Pattern matching comparisons

Question

I was creating a case statement and reached a case where I thought it might be nice/applicable to do a pattern matching comparison. I can't seem to find anything online about it. Take the following hypothetical example:

person = %Person{first_name: "Test", last_name: "example}
person2 = %Person{first_name: "another", last_name: "person"}

case list do
   [] -> 
     :empty

   [person, person2] == [%Person{} | _] ->
     :true

   [_] -> 
     :no_Person_struct
end

Granted, this would only check the head of the list but is there anything like this or a way to do it?

Also negation of it would be nice if possible. i.e.

[person, person2] == [%NotAPerson{} | _] == false -> :true

Syntax is very likely wrong.

EDIT: How about pattern matching in the arguments at least?

def([%Person{} | _] = people) do

Have you looked into using guards? hexdocs.pm/elixir/master/guards.html — pdexter
Not 100% sure about what you're asking but I think you could use cond with match? here? — Dogbert
@pdexter yeah, that's what I'm currently using and it works fine but I think this would be a nice addition if possible instead of guards. At the moment I'm just using ...when is_list(list) do. I'll edit the post. Kind of have a related side thought, — shmink
@Dogbert Comparing a list with a pattern of it. If they match, it returns a true. — shmink

Aleksei Matiushkin Aleksei Matiushkin · Accepted Answer · 2018-09-06T09:13:32

You are overcomplicating things.

case list do
   [] -> :empty
   [%Person{} = _person | _] -> :first_is_a_person
   [_ | _] -> :first_is_not_a_person # because the previous clause did not match
end

To check all elements in a list, use Enum.all?/2:

Enum.all?(list, fn
  %Person{} -> true
  _ -> false
end)

Pattern matching comparisons

1 Answers