Pattern Match Keyword List in Function Def

TIL that you can pattern match a keyword list in a function definition.

Sometimes you'll receive the last argument as an empty keyword list; in this case, we're calling it opts. You can pattern match key-values by defining the shape in a function.

  defmodule Example do
    def hello(opts \\ [])
 
   # will match when message is the only keyword included in opts
    def hello([message: message]) do
      IO.puts("Hello #{message}")
    end

    # will match when there are multiple keywords but message is the first
    def hello([{:message, message} | _rest] = opts) do
      IO.puts("Hello #{message}")
    end
  end

> Example.hello(message: "World")
Hello World
:ok

# could also call in the more verbose way
> Example.hello([message: "World"])
Hello World
:ok

# :message but with other args after
> Example.hello(message: "World", something_else: "hi")
Hello World
:ok

Note: As a TIL reader pointed out, pattern matching Keywords will make your function args order dependent. The following would not work:

> Example.hello(foo: "bar", message: "World")
** (FunctionClauseError) no function clause matching in Example.hello/1

If you need them to be order independent, use a map or just match on single argument, then check for each option appropriately with the Keyword module.

https://elixir-lang.org/getting-started/keywords-and-maps.html

Today I Learned

A Hashrocket project Follow on Twitter

Pattern Match Keyword List in Function Def