queue (stdlib v3.15.2)

This module provides (double-ended) FIFO queues in an efficient manner.

All functions fail with reason badarg if arguments are of wrong type, for example, queue arguments are not queues, indexes are not integers, and list arguments are not lists. Improper lists cause internal crashes. An index out of range for a queue also causes a failure with reason badarg.

Some functions, where noted, fail with reason empty for an empty queue.

The data representing a queue as used by this module is to be regarded as opaque by other modules. Any code assuming knowledge of the format is running on thin ice.

All operations have an amortized O(1) running time, except all/2, any/2, delete/2, delete_r/2, delete_with/2, delete_with_r/2, filter/2, filtermap/2, fold/3, join/2, len/1, member/2, split/2 that have O(n). To minimize the size of a queue minimizing the amount of garbage built by queue operations, the queues do not contain explicit length information, and that is why len/1 is O(n). If better performance for this particular operation is essential, it is easy for the caller to keep track of the length.

Queues are double-ended. The mental picture of a queue is a line of people (items) waiting for their turn. The queue front is the end with the item that has waited the longest. The queue rear is the end an item enters when it starts to wait. If instead using the mental picture of a list, the front is called head and the rear is called tail.

Entering at the front and exiting at the rear are reverse operations on the queue.

This module has three sets of interface functions: the "Original API", the "Extended API", and the "Okasaki API".

The "Original API" and the "Extended API" both use the mental picture of a waiting line of items. Both have reverse operations suffixed "_r".

The "Original API" item removal functions return compound terms with both the removed item and the resulting queue. The "Extended API" contains alternative functions that build less garbage and functions for just inspecting the queue ends. Also the "Okasaki API" functions build less garbage.

The "Okasaki API" is inspired by "Purely Functional Data Structures" by Chris Okasaki. It regards queues as lists. This API is by many regarded as strange and avoidable. For example, many reverse operations have lexically reversed names, some with more readable but perhaps less understandable aliases.

Link to this section Summary

Types

-type queue() :: term().

-type queue(Arg1) :: term().

As returned by new/0.

Functions

all/2

Returns true if Pred(Item) returns true for all items Item in Q, otherwise false.

any/2

Returns true if Pred(Item) returns true for at least one item Item in Q, otherwise false.

cons/2

Inserts Item at the head of queue Q1. Returns the new queue Q2.

daeh/1

Returns the tail item of queue Q.

delete/2

Returns a copy of Q1 where the first item matching Item is deleted, if there is such an item.

delete_r/2

Returns a copy of Q1 where the last item matching Item is deleted, if there is such an item.

delete_with/2

Returns a copy of Q1 where the first item for which Pred returns true is deleted, if there is such an item.

delete_with_r/2

Returns a copy of Q1 where the last item for which Pred returns true is deleted, if there is such an item.

drop/1

Returns a queue Q2 that is the result of removing the front item from Q1.

drop_r/1

Returns a queue Q2 that is the result of removing the rear item from Q1.

filter/2

Returns a queue Q2 that is the result of calling Fun(Item) on all items in Q1.

filtermap/2

Returns a queue Q2 that is the result of calling Fun(Item) on all items in Q1.

fold/3

Calls Fun(Item, AccIn) on successive items Item of Queue, starting with AccIn == Acc0. The queue is traversed in queue order, that is, from front to rear. Fun/2 must return a new accumulator, which is passed to the next call. The function returns the final value of the accumulator. Acc0 is returned if the queue is empty.

from_list/1

Returns a queue containing the items in L in the same order; the head item of the list becomes the front item of the queue.

get/1

Returns Item at the front of queue Q.

get_r/1

Returns Item at the rear of queue Q.

head/1

Returns Item from the head of queue Q.

in/2

Inserts Item at the rear of queue Q1. Returns the resulting queue Q2.

in_r/2

Inserts Item at the front of queue Q1. Returns the resulting queue Q2.

init/1

Returns a queue Q2 that is the result of removing the tail item from Q1.

is_empty/1

Tests if Q is empty and returns true if so, otherwise false.

is_queue/1

Tests if Term is a queue and returns true if so, otherwise false.

join/2

Returns a queue Q3 that is the result of joining Q1 and Q2 with Q1 in front of Q2.

lait/1 deprecated

Returns a queue Q2 that is the result of removing the tail item from Q1.

last/1

Returns the tail item of queue Q.

len/1

Calculates and returns the length of queue Q.

liat/1

Returns a queue Q2 that is the result of removing the tail item from Q1.

member/2

Returns true if Item matches some element in Q, otherwise false.

new/0

Returns an empty queue.

out/1

Removes the item at the front of queue Q1. Returns tuple {{value, Item}, Q2}, where Item is the item removed and Q2 is the resulting queue. If Q1 is empty, tuple {empty, Q1} is returned.

out_r/1

Removes the item at the rear of queue Q1. Returns tuple {{value, Item}, Q2}, where Item is the item removed and Q2 is the new queue. If Q1 is empty, tuple {empty, Q1} is returned.

peek/1

Returns tuple {value, Item}, where Item is the front item of Q, or empty if Q is empty.

peek_r/1

Returns tuple {value, Item}, where Item is the rear item of Q, or empty if Q is empty.

reverse/1

Returns a queue Q2 containing the items of Q1 in the reverse order.

snoc/2

Inserts Item as the tail item of queue Q1. Returns the new queue Q2.

split/2

Splits Q1 in two. The N front items are put in Q2 and the rest in Q3.

tail/1

Returns a queue Q2 that is the result of removing the head item from Q1.

to_list/1

Returns a list of the items in the queue in the same order; the front item of the queue becomes the head of the list.

