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Earlier we said a list may either be empty or consist of a head and a tail. The head contains a value and the tail is itself a list. This is a recursive definition.

We’ll represent the empty list like this: [ ].

Let’s imagine we could represent the split between the head and the tail using a pipe character, |. The single element list we normally write as [3] can be written as the value 3 joined to the empty list:

 ​ [ 3 | [] ]

When we see the pipe character, we say that what is on the left is the head of a list and what’s on the right is the tail.

Let’s look at the list [2, 3]. The head is 2, and the tail is the single-element list containing 3. And we know what that list looks like—it is our previous example. So we could write ...

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