It can be. Pure functions which exclusively operate upon the stacks are often considered idiomatic. If you choose, however, you can reference and alter named gl

It can be. Pure functions which exclusively operate upon the stacks are often considered idiomatic. If you choose, however, you can reference and alter named global variables within a function, and some dialects offer a facility for named local variables. You might argue that the 'i' and 'j' words, which interact with loop constructs to give you a copy of the current and outer loop induction variable, count as references to named variables, even if they aren't strictly the same thing. As with most languages it comes down to the programmer.