Pattern matching on lists

Matching on lists is akin to matching on tuples. Here's a simple example:

iex> [first, second, third] = ["α", "β", "γ"]
["α", "β", "γ"]
iex> first
"α"
iex> second
"β"
iex> third
"γ"

There's nothing new here. What if, for instance, we don't care about the second element of the list? That's where the  _ (underscore) anonymous variable is convenient:

iex> [first, _, third] = ["δ", "ε", "ζ"]
["δ", "ε", "ζ"]
iex> first
"δ"
iex> third
"ζ"

We're again matching a list with three elements, but now bind the second element to the _ variable, which means that we accept anything in that position and we won't use its value.

As we've mentioned on the data types section, you can use the hd and tl functions from the Kernel module to get the head and the tail of a list. You can do the same with pattern matching:

iex> [first | rest_of_list] = ["α", "β", "γ"]
["α", "β", "γ"]
iex> first
"α"
iex> rest_of_list
["β", "γ"] 

While in this contrived example, this approach yields no benefit, this technique is a fundamental piece to operate on a list using recursion. We'll look at this in greater detail in the Working with collections section.