- package provide package ?version?
In response to a question about finding out versions of Tcl or Expect, Donald Porter writes:
brentnye <[email protected]> wrote: > There is a tcl command to find the version of tcl, i.e. > info tclversionIck. Don't use that. And don't use the even worse $::tcl_version. Use [package provide Tcl]. (RS uses most often "info pa" for the added detail). Several people have asked why I recommend [package provide Tcl] rather than [package require Tcl]. In the particular example of the Tcl package, it really doesn't matter, because the Tcl package is (practically) always provided.However, I wanted the example to generalize cleanly to other packages. [package provide foo] and [package require foo] differ in how they react when no foo package is currently in the interpreter. [package provide foo] returns an empty string, indicating no version of foo is provided. [package require foo], however, tries to go find the package foo and bring it into the interpreter. This is no longer introspection, but an action. Also, if [package require foo] fails to find foo among the installed packages, it will raise an error. The better choice for simple introspection is [package provide foo].To add yet another wrinkle, [package present foo] occupies a middle ground. Unlike [package require foo], it will not attempt to load foo, but like [package require foo], it will raise an error if no package foo is provided in the interp. I still prefer [package provide foo] for introspection because I don't have to [catch] it. (Although I do have to be careful about passing its return value to another command expecting a version number.)CJU - Forgive my skepticism, but I notice that you've provided an argument for [package provide] but nothing in particular against [info tclversion]. (I found the original thread mentioned above and searched the wiki to try to find an explanation but came up empty-handed.) What's so wrong with [info tclversion]? I, like so many newbies, would assume upon seeing [package provide] that the author of the code is trying to provide a package rather than just checking a version number. If I were to use [package provide] in this manner, I would always feel the need to add a comment to explain this non-intuitive use of the command. I'm sure you know what you're doing, but an explanation would greatly help before discarding the much more readable [info tclversion]. Additionally, the Tcl Usage FAQ  (question Q.B21) suggests either one of $tcl_version or [info tclversion], with no mention of [package]. The FAQ should be changed if neither of these are the optimal solutions.DGP 2004 Feb 16 I haven't read the Tcl Usage FAQ in more than 5 years. Has anyone? If it does not mention [package], then it probably has not been updated since Tcl 7.4. Sorry, but it just isn't part of my Tcl world.I think of Tcl as one of possibly many packages -- that is collections of commands following a documented interface. From that perspective, [package provide Tcl] is the specific instance of the general tool [package provide pkgName] to ask which version of the interface is being provided.