:   1}, user) #=> do_first(1, user)

So how does do_first/2 know whether the 1 refers to a sales_id or to a company_id?

1 Like
6

In my particular app it doesn’t actually matter because all of our ID’s are global UUIDS. This is an Absinthe Batching function if that makes more sense. My resolvers are calling &SomeModule.first/3

1 Like