Documentation can be found at http://tcllib.sourceforge.net/doc/queue.html .

This module of the [tcllib] data [struct]ures collection provides
the data structure portion of a queue.


See [Stacks and Queues] for a description about what a queue is.

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[SEH] 10 Dec 04 -- I have found out to my regret that it is not really possible to use [[queue]] in loop structures, because the "peek" and "get" commands acts differently when the queue size is 1 than when it is any number > 1.

It would be nice to be able to do something like:

 foreach item [$queue peek [$queue size]] {<do stuff>}

but if [[$queue size]] is 1, then the value is returned as a string value, whereas if it's 2, the values are returned as a list.  So you can't use the above construction unless you know all queue values will look the same as strings or lists.  For example:

 %set q [struct::queue]
 %$q put "a b"
 %foreach item [$q peek [$q size]] {puts $item}
 a
 b
 %$q put "c d"
 %foreach item [$q peek [$q size]] {puts $item}
 a b
 c d

This behavior is documented and hence not actually a bug, but as a practical matter it makes the [[queue]] structure useless to me.

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[schlenk] I think the original queue author thought of that as a convenience. Maybe he should just add an lpeek subcommand that does what you want.

This should implement it:
 proc ::struct::queue::_lpeek {name {count 1}} {
    variable queues
    if { $count < 1 } {
	error "invalid item count $count"
    }

    if { $count > [llength $queues($name)] } {
	error "insufficient items in queue to fill request"
    }

    set index [expr {$count - 1}]
    return [lrange $queues($name) 0 $index]
 }

(btw. the current queue implementation is smart enough so you just define this after package require ::struct::queue and have lpeek working for your code, but its a hack nontheless)

[SEH] -- Nice!  I nominate this for inclusion into [tcllib].

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[[
[Arts and Crafts of Tcl-Tk Programming] |
[Category Command], package [tcllib], module [struct] |
[Category Data Structure] |
]]