Link to this section Types

-type queue() :: term().

Specs

queue() :: queue(_).

-type queue(Arg1) :: term().

(opaque)

Specs

queue(Item)

As returned by new/0.

Link to this section Functions

all/2

(since OTP 24.0)

Specs

all(Pred, Q :: queue(Item)) -> boolean() when Pred :: fun((Item) -> boolean()).

Returns true if Pred(Item) returns true for all items Item in Q, otherwise false.

any/2

(since OTP 24.0)

Specs

any(Pred, Q :: queue(Item)) -> boolean() when Pred :: fun((Item) -> boolean()).

Returns true if Pred(Item) returns true for at least one item Item in Q, otherwise false.

cons/2

Specs

cons(Item, Q1 :: queue(Item)) -> Q2 :: queue(Item).

Inserts Item at the head of queue Q1. Returns the new queue Q2.

daeh/1

Specs

daeh(Q :: queue(Item)) -> Item.

Returns the tail item of queue Q.

Fails with reason empty if Q is empty.

delete/2

(since OTP 24.0)

Specs

delete(Item, Q1) -> Q2 when Item :: T, Q1 :: queue(T), Q2 :: queue(T), T :: term().

Returns a copy of Q1 where the first item matching Item is deleted, if there is such an item.

delete_r/2

(since OTP 24.0)

Specs

delete_r(Item, Q1) -> Q2 when Item :: T, Q1 :: queue(T), Q2 :: queue(T), T :: term().

Returns a copy of Q1 where the last item matching Item is deleted, if there is such an item.

delete_with/2

(since OTP 24.0)

Specs

delete_with(Pred, Q1) -> Q2
               when
                   Pred :: fun((Item) -> boolean()),
                   Q1 :: queue(Item),
                   Q2 :: queue(Item),
                   Item :: term().

Returns a copy of Q1 where the first item for which Pred returns true is deleted, if there is such an item.

delete_with_r/2

(since OTP 24.0)

Specs

delete_with_r(Pred, Q1) -> Q2
                 when
                     Pred :: fun((Item) -> boolean()),
                     Q1 :: queue(Item),
                     Q2 :: queue(Item),
                     Item :: term().

Returns a copy of Q1 where the last item for which Pred returns true is deleted, if there is such an item.

drop/1

Specs

drop(Q1 :: queue(Item)) -> Q2 :: queue(Item).

Returns a queue Q2 that is the result of removing the front item from Q1.

Fails with reason empty if Q1 is empty.

drop_r/1

Specs

drop_r(Q1 :: queue(Item)) -> Q2 :: queue(Item).

Returns a queue Q2 that is the result of removing the rear item from Q1.

Fails with reason empty if Q1 is empty.

filter/2

Specs

filter(Fun, Q1 :: queue(Item)) -> Q2 :: queue(Item) when Fun :: fun((Item) -> boolean() | [Item]).

Returns a queue Q2 that is the result of calling Fun(Item) on all items in Q1.

If Fun(Item) returns true, Item is copied to the result queue. If it returns false, Item is not copied. If it returns a list, the list elements are inserted instead of Item in the result queue.

So, Fun(Item) returning [Item] is thereby semantically equivalent to returning true, just as returning [] is semantically equivalent to returning false. But returning a list builds more garbage than returning an atom.

filtermap/2

(since OTP 24.0)

Specs

filtermap(Fun, Q1) -> Q2
             when
                 Fun :: fun((Item) -> boolean() | {true, Value}),
                 Q1 :: queue(Item),
                 Q2 :: queue(Item | Value),
                 Item :: term(),
                 Value :: term().

Returns a queue Q2 that is the result of calling Fun(Item) on all items in Q1.

If Fun(Item) returns true, Item is copied to the result queue. If it returns false, Item is not copied. If it returns {true, NewItem}, the queue element at this position is replaced with NewItem in the result queue.

fold/3

(since OTP 24.0)

Specs

fold(Fun, Acc0, Q :: queue(Item)) -> Acc1
        when
            Fun :: fun((Item, AccIn) -> AccOut),
            Acc0 :: term(),
            Acc1 :: term(),
            AccIn :: term(),
            AccOut :: term().

