#135 Global namespace in tcl

#135 Global namespace in tcl

It is possible to reference global variables inside procedures using global namespace indicator:

set ::element_list "1 2 3 4 5";
proc ::element_info {} {
   puts $::element_list;
   set ::node_list "10 20 30 40";
   puts $::node_list;
}
element_info;
1 2 3 4 5
10 20 30 40

puts $::element_list;
1 2 3 4 5

puts $::node_list;
10 20 30 40

#Script begins.

#Define a variable in the global scope.
set f "This is a global variable";

#This global procedure will generate an error because the
#global variable f is not in the scope.
proc testproc1 {} {
   catch {puts $f} g;
   puts $g;
}

#This procedure will not generate an error because the
#global variable f is localized with the global command.
proc testproc2 {} {
   global f;
   catch {puts $f} g;
   puts $g;
}

#A call to delete the namespace clears variables within
#that namespace.
catch {namespace delete ::testspace};

#The namespace eval command creates a new namespace.
namespace eval ::testspace {
   variable fg "This is a namespace variable";
   
   #Export a single procedure for use later.
   namespace export tproc3
}

#Procedures can be organized into namespaces.
proc ::testspace::tproc1 {} {
   #Global procedures do not require a namespace indicator.
   testproc1;
   testproc2;

   #Procedures within the same scope do not require a 
   #namespace indicator.
   tproc2;
   tproc3;

   #Procedures from other namespaces must be explicitly declared
   #unless a namespace import is used.
   catch {oproc1} ds;
   puts $ds;
   ::otherspace::oproc1;
}

#This namespace procedure will generate an error because the
#namespace variable is not in the scope.
proc ::testspace::tproc2 {} {
   #The catch command is used to prevent the script from
   #exiting with an error.
   catch {puts $fg} g;
   puts $g;
}

#This namespace procedure will not generate an error because the
#namespace variable is localized.
proc ::testspace::tproc3 {} {
   #The variable command, if referring to an existing
   #namespace variable and not making an assignment,
   #allows local access to the variable.
   variable fg;
   catch {puts $fg} g;
   puts $g;
}

#A call to delete the namespace clears variables within
#that namespace.
catch {namespace delete ::otherspace};

#Creating a second namespace.
namespace eval ::otherspace {
   variable fh "This is a different namespace variable";
}

proc ::otherspace::oproc1 {} {
   #The namespace import command can be used to
   #localize procedures.
   namespace import ::testspace::tproc3;
   variable fh;
   catch {puts $fh} h;
   puts $h;
   tproc3;

   #namespace origin returns original namespace of the proc.
   puts [namespace origin tproc3];
}

#Run the primary procedure
::testspace::tproc1

Output:

can't read "f": no such variable
This is a global variable

can't read "fg": no such variable
This is a namespace variable

invalid command name "oproc1"
This is a different namespace variable

This is a namespace variable

::testspace::tproc3

Last Updated on 2020-11-12 by gantovnik