26 June 2021

TIL: Elixir's match?/2 turns a match result into a predicate

TIL about Elixir’s Kernel.match?/2.

iex> list = [a: 1, b: 2, a: 3]
iex> Enum.filter(list, &match?({:a, _}, &1))
[a: 1, a: 3]

So many times I’ve hacked around not knowing about this. 🤯

Where this gets really useful is in tests where you want to assert part of the shape of a map. For instance:

assert Enum.count(
         &match?(%{id: ^my_id, inserted_at: ^time}, &1)
       ) == 1