[NEM] ''26 Aug 2006'': [C#] has a nice little construct for automatically
closing a resource once you are finished with it. Here's a Tcl version
of this ''using'' command:
======
proc using {varName chan script} {
    upvar 1 $varName var
    set var $chan
    set rc [catch { uplevel 1 $script } result]
    catch { close $chan }
    return -code $rc $result
}
======
Which you can then use like:
======
using fd [open somefile.txt] { puts [read $fd] }
======
or
======
foreach host $hosts {
    using sock [socket $host $port] { ... do stuff on $host ... }
}
======

See also [Playing with with] and [withOpenFile].

----
<<discussion>>
[RS] I'm not so happy with the name ''using'': it means other things in other languages (like with packages or namespaces) and does not indicate that it's for channels. Something like [LISP]'s WITH-OPEN-FILE might be clearer to understand :)

[KPV] Actually the [C#] construct is quite useful and not just for
files but for many types of objects which grab some scarce resource.
(Technically, it works with any object which implements the IDispose
interface).

[NEM]: Yes, the name is more appropriate in C# where it refers to any
kind of (disposeable) resource. I limited it to channels in Tcl for simplicity. 
Despite the potential ambiguity of the name, in actual usage it seems pretty
clear to me. Ideally, you could put it in the [chan] ensemble.

----
[CMcC] 2007-1-13: [jcw]'s [oomk] interface to [metakit] has a similar concept for scoping.  the '''as''' method on some objects constructs an instance and assigns its name to a variable in the calling namespace.  Using [trace], the lifetime of the constructed instance is associated with that of the scoped variable: when the [unset] [trace] fires, the object's destructor is called.  Thought it might be relevant to mention.

----
[AMG]: This reminds me of the [sqlite] '''transaction''' subcommand: [http://www.sqlite.org/tclsqlite.html#transaction].  "The transaction is committed when the script completes, or it rolls back if the script fails."

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