About the | operator in erlang.

Question

We can make a nested list in erlang by writing something like this:

 NL = [[2,3], [1]]. 
 [[2,3],[1]]

But assume we wrote it like this instead:

 OL = [[2,3]|1].
 [[2,3]|1]

Is OL still a list? Can someone please elaborate more what OL is?

Answer 1

This is called an improper list and should typically not be used. I think most library functions expects proper lists (e.g. length([1|2]) throws bad argument exception). Pattern matching with improper lists works though.

For some use cases, see Practical use of improper lists in Erlang (perhaps all functional languages)