A module definition may contain a declaration of one or more global variables. These are exactly analogous to global variables of procedures. Each expression glo1, glo2, ... must be a symbol.

Changes (by assignment) to the value of a global variable made anywhere within a module definition are visible at global scope, once the code effecting the change executes.

Note: The body of a module definition executes when the module definition is evaluated (during instantiation), while the bodies of (exported) procedures execute when they are called.

Declaring a variable to be global in a module prevents the implicit scoping rules from causing it to be declared local implicitly. Even if implicit scoping rules do not force a variable to be declared local implicitly, you must always declare all of your global variables.

m := module()
    global    `convert/pair`, `print/PAIR_`;
    export    cons, car, cdr;
    `convert/pair` := proc( expr )
        if type( expr, [ 'anything', 'anything' ] ) then
            cons( op( expr ) )
        else
            error "cannot convert %1 to a pair data structure", expr
        end if
    end proc:
    `print/PAIR_` := proc()
        local ``;
        ``( args )
    end proc:
    cons := (a,b) -> 'PAIR_'( a, b );
    car := curry( op, 1 );
    cdr := curry( op, 2 );
end module:

$\mathrm{convert}\left(\left[u\,v\right]\,'\mathrm{pair}'\right)$

$\left(u\,v\right)$

$m:-\mathrm{cdr}\left(\right)$

$v$