The functions in this section are provided by
(use-modules (ice-9 q))
This module implements queues holding arbitrary scheme objects and designed for efficient first-in / first-out operations.
make-q creates a queue, and objects are entered and removed
with enq! and deq!. q-push! and q-pop!
can be used too, treating the front of the queue like a stack.
Return a new queue.
Return #t if obj is a queue, or #f if not.
Note that queues are not a distinct class of objects but are
implemented with cons cells. For that reason certain list structures
can get #t from q?.
Add obj to the rear of q, and return q.
Remove and return the front element from q. If q is
empty, a q-empty exception is thrown.
deq! and q-pop! are the same operation, the two names
just let an application match enq! with deq!, or
q-push! with q-pop!.
Add obj to the front of q, and return q.
Return the number of elements in q.
Return true if q is empty.
Throw a q-empty exception if q is empty.
Return the first element of q (without removing it). If q
is empty, a q-empty exception is thrown.
Return the last element of q (without removing it). If q
is empty, a q-empty exception is thrown.
Remove all occurrences of obj from q, and return q.
obj is compared to queue elements using eq?.
The q-empty exceptions described above are thrown just as
(throw 'q-empty), there’s no message etc like an error throw.
A queue is implemented as a cons cell, the car containing a
list of queued elements, and the cdr being the last cell in
that list (for ease of enqueuing).
(list . last-cell)
If the queue is empty, list is the empty list and
last-cell is #f.
An application can directly access the queue list if desired, for instance to search the elements or to insert at a specific point.
Recompute the last-cell field in q.
All the operations above maintain last-cell as described, so
normally there’s no need for sync-q!. But if an application
modifies the queue list then it must either maintain
last-cell similarly, or call sync-q! to recompute it.