Behavior of | with lists

iex(17)> mlist = [2,3,4]
[2, 3, 4]
iex(18)> new = [mylist | 7]
[[2, 3, 4] | 7]
iex(19)> new = [7 | mylist]
[7, 2, 3, 4]

Can someone help me understand why the different behavior in the two cases?

You should push from the head not the tail

this one is correct

[7 | mylist]

This one not, because You push a list from the left, into… 7

[mylist | 7]

In both (18) and (19) you are cons-ing a new head onto the list. In (18) the new head of the list is the “element” “[2,3,4]”. In (19) the new head is the “element” “7”.

See also Basic Types: Lists or Tuples.

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As a simple way to remember, a list can be decomposed as

[head | tail]

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

This way it is easy to remember that left side of | is an element, and right side is a list.

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Thank you for your quick response and the reference to cons, and the associated Elixir docs.

Thank you for your quick and helpful responses. I’m quite amazed by the helpfulness of the Elixir community.

Just a nitpick - strictly speaking it is a pattern match where head matches the head element while tail matches the remainder of the list. The big difference to a decomposition (or rather destructuring) is that pattern matching [h|t] cannot match [] while destructuring would simply set h and t to nil. Meanwhile a [h|t] pattern match will match a single element list by binding the single element to h and the empty list [] to t.

Pattern matching is a conditional construct and destructuring isn’t.

It is true that my terminology is incorrect

I have been doing too much JS these days :slight_smile:

I am still using destructuring while pattern matching should be used almost everywhere in Elixir

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As an additional note, the cons article mentions another useful list operator, append, which concatenates two lists. I see that ++ is Elixir for concatenating two lists, reference

iex(24)> [7 | mylist] = [7] ++ mylist
[7, 2, 3, 4]

In that context have a look at The Eight Myths of Erlang Performance: 4. Myth: Operator “++” is Always Bad.

i.e. be mindful of what you are doing when you are using Kernel.++/2