'''package present''' ''?-'''''exact'''''? package ?version?''

"This command is equivalent to '''[package require]''' except that it does not try and load the [package] if it is not already loaded."
Its canonical description appears in the [man] page
[http://www.tcl.tk/man/tcl8.4/TclCmd/package.htm#M8].
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[etdxc] Sorry for being so literal, but I presume this means that it tries to load the package only when it is not already loaded.

[CL] agrees that the man page is unsatisfying.  I submit this alternative:
   * If '''package''' is loaded, the command returns its version;
   * If '''package''' is not loaded, the command branches to '''error "package $package not present"'''.

[LV] Here's an example:
 $ tclsh
 % package names
 Tcl
 % package present Tk
 package Tk is not present
 % package require Tk
 8.5
 % package present Tk
 8.5
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[DGP] has been heard to say, "the one place I find [[package present]] useful as
defined is in the [script] portion of a package that has both [C] and script portions.
I have the C initialization call Tcl_PkgProvide(),
and the script part does [[package present -exact Foo 1.2.3]] to verify it got loaded in a C part from the same release. 
[[This]] reports possible install errors."
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To use [[package present]] in a script, it is useful to enclose it in a catch:

 proc is_Tk_present {} {
   expr ![catch {package present Tk}]
 }
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[EF] This means that if you want to (dynamically) know all the packages that a script has loaded in, you can do the following:
    foreach n [package names] {
	if { ! [catch {package present $n} v] } {
	    puts $n
	}
    }
[DKF]: Maybe this would be simpler (and more useful, since it prints the version number too):
 foreach n [package names] {
    catch {puts "$n [package present $n]"}
 }
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See also:

   * [package]
   * [package provide]
   * [package require]

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