In today’s Erlang Thursday we continue with the queue module’s Okasaki API, and look at queue:tail/1.

queue:tail/1 takes a non-empty queue as its argument, and returns a new queue with the first element removed.

Queue = queue:from_list([1, 2, 3, 4, 5]).
% {[5,4],[1,2,3]}
Tail = queue:tail(Queue).
% {[5,4],[2,3]}
Queue.
% {[5,4],[1,2,3]}
queue:head(Tail).
% 2
queue:to_list(Tail).
% [2,3,4,5]

We can see above that calling queue:tail/1 is not a destructive operation as might happen in other languages, and does indeed leave the original queue intact.

As part of the Okasaki API, which treats a queue as a double ended, queue:tail/1 has a counterpart function queue:liat/1 which will return a new queue with last item removed. queue:liat/1 also has an alias in the Okasaki API of queue:init/1.

queue:liat(Queue).
% {[4],[1,2,3]}
queue:init(Queue).
% {[4],[1,2,3]}
Queue.
% {[5,4],[1,2,3]}

Note that the Erlang documentation also shows that there is an alias queue:lait/1 which it points out should not be used because it is a misspelling.

And because we want to try to break things and see what we can learn, let’s try to call the different tail functions we have covered so far with an empty queue to see what happens.

EmptyQueue = queue:new().
% {[],[]}
queue:tail(EmptyQueue).
% ** exception error: empty
%      in function  queue:drop/1
%         called as queue:drop({[],[]})
queue:liat(EmptyQueue).
% ** exception error: empty
%      in function  queue:drop_r/1
%         called as queue:drop_r({[],[]})
queue:init(EmptyQueue).
% ** exception error: empty
%      in function  queue:drop_r/1
%         called as queue:drop_r({[],[]})

Looks like we get exception errors in queue:drop/1 and queue:drop_r/1 when we call queue:tail/1 and queue:liat/1 respectively.

And when we look at the behavior of queue:drop/1 and queue:drop_r/1 with a queue with items in it, it looks like queue:tail/1 is just an alias for queue:drop/1, and queue:liat/1 and queue:init/1 are just aliases for queue:drop_r/1.

queue:drop(Queue).
% {[5,4],[2,3]}
queue:drop_r(Queue).
{[4],[1,2,3]}

–Proctor