View Source queue (stdlib v6.0)

Abstract data type for FIFO queues.

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. In abstract terms, the representation is a composite type of existing Erlang terms. See note on data types. 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.

Summary

Types

As returned by new/0.

Original API

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

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

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

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

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

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

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

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

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.

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.

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

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

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

Tests if Term is a queue and returns true if so, otherwise false. Note that the test will return true for a term coinciding with the representation of a queue, even when not constructed by thus module. See also note on data types.

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

Calculates and returns the length of queue Q.

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

Returns an empty queue.

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.

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.

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

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

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.

Extended API

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

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

Returns Item at the front of queue Q.

Returns Item at the rear of queue Q.

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

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

Okasaki API

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

Returns the tail item of queue Q.

Returns Item from the head of queue Q.

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

lait(Q1) deprecated

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

Returns the tail item of queue Q.

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

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

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

Types

-type queue() :: queue(_).
-opaque queue(Item)

As returned by new/0.

Original API

Link to this function

all(Pred, Q)

View Source (since OTP 24.0)
-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
2> queue:all(fun (E) -> E > 3 end, Queue).
false
3> queue:all(fun (E) -> E > 0 end, Queue).
true
Link to this function

any(Pred, Q)

View Source (since OTP 24.0)
-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
2> queue:any(fun (E) -> E > 10 end, Queue).
false
3> queue:any(fun (E) -> E > 3 end, Queue).
true
Link to this function

delete(Item, Q1)

View Source (since OTP 24.0)
-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
2> Queue1 = queue:delete(3, Queue).
3> queue:member(3, Queue1).
false
Link to this function

delete_r(Item, Q1)

View Source (since OTP 24.0)
-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,3,5]).
2> Queue1 = queue:delete_r(3, Queue).
3> queue:to_list(Queue1).
[1,2,3,4,5]
Link to this function

delete_with(Pred, Q1)

View Source (since OTP 24.0)
-spec 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.

Example:

1> Queue = queue:from_list([100,1,2,3,4,5]).
2> Queue1 = queue:delete_with(fun (E) -> E > 0, Queue).
3> queue:to_list(Queue1).
[1,2,3,4,5]
Link to this function

delete_with_r(Pred, Q1)

View Source (since OTP 24.0)
-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5,100]).
2> Queue1 = queue:delete_with(fun (E) -> E > 10, Queue).
3> queue:to_list(Queue1).
[1,2,3,4,5]
-spec 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.

Example 1:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:filter(fun (E) -> E > 2 end, Queue).
{[5],[3,4]}
3> queue:to_list(Queue1).
[3,4,5]

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.

Example 2:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:filter(fun (E) -> [E, E+1] end, Queue).
{[6,5,5,4,4,3],[1,2,2,3]}
3> queue:to_list(Queue1).
[1,2,2,3,3,4,4,5,5,6]
Link to this function

filtermap(Fun, Q1)

View Source (since OTP 24.0)
-spec 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.

Example 1:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:filtermap(fun (E) -> E > 2 end, Queue).
{[5],[3,4]}
3> queue:to_list(Queue1).
[3,4,5]
4> Queue1 = queue:filtermap(fun (E) -> {true, E+100} end, Queue).
{"ihg","ef"}
5> queue:to_list(Queue1).
"efghi
Link to this function

fold(Fun, Acc0, Q)

View Source (since OTP 24.0)
-spec fold(Fun, Acc0, Q :: queue(Item)) -> Acc1
        when
            Fun :: fun((Item, AccIn) -> AccOut),
            Acc0 :: term(),
            Acc1 :: term(),
            AccIn :: term(),
            AccOut :: term().

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.

Example:

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

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

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

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:in(100, Queue).
{[100,5,4,3],[1,2]}
3> queue:to_list(Queue1).
[1,2,3,4,5,100]
-spec in_r(Item, Q1 :: queue(Item)) -> Q2 :: queue(Item).

