I recently wrote a small 16-bit Forth for 8086 PCs running DOS. I built the basic one-liner loop words that can trivially be built with just “branch if zero” and “goto”: begin, while, repeat, until, again. But I held off on implementing do / loop at first.

It didn't seem like too much of a hardship. In a previous Forth I'd built, I'd implemented do / loop using the return stack, but it was... ugly. The code to implement it was ugly, the code it generated was ugly (and large!), and I didn't find a lot of places where it was actually much nicer to use than explicit begin-based loops. I was able to implement an 8086 assembler and a Minesweeper game without bothering to build do / loop. I didn't really miss it, but I had a design percolating in the back of my mind that I wanted to try.

At some point I came across some writing that suggested that Forth had a “loop control stack”. Wouldn't it be nice if I could implement some kind of loop control stack that worked for all kinds of iteration?

The thing I built has blown me away with how flexible, composable, and useful it's turned out to be. It's way more powerful than I was expecting. And the code that leverages it is inevitably much simpler and easier to read.

I fairly frequently see people who are taking an interest in Forth struggle with the idea of programming without local variables. I struggled with it when I started writing Forth! I feel like there's an unspoken assumption for people coming to Forth from other languages, and if I were to speak it aloud, it would sound something like “temporary data should go on the stack”.

Because... functions should be re-entrant by default! They should clean up after themselves! Global variables are bad and must be avoided at all costs! Functions should be “pure” and take all of their inputs as parameters, avoiding hidden dependencies!

All of these ideas of what “good code” looks like are wrong in Forth.

It is actually extremely common for Forth words to rely on implicit context, which is globally accessible through other Forth words. This is often how you build DSLs!