In the previous article [here], I wrote about namespaces and how - as our program grows - we need to keep named pieces of code and globally accessible variable in their own groups. And we do so using a structure that's very much like family names and forenames. As our program grows, we also need to separate out various sections of the code into different files, so they can be maintained by a different person, perhaps on a different maintainance cycle, and so that common code can be called into severl programs without it being duplicated.

So every language that we teach (there's a full list of our public courses here) has some sort of capability to load code from multiple places into a single program. In Tcl, separate loading can be done on an adhoc basis via source or more flexibly and formalised via package require.

The package mechanism works as follows.

1. In the script file which contains the code for the package, use a package provide - for example:
  package provide table 1.0

2. Provide an index file in the folder in which the package is stored, called pkgIndex.tcl. In this file will be a whole load of statements which define which packages are held in which file in the directory. You could write it manually, but plese don't - there's a Tcl command called pkg_mkIndex to do it for you - and you run it in a Tcl shell as follows:
  pkg_mkIndex . *.tcl

3. In your program which requires the package, add the directory in which the package may by found to the variable called auto_path ...
  lappend auto_path furniture

4. And (finally) package require the package:
  package require table

There are complete source files from yesterday's training on our site - [the package code] and [a calling program]. You'll note that the package file defines a namesapce. Although namespaces and packages are different facilities, its natural to use them together, with a package loading in a whole bundle of pieces of code neatly wrapped in a namespace.
(written 2011-09-03)

 T213 - Tcl/Tk - Libraries, Packages and Namespaces
  [3417] What is a namespace and why do we need them? - (2011-09-03)
  [3066] Separating groups of variables into namespaces - (2010-11-24)
  [2476] Tcl - uplevel to run code at calling level - (2009-10-24)
  [1529] Tcl - learning how to use namespaces, packages and libraries together - (2008-02-02)
  [1339] Packages and Namespaces are like Suitcases and Aircraft - (2007-09-09)
  [146] example of Tcl namespaces and packages - (2004-12-10)

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Forward to Data that we use during our training courses, and other training resources

Assigning values to variables within other statements - Ruby
Ruby off the Rails?
Making best use of the new enthusiasm for Melksham
Data that we use during our training courses, and other training resources
Tcl packages, pkg_mkIndex, pkgIndex.tcl -what are they and why use them.
Storing Tcl source code encoded, and running via your own C program
User defined sorting and other uses of callbacks in Tcl and Tk
Passing back multiple results in Tcl - upvar and uplevel
If its Sunday, must it be Weymouth?