Learn Functional Programming with Elixir/

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Unpacking Values from Lists, Maps, and Structs

Unpacking Values from Lists, Maps, and Structs

Explore pattern matching with lists, maps, and structs.

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Matching lists

Tuples represent collections of a few items. We’ve used pattern matching on them, taken values, and put them in variables. But tuples have one limitation: they’re stored contiguously in memory. We need to know in advance how many elements are inside of them. It’s a problem because we can’t always predict a collection size, and it’s impractical to write all the items of a vast collection in an expression. To address these problems, Elixir uses the list data type. In Elixir, lists are linked lists. That means each item of the collection contains a value and an implicit reference to the next element. For a list [:a, :b, :c, :d, :e], we’ll have something like this in memory:

%0 node_1 :a node_2 :b node_1->node_2 node_3 :c node_2->node_3 node_1629409105973 :d node_3->node_1629409105973 node_1629409084922 :e node_1629409105973->node_1629409084922 node_1629409069935 [] node_1629409084922->node_1629409069935
Elixir list in memory

A list ends by linking to an empty list, turning the list into a proper list. It’s useful to avoid infinite loops by checking if the last item is an empty list and stopping a recursive iteration. In some rare cases, we can face an improper list, one that doesn’t link to an empty list at its end.

Like with tuples, we can create pattern-matching expressions to extract values from the collection and put them into variables or check if the list items follow some pattern. For representing lists, we use the [] syntax. Let’s start our exploration by creating an expression that tells if the items are the same. Try it in the IEx below:

iex> [a, a, a] = [1, 1, 1]
#Output -> [1, 1, 1]

iex> [a, a, a] = [1, 2, 1]
#Output -> ** (MatchError) no match of right hand side value: [1, 2, 1] 

iex> [a, b, a] = [1, 2, 1]
#Output -> [1, 2, 1]

iex> [a, a, a] = ["apples", "apples", "apples"] 
#Output -> ["apples", "apples", "apples"]

The pattern [a, a, a] means that a list must have three elements with the same value. We see this because we’re using the variable a for the three items. Variables have a unique value in an expression; the variable a can’t be the number 1 and 2 simultaneously. That’s why the list [1, 2, 1] results in a MatchError with [a, a, a] but succeeds with [a, b, a]. We can create complex checks like this one:

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