Queue/Definition: Difference between revisions

=={{header|Erlang}}==

The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.

<lang Erlang>

-module(fifo).

-export([new/0, push/2, pop/1, empty/1]).

new() -> {fifo, [], []}.

push({fifo, In, Out}, X) -> {fifo, [X|In], Out}.

pop({fifo, [], []}) -> erlang:error('empty fifo');

pop({fifo, In, []}) -> pop({fifo, [], lists:reverse(In)});

pop({fifo, In, [H|T]}) -> {H, {fifo, In, T}}.

empty({fifo, [], []}) -> true;

empty({fifo, _, _}) -> false.

</lang>