Pattern Matching in

About Pattern Matching

When writing Elixir functions, we can make use of an assertive style with pattern matching:

def read_file() do
  {:ok, contents} = File.read("hello.txt")
  contents
end

• Pattern matching is explicitly performed using the match operator, =/2.

  • Matches succeed when the shape of the data on the left side of the operator matches the right side.

  • When matches succeed, variables on the left are bound to the values on the right.

  • Using an underscore, _, allows us to disregard the values in those places.

    {:ok, number, _} = {:ok, 5, [4.5, 6.3]}
    number
    # => 5 is bound to this variable
    

  • The pin operator ^ can be used to prevent rebounding a variable and instead pattern match against its existing value.

    number = 10
    {:ok, ^number, _} = {:ok, 5, [4.5, 6.3]}
    # => ** (MatchError) no match of right hand side value: {:ok, 5, [4.5, 6.3]}
    

• Pattern matches may also occur in a function clause head, so that only arguments that match the pattern will invoke the function.

• Variables can be bound in a function clause pattern match.

  defmodule Example do
    def named_function(:a = atom_variable) do
      {atom_variable, 1}
    end
  end
  
  Example.named_function(:a)
  # => {:a, 1}
  # The first function clause matches, so it is invoked
  
  Example.named_function(:b)
  # => ** (FunctionClauseError) no function clause matching in Example.named_function/1