User:LABoyd2/special variables to manual 160316

Special variables

Special variables provide an alternate means of passing arguments to modules and functions. All user, or OpenSCAD, defined variables starting with a '$' are special variables, similar to special variables in lisp. Modules and function see all outside variables in addition to those passed as arguments or defined internally.

The value for a regular variable is assigned at compile time and is thus static for all calls.

Special variables pass along their value from within the scope (see scope of variables) from which the module or function is called. This means that special variables can potentially have a different value each time a module or function is called.

regular  = "regular global";
$special = "special global";
module show() echo("         in show    ", regular,"   ", $special );

echo ("         outside    ", regular,"   ", $special );
          // ECHO: "         outside    ", "regular global", "   ", "special global"
  
for ( regular = [0:1] ){ echo("in regular loop     ", regular,"   ", $special ); show();}
          // ECHO: "in regular loop     ", 0, "   ", "special global"
          // ECHO: "         in show    ", "regular global", "   ", "special global"
          // ECHO: "in regular loop     ", 1, "   ", "special global"
          // ECHO: "         in show    ", "regular global", "   ", "special global"

for ( $special = [5:6] ){ echo("in special loop     ", regular,"   ", $special ); show();}
          // ECHO: "in special loop     ", "regular global", "   ", 5
          // ECHO: "         in show    ", "regular global", "   ", 5
          // ECHO: "in special loop     ", "regular global", "   ", 6
          // ECHO: "         in show    ", "regular global", "   ", 6

show();
          // ECHO: "         in show    ", "regular global", "   ", "special global"

This is useful when multiple arguments need to be passed thru several layers of module calls.

Several special variables are already defined by OpenSCAD.