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

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue1 = queue:in_r(100, Queue).
{[5,4,3],[100,1,2]}
3> queue:to_list(Queue1).
[100,1,2,3,4,5]
-spec is_empty(Q :: queue()) -> boolean().

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

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

Tests if Term is a queue and returns true if so, otherwise false. Note that the test will return true for a term coinciding with the representation of a queue, even when not constructed by thus module. See also note on data types.

-spec 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.

Example:

1> Queue1 = queue:from_list([1,3]).
{[3],[1]}
2> Queue2 = queue:from_list([2,4]).
{[4],[2]}
3> queue:to_list(queue:join(Queue1, Queue2)).
[1,3,2,4]
-spec len(Q :: queue()) -> non_neg_integer().

Calculates and returns the length of queue Q.

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

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

-spec new() -> queue(none()).

Returns an empty queue.

-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> {{value, 1=Item}, Queue1} = queue:out(Queue).
{{value,1},{[5,4,3],[2]}}
3> queue:to_list(Queue1).
[2,3,4,5]
-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> {{value, 5=Item}, Queue1} = queue:out_r(Queue).
{{value,5},{[4,3],[1,2]}}
3> queue:to_list(Queue1).
[1,2,3,4]
-spec reverse(Q1 :: queue(Item)) -> Q2 :: queue(Item).

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

-spec 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.

-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> List == queue:to_list(Queue).
true

Extended API

-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue = queue:drop(Queue).
{[5,4,3],[2]}
3> queue:to_list(Queue1).
[2,3,4,5]
-spec 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.

Example:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> Queue = queue:drop_r(Queue).
{[4,3],[1,2]}
3> queue:to_list(Queue1).
[1,2,3,4]
-spec get(Q :: queue(Item)) -> Item.

Returns Item at the front of queue Q.

Fails with reason empty if Q is empty.

Example 1:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> 1 == queue:get(Queue).
true
-spec get_r(Q :: queue(Item)) -> Item.

Returns Item at the rear of queue Q.

Fails with reason empty if Q is empty.

Example 1:

1> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
2> 5 == queue:get_r(Queue).
true
-spec 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.

Example 1:

1> queue:peek(queue:new()).
empty
2> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
3> queue:peek(Queue).
{value, 1}
-spec 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.

Example 1:

1> queue:peek_r(queue:new()).
empty
2> Queue = queue:from_list([1,2,3,4,5]).
{[5,4,3],[1,2]}
3> queue:peek_r(Queue).
{value, 5}

Okasaki API

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

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

Example:

1> Queue = queue:cons(0, queue:from_list([1,2,3])).
{[3,2],[0,1]}
2> queue:to_list(Queue).
[0,1,2,3]
-spec daeh(Q :: queue(Item)) -> Item.

Returns the tail item of queue Q.

Fails with reason empty if Q is empty.

Example 1:

1> queue:daeh(queue:from_list([1,2,3])).
3
-spec head(Q :: queue(Item)) -> Item.

Returns Item from the head of queue Q.

Fails with reason empty if Q is empty.

Example 1:

1> queue:head(queue:from_list([1,2,3])).
1
-spec 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.

Example:

1> Queue = queue:init(queue:from_list([1,2,3])).
{[2],[1]}
2> queue:to_list(Queue).
[1,2]
This function is deprecated. queue:lait/1 is deprecated; use queue:liat/1 instead.
-spec 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.

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

Returns the tail item of queue Q.

Fails with reason empty if Q is empty.

Example:

1> queue:last(queue:from_list([1,2,3])).
3
-spec 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.

Example:

1> Queue = queue:liat(queue:from_list([1,2,3])).
{[2],[1]}
2> queue:to_list(Queue).
[1,2]
-spec snoc(Q1 :: queue(Item), Item) -> Q2 :: queue(Item).

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

Example:

1> Queue = queue:snoc(queue:from_list([1,2,3]), 4).
{[4,3,2],[1]}
2> queue:to_list(Queue).
[1,2,3,4]
-spec 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.