A beginner example to start with package on TCL : HelloWorld

Creation of the package

To create a package with TCL, you need two files :

• the package file which contain your code
• the index package file to declare your package

The package file

A package use the notion of namespace to avoid name collision. This happens when two pieces of code try to define a procedure or variable with the same name. In Tcl when this occurs the old procedure or variable is simply overwritten. This is sometimes a useful feature, but more often it is the cause of bugs if the two definitions are not compatible.

Each namespace can contain :

• local commands
• local variables

When a command in a namespace is invoked it can see all the other commands and variables in its namespace, as well as those in the global namespace.

Items in a namespace can be accessed by creating a path to the item. This is done by joining the names of the items with ::. For instance, to access the variable bar in the namespace foo, you could use the path foo::bar.

The command “namespace export pattern pattern …” adds any procedure matching one of the patterns to the list of commands.

Example :

# Create the namespace
namespace eval ::HelloWorld {
    	
	# Export proc
    	namespace export Talk
	
	# Set up Variable
	set version 1.0
	set HelloWorldDescription "HelloWorld"

        # Variable for the path of the script
        variable home [file join [pwd] [file dirname [info script]]]

}

# Definition of the procedure talk
proc ::HelloWorld::Talk {} {
	puts $HelloWorld::HelloWorldDescription
}
# Definition of the procedure getpath
proc ::HelloWorld::GetPath {} {
	variable home
        return $home
}
package provide HelloWorld $HelloWorld::version
package require Tcl      8.0

The index package file

This file tells Tcl how to load your package. In essence the index file is simply a Tcl file which is loaded into the interpreter when Tcl searches for packages. It should use the package ifneeded command register a script which will load the package when it is required.

The index package must be :

• named : pkgIndex.tcl
• located in the directories define by the tcl_pkgPath or auto_path global variables list.

package ifneeded HelloWorld 1.0 [list source [file join $dir helloworld.tcl]]
package ifneeded OtherPackage 1.0 [list source [file join $dir otherpackage.tcl]]

OMB+> package ifneeded Pk_ExternalTable 1.0
source C:/EclipseWorkspace/OMB/Package_ExternalTableProcedures.tcl

The main script

All the script above (helloworld.tcl and pkgIndex.tcl) are in the directory “C:/Dir/Dir With Space/test”.

# Declare the auto_path variable to allow tcl to find the files
lappend auto_path "C:/Dir/Dir With Space/test"

# Ask for the package
package require HelloWorld 1.0

# Call the procedure HelloWorld::Talk
puts [HelloWorld::Talk]

The result :

OMB*Plus: Release 10.2.0.3.33
Copyright (c) 2000, 2006, Oracle. All rights reserved.
OMB+> lappend auto_path "C:/Dir/Dir With Space/test"
resource:/tcl/lang/library {C:/Dir/Dir With Space/test}
OMB+> package require HelloWorld 1.0
1.0
OMB+> puts [HelloWorld::Talk]
HelloWorld

Package Management

You can manage the package (loads one or more binary or script files designed to extend the tcl functionality) with several command Package Command.

To reload a package, you can just load it with the command source

source C:/MyPackage.tcl

Support

If the version of package and/or the TCL version are not correct, this message is fired :

version conflict for package "HelloWorld": have 0.1, need 1.0
version conflict for package "Tcl": have 8.0, need 8.5

Reference

• See Building reusable libraries - packages and namespaces to know more