Tcl namespace Command

last modified April 3, 2025

The Tcl namespace command provides encapsulation and organization for variables and procedures. It helps prevent naming conflicts in large applications. Namespaces create separate scopes for related code.

Basic Definition

A namespace is a container for commands and variables that creates a separate naming context. It helps organize code and avoid name collisions between different components.

Syntax: namespace eval name ?arg...? script. This evaluates the script in the specified namespace context. Names can be hierarchical using ::.

Creating a Simple Namespace

This example demonstrates creating a basic namespace and adding a variable.

basic_namespace.tcl

namespace eval math {
    variable PI 3.14159
}

puts $math::PI

We create a math namespace containing a PI variable. The variable is accessed using the namespace qualifier ::.

Namespace with Procedures

Namespaces can contain procedures that are only visible within that namespace.

namespace_proc.tcl

namespace eval geometry {
    proc area {radius} {
        variable PI 3.14159
        return [expr {$PI * $radius * $radius}]
    }
}

puts [geometry::area 5]

This creates a geometry namespace with an area procedure. The procedure uses a namespace variable to calculate circle area.

Nested Namespaces

Namespaces can be nested to create hierarchical organization structures.

nested_namespace.tcl

namespace eval company {
    namespace eval department {
        variable employees 25
    }
}

puts $company::department::employees

This creates a nested namespace structure. The employees variable is accessed through the full namespace path using :: separators.

Exporting Commands

Namespace commands can be exported to make them available without qualifiers.

namespace_export.tcl

namespace eval utils {
    namespace export greet
    proc greet {name} {
        return "Hello, $name!"
    }
}

namespace import utils::greet
puts [greet "John"]

The greet procedure is exported from the utils namespace and then imported into the global namespace. This allows calling it without the namespace qualifier.

Namespace Variables

Variables in namespaces have different scoping rules than global variables.

namespace_vars.tcl

namespace eval counter {
    variable count 0
    
    proc increment {} {
        variable count
        incr count
    }
    
    proc get {} {
        variable count
        return $count
    }
}

counter::increment
counter::increment
puts [counter::get]

This implements a simple counter using namespace variables. The count variable is only accessible within the counter namespace.

Namespace Current

The namespace current command returns the fully-qualified name of the current namespace.

namespace_current.tcl

namespace eval outer {
    namespace eval inner {
        puts [namespace current]
    }
}

This demonstrates how namespace current returns the full path of the current namespace context. The output will be ::outer::inner.

Best Practices

Organization: Use namespaces to group related functionality.
Naming: Choose clear, descriptive namespace names.
Exporting: Only export commands that need global access.
Variables: Declare namespace variables with variable.
Hierarchy: Use nested namespaces for complex systems.

This tutorial covered the Tcl namespace command with practical examples showing its usage for code organization and encapsulation.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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