Example:

> queue:fold(fun(X, Sum) -> X + Sum end, 0, queue:from_list([1,2,3,4,5])).
15
> queue:fold(fun(X, Prod) -> X * Prod end, 1, queue:from_list([1,2,3,4,5])).
120

from_list/1

Specs

from_list(L :: [Item]) -> queue(Item).

Returns a queue containing the items in L in the same order; the head item of the list becomes the front item of the queue.

get/1

Specs

get(Q :: queue(Item)) -> Item.

Returns Item at the front of queue Q.

Fails with reason empty if Q is empty.

get_r/1

Specs

get_r(Q :: queue(Item)) -> Item.

Returns Item at the rear of queue Q.

Fails with reason empty if Q is empty.

head/1

Specs

head(Q :: queue(Item)) -> Item.

Returns Item from the head of queue Q.

Fails with reason empty if Q is empty.

in/2

Specs

in(Item, Q1 :: queue(Item)) -> Q2 :: queue(Item).

Inserts Item at the rear of queue Q1. Returns the resulting queue Q2.

in_r/2

Specs

in_r(Item, Q1 :: queue(Item)) -> Q2 :: queue(Item).

Inserts Item at the front of queue Q1. Returns the resulting queue Q2.

init/1

Specs

init(Q1 :: queue(Item)) -> Q2 :: queue(Item).

Returns a queue Q2 that is the result of removing the tail item from Q1.

Fails with reason empty if Q1 is empty.

is_empty/1

Specs

is_empty(Q :: queue()) -> boolean().

Tests if Q is empty and returns true if so, otherwise false.

is_queue/1

Specs

is_queue(Term :: term()) -> boolean().

Tests if Term is a queue and returns true if so, otherwise false.

join/2

Specs

join(Q1 :: queue(Item), Q2 :: queue(Item)) -> Q3 :: queue(Item).

Returns a queue Q3 that is the result of joining Q1 and Q2 with Q1 in front of Q2.

lait/1

This function is deprecated. queue:lait/1 is deprecated; use queue:liat/1 instead.

Specs

lait(Q1 :: queue(Item)) -> Q2 :: queue(Item).

Returns a queue Q2 that is the result of removing the tail item from Q1.

Fails with reason empty if Q1 is empty.

The name lait/1 is a misspelling - do not use it anymore.

last/1

Specs

last(Q :: queue(Item)) -> Item.

Returns the tail item of queue Q.

Fails with reason empty if Q is empty.

len/1

Specs

len(Q :: queue()) -> non_neg_integer().

Calculates and returns the length of queue Q.

liat/1

Specs

liat(Q1 :: queue(Item)) -> Q2 :: queue(Item).

Returns a queue Q2 that is the result of removing the tail item from Q1.

Fails with reason empty if Q1 is empty.

member/2

Specs

member(Item, Q :: queue(Item)) -> boolean().

Returns true if Item matches some element in Q, otherwise false.

new/0

Specs

new() -> queue().

Returns an empty queue.

out/1

Specs

out(Q1 :: queue(Item)) -> {{value, Item}, Q2 :: queue(Item)} | {empty, Q1 :: queue(Item)}.

Removes the item at the front of queue Q1. Returns tuple {{value, Item}, Q2}, where Item is the item removed and Q2 is the resulting queue. If Q1 is empty, tuple {empty, Q1} is returned.

out_r/1

Specs

out_r(Q1 :: queue(Item)) -> {{value, Item}, Q2 :: queue(Item)} | {empty, Q1 :: queue(Item)}.

Removes the item at the rear of queue Q1. Returns tuple {{value, Item}, Q2}, where Item is the item removed and Q2 is the new queue. If Q1 is empty, tuple {empty, Q1} is returned.

peek/1

Specs

peek(Q :: queue(Item)) -> empty | {value, Item}.

Returns tuple {value, Item}, where Item is the front item of Q, or empty if Q is empty.

peek_r/1

Specs

peek_r(Q :: queue(Item)) -> empty | {value, Item}.

Returns tuple {value, Item}, where Item is the rear item of Q, or empty if Q is empty.

reverse/1

Specs

reverse(Q1 :: queue(Item)) -> Q2 :: queue(Item).

Returns a queue Q2 containing the items of Q1 in the reverse order.

snoc/2

Specs

snoc(Q1 :: queue(Item), Item) -> Q2 :: queue(Item).

Inserts Item as the tail item of queue Q1. Returns the new queue Q2.

split/2

Specs

split(N :: non_neg_integer(), Q1 :: queue(Item)) -> {Q2 :: queue(Item), Q3 :: queue(Item)}.

Splits Q1 in two. The N front items are put in Q2 and the rest in Q3.

tail/1

Specs

tail(Q1 :: queue(Item)) -> Q2 :: queue(Item).

Returns a queue Q2 that is the result of removing the head item from Q1.

Fails with reason empty if Q1 is empty.

to_list/1

Specs

to_list(Q :: queue(Item)) -> [Item].

Returns a list of the items in the queue in the same order; the front item of the queue becomes the head of the list.