Today’s Erlang Thursday digs a little into the queue module, and we cover queue:cons/2 from the Okasaki API.

queue:cons/2 takes a item and a queue, and will return a new queue with the item at the head of the queue.

queue:cons(7, queue:new()).
% {[],[7]}
queue:cons(3, queue:cons(7, queue:new())).
% {[7],[3]}
queue:cons(nil, queue:new()).
% {[],[nil]}
queue:cons(5, queue:from_list([7, 9, 13, 21])).
% {[21],[5,7,9,13]}

If we try to pass a list in to queue:cons/2, we see that it does want a queue, and will not do an implicit conversion of a list to a queue.

queue:cons(5, [1, 2, 3, 4]).
% ** exception error: bad argument
%      in function  queue:in_r/2
%         called as queue:in_r(5,[1,2,3,4])

As the queue is setup to be a double ended queue, the Okasaki API also provides a counter function queue:snoc/2, that adds an item to the tail of the queue passed in. Note that the argument order is swapped between queue:snoc/2 and queue:cons/2; queue:snoc/2 takes the queue as the first argument, and the item to add at the tail as the second argument.

queue:snoc(queue:new(), 5).
% {[5],[]}
queue:snoc(queue:from_list([7]), 5).
% {[5],[7]}
queue:snoc(queue:snoc(queue:new(), 7), 5).
% {[5],[7]}
queue:snoc(queue:from_list([7, 9, 13, 21]), 5).
% {[5,21],[7,9,13]}

–Proctor