Practical use of improper lists in Erlang (perhaps all functional languages)

Question

I\'ve been reading Erlang and OTP in action, and stumbled upon a question with regards to improper lists.

Don’t be tempted to use list cel

Answer 1

One valid use is described at the very bottom of the Eunit user manual, in the section about lazy generators. This code example is supposed to create a very long list that will be consumed one element at a time, so instead of generating the entire list at once it creates an improper list whose tail describes how to generate the rest of the list:

lazy_test_() ->
    lazy_gen(10000).
lazy_gen(N) ->
    {generator,
     fun () ->
         if N > 0 ->
                [?_test(...)
                 | lazy_gen(N-1)];
            true ->
                []
         end
     end}.

In other words, it's a lazy list, which Erlang itself doesn't give you.