Tcl rename Command

last modified April 3, 2025

The Tcl rename command is used to change the name of existing commands. It's a powerful feature for command aliasing and modification. The command can also be used to delete commands when renaming to an empty string.

Basic Definition

The rename command changes the name under which a command is invoked. It can create command aliases or completely remove commands.

Syntax: rename oldName newName. If newName is an empty string, the command deletes oldName. Both names must be valid Tcl command names.

Basic Command Renaming

This shows the simplest usage of rename to create a command alias.

basic_rename.tcl

proc hello {} {
    puts "Hello, World!"
}

rename hello greet
greet

This renames the hello command to greet. After renaming, calling hello would raise an error, while greet works as the original command.

Deleting Commands

The rename command can delete commands by specifying an empty string as the new name.

rename_delete.tcl

proc temp {} {
    puts "This is a temporary command"
}

temp
rename temp ""
# temp ; # This would now raise an error

Here we create a temporary command and then delete it using rename. After deletion, attempting to call the command would result in an error.

Command Aliasing

rename is often used to create alternative names for commands.

rename_alias.tcl

proc calculate {x y} {
    return [expr {$x + $y}]
}

rename calculate add
puts [add 5 7]

This creates an alias add for the calculate command. Both names would refer to the same implementation until one is renamed again.

Command Wrapping

rename is essential for command wrapping patterns where you want to modify command behavior.

rename_wrap.tcl

proc original {x} {
    return [expr {$x * 2}]
}

rename original original_impl
proc original {x} {
    puts "Calling original with $x"
    set result [original_impl $x]
    puts "Result is $result"
    return $result
}

original 10

This demonstrates command wrapping. We first rename the original command, then create a new command with the original name that adds logging functionality while delegating to the original implementation.

Namespace Management

rename can be used to move commands between namespaces.

rename_namespace.tcl

namespace eval ns1 {
    proc func {} {
        puts "Function in ns1"
    }
}

rename ns1::func ns2::func
ns2::func
# ns1::func ; # This would now raise an error

This moves a command from one namespace to another. The command is no longer available in the original namespace after the rename operation.

Command Hiding

rename can be used to temporarily hide built-in commands.

rename_hide.tcl

rename puts original_puts
proc puts {args} {
    if {[llength $args] == 1} {
        original_puts "Modified: [lindex $args 0]"
    } else {
        eval original_puts $args
    }
}

puts "Hello"
rename puts ""
rename original_puts puts
puts "Back to normal"

This example temporarily replaces the built-in puts command with a modified version, then restores the original. This pattern is useful for debugging or adding functionality to built-in commands.

Best Practices

Documentation: Always document when renaming commands.
Restoration: Plan how to restore original commands.
Namespaces: Use namespaces to avoid naming conflicts.
Testing: Thoroughly test after renaming commands.
Safety: Be cautious when renaming built-in commands.

This tutorial covered the Tcl rename command with practical examples showing its usage in different scenarios.

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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