Is ++ operator more expensive than | operator in Erlang? -

i reading learn erlang , came upon example in recursion chapter.

tail_sublist(_, 0, sublist) -> sublist; tail_sublist([], _, sublist) -> sublist; tail_sublist([h|t], n, sublist) when n > 0 ->  tail_sublist(t, n-1, [h|sublist]). 

as author goes on explain there fatal flaw in our code. being sub lists hence produced reverse , have re-reverse them correct output. in contrast, did use ++ operator avoid reversing list later.

sublist_tail([],_,acc) -> acc; sublist_tail(_,0,acc) -> acc; sublist_tail([h|t],n,acc) -> sublist_tail(t,n-1,acc++[h]). 

my question is, is ++ operator more expensive | operator? , if is, would solution (using ++ operator) still slow compared author's solution (including reversing list correct output)?

is ++ operator more expensive | operator?

that depends. if use correctly, no. ++ dangerous when have big left-hand-side operand.

each time "++"-operator invoked on left-hand list (like: list1 ++ list2), creating new list, copy of left-hand operand (list1). each copy operation has runtime, dependent on length of list1 (which keeps growing iterations).

so, if prepend values 'head first', don't have perform copy-operation on whole list in each step. means, accumulation ++ @ head of list wouldn't bad either, since "h"-value copied once in each iteration:

sublist_tail([h|t],n,acc) -> sublist_tail(t,n-1,[h]++acc). 

but if accumulating head-first (and have reverse later anyhow), can cons-operator (|)

sublist_tail([h|t],n,acc) -> sublist_tail(t,n-1,[h|acc]). 

this 'proper' way, since (please correct me if wrong) ++ syntactic sugar , implemented internally cons-operator (|).