Factor – A practical stack language

You get Lisp or Fortran by adding discipline and using the stack language as a back end. Then instead of "pop three things off the stack and put one back", that stuff is hidden and generated by the compiler, so (on the plus side) the stack pointer is never wrong. You don't have to worry about: if I take this branch of the conditional, will the stack be left at a different height. Or worse: be tempted to exploit that somehow as a feature. The stack basically disappears from view. And then (on the minus side) you can't do things any more like replace a function that pops three things, by a sequence of two functions where each pops two and puts one back. On the plus side again, because the stack has disappeared, you can really change how it is represented. Like pass and return in registers; aggressively inline functions, etc.

It helps to be familiar with the words (Factor term for functions) in use. You can use http://docs.factorcode.org to look them up. All the help is also available from within Factor itself. You can click on word definitions and it comes up.

In this case there is a word called '' being defined. It takes no arguments off the stack and leaves one, gadget, on it.

Now working from left to right, 'vertical' is a constant. '' creates a new GUI object called a track that requires the orientation to be on the stack. That what 'vertical' is. It's the orientation.

Now that there is a track object there we fill in the slots of that object. That's what '>>foo' does. It stores something in the 'foo' slot of the object on the stack. So "{ 5 5 } >>gap" stores the 2 element vector containing the integer 5 twice, into the 'gap' slot.

'' creates a gadget and a model that is left on the stack. This is what 'bi-star' is now operating on.

'bi-star' is a combinator. It requires two objects and two quotations (quotations are factors term for anonymous functions or closures). It applies the first quotation to the first object, and the second to the second object. This system of combinators is described in the Joy language writings, although I think under different names.

"[ ... ]" is the syntax for the quotation. So "[ f track-add ]" is the first quotation. "bi-star" call this with the 'track' on the stack since that's the first object that "bi-star" gets access to.

The rest of the code can be worked through in a similar manner. In general you learn to recognise things like "" for creating objects, ">>foo" for setting slots, "[ ... ]" for quotations and combinators to reduce stack management (like dup, rot, pick, etc).

I did not know about the bcache thing - interesting!

Firstly, I think that some problems just do not map well to the concatenative space. E.g., complex mathematical expressions are a total bitch to try to do entirely with stack shuffling and whatnot. Thankfully, the Factor team agrees, so Factor has a full lexical variable vocabulary that performs just as well as the concatenative variety, and which is used even in the stdlib to do exactly this kind of thing.

Second, there's a mental hurdle to overcome to fight the Yoda speak, which is making sure that all of your words (i.e., functions) are tiny and well-factored. In most languages, having functions that run ten lines isn't a big deal. In Factor, that's a major problem I'd reject in code review. My best functions only run one line. Two is tolerable. In very specific circumstances, if it really makes sense, I'll bust out lexical variables and allow an extra line or two. Anything longer than that means I seriously messed up.

This makes a huge difference, because the thing about having such short functions is that the Yoda speak isn't any worse than when you're doing a builder pattern in Java or the like. There isn't really that big a difference between

    foo.Where(x => x > 0).Select(x -> Math.pow(x, 2));

and

    (->> foo (filter positive?) (map #(^ _ 2)))

and

    foo [ > 0 ] filter [ 2 pow ] map

These two things combine to make Factor downright pleasant to work in for me.

Maybe of interest too : oforth language