What is the benefit of Keyword Lists?

Question

In elixir we have Maps:

> map = %{:a => \"one\", :b => \"two\"} # = %{a: \"one\", b: \"two\"}
> map.a                             # = \"one\"
>         


        

Answer 1

The main benefit of keyword lists is a backward compatibility with existing elixir and erlang codebase.

They also adds syntax sugar if used as functions arguments which resembles for e.g. a ruby syntax:

def some_fun(arg, opts \\ []), do: ...
some_fun arg, opt1: 1, opt2: 2

The main drawback of using keyword lists is that it's not possible to perform a partial pattern matching on them:

iex(1)> m = %{a: 1, b: 2}
%{a: 1, b: 2}
iex(2)> %{a: a} = m
%{a: 1, b: 2}
iex(3)> a
1
iex(4)> k = [a: 1, b: 2]
[a: 1, b: 2]
iex(5)> [a: a] = k
** (MatchError) no match of right hand side value: [a: 1, b: 2]

Let's extend it to function arguments. Imagine we need to handle a multiclause function based on a value of one of the options:

def fun1(arg, opt1: opt1) when is_nil(opt1), do: do_special_thing
def fun1(arg, opts), do: do_regular_thing

def fun2(arg, %{opt1: opt1}) when is_nil(opt1), do: do_special_thing
def fun2(arg, opts), do: do_regular_thing

This will never execute the do_special_thing:

fun1("arg", opt1: nil, opt2: "some value")
doing regular thing  

With map arguments it will work:

fun2("arg", %{opt1: nil, opt2: "some value"})
doing special thing