The preceding two chapters described the basics of lists and maps. But you may have noticed that, although I talked about them as types, I didn’t really say what I meant.
The first thing to understand is that the primitive data types are not necessarily the same as the types they can represent. For example, a primitive Elixir list is just an ordered group of values. We can use the […] literal to create a list, and the | operator to deconstruct and build lists.
Then there’s another layer. Elixir has the List module, which provides a set of functions that operate on lists. Often these functions simply use recursion and the | operator to add this extra functionality.
In my mind, there’s a difference between the primitive list and the functionality of the List module. The primitive list is an implementation, whereas the List module adds a layer of abstraction. Both implement types, but the type is different. Primitive lists, for example, don’t have a flatten function.
Maps are also a primitive type. And, like lists, they have an Elixir module that implements a richer, derived map type.
The code that provides the Keyword type is an Elixir module. But the type is represented as a list of tuples:
| | options = [ {:width, 72}, {:style, "light"}, {:style, "print"} ] |
Clearly this is still a list, and all the list functions will work on it. But Elixir adds functionality to give you dictionary-like behavior.
| | iex> options = [ {:width, 72}, {:style, "light"}, {:style, "print"} ] |
| | [width: 72, style: "light", style: "print"] |
| | iex> List.last options |
| | {:style, "print"} |
| | iex> Keyword.get_values options, :style |
| | ["light", "print"] |
In a way, this is a form of the duck typing that is talked about in dynamic object-oriented languages.[17] The Keyword module doesn’t have an underlying primitive data type. It simply assumes that any value it works on is a list that has been structured a certain way.
This means the APIs for collections in Elixir are fairly broad. Working with a keyword list, you have access to the APIs in the primitive list type, and the List and Keyword modules. You also get Enum and Collectable, which we talk about next.