Introducing Elixir

Chapter 7. Name-Value Pairs

Tuples and lists are powerful tools for creating complex data structures, but there are two key pieces missing from the story so far. Tuples are relatively anonymous structures. Relying on a specific order and number of components in tuples can create major maintenance headaches. Lists have similar problems: the usual approaches to list processing in Elixir assume that lists are just a sequence of (often) similar parts.

Sometimes you want to call things out by name instead of number, or pattern match to a specific location. Elixir has many different options for doing just that.

Note

Maps and structs appeared late in Elixir’s development. They layer directly on features Erlang introduced in R17. In the long run, maps and structs will probably become the key pieces to know, but you may need the rest for compatibility with older Erlang code.

Keyword Lists

Sometimes you need to process lists of tuples containing two elements that can be considered as a “key and value” pair, where the key is an atom. Elixir displays them in keyword list format, and you may enter them in that format as well:

iex(1)> planemo_list = [{:earth, 9.8}, {:moon, 1.6}, {:mars, 3.71}]
[earth: 9.8, moon: 1.6, mars: 3.71]
iex(2)> atomic_weights = [hydrogen: 1.008, carbon: 12.011, sodium: 22.99]
[hydrogen: 1.008, carbon: 12.011, sodium: 22.99]
iex(3)> ages = [david: 59, simon: 40, cathy: 28, simon: 30]
[david: 59, simon: 40, cathy: 28, simon: 30]

A keyword list is always sequential ...