Question

Why is the following 2 expressions that can create a list return different results in Erlang?

> ["A" | []]
// returns ["A"]
> [[] | "A"]
// returns [[], 65]

I was expecting that the second expression returns the same result as the first one. Can anyone explain why is this happening?

Answer 1

To properly understand how this works, keep in mind that all these expressions follow this pattern:

[head | tail]

where head is a single element and tail is another list.

Also note that strings are list of characters, that is, "A" is actually equal to [65].

Hence, in the first case, a list whose head is "A" and whose tail is [] is created and that turns, as expected, into ["A"].

However, in the second case, the head is [] and the tail is "A", which is equal to [65]. Therefore, the result is the head element ([]) plus all the elements in the tail (65).