Pattern matching in Elixir can be useful to not only match values, but destructuring data. This is useful in several cases against maps and structs (also maps 🤪), for example:
1. Getting the struct type
iex> %struct{} = URI.parse("google.com")
iex> struct
URI2. Matching the struct type dynamically
iex> type = URI
iex> %^type{} = %URI{}
%URI{}
iex> %^type{} = %User{}
** (MatchError) no match of right hand side value: %User{}3. Ensuring an object is a struct
iex> %_{} = %URI{}
%URI{}
iex> %_{} = %{}
** (MatchError) no match of right hand side value: %{}4. Ensuring the map has specific keys
iex> map = %{first_name: "Vinicius", last_name: "Brasil", city: "Curitiba"}
iex> %{first_name: _, last_name: _} = map
%{first_name: "Vinicius", last_name: "Brasil", city: "Curitiba"}
iex> %{age: _} = map
** (MatchError) no match of right hand side value: %{city: "Curitiba", first_name: "Vinicius", last_name: "Brasil"}5. Ensuring a map is not empty
This one is tricky because %{} matches against any map value, empty or not. For instance:
iex> empty_map = %{}
iex> filled_map = %{age: 21}
iex> %{} = empty_map
%{}
iex> %{} = filled_map
%{age: 21}To ensure a map actually has keys in a guard clause, you can use Kernel.map_size/1 as the example below:
iex> defmodule Empty do
...> def is_nil_or_empty?(nil), do: true
...> def is_nil_or_empty?(map) when map_size(map) == 0, do: true
...> def is_nil_or_empty?(%{}), do: false
...> end
iex> Empty.is_nil_or_empty?(nil)
true
iex> Empty.is_nil_or_empty?(%{})
true
iex> Empty.is_nil_or_empty?(%{a: 1})
false