Comment by zelphirkalt | Hacker News Reader

Comment by zelphirkalt | Hacker News Reader

I used some GForth to solve a few Advent of Code puzzles, but had to stop, when I couldn't figure out how to properly read in lines of different lengths from a file. I had no idea how to use the pad and had not progressed that far in learning. But I have another question about Forth in general, and maybe someone can explain it to me:

With Forth's stack based nature, how is it possible to ever have performant data structures? If I only ever do things by pushing and popping from the stack, then I would think, that data structures are inherently limited to linear access times. But there are libraries implementing arrays and so on. I don't understand how this is possible in a performant way. How those structures are made, so that they are performantly accessible. Or perhaps Forth is really seriously lacking performant data structures? But that seems crazy unlikely.

Forth supports direct memory access; you can request arbitrarily sized chunks of heap memory and retain pointers either on the stack or aliased by dictionary words.

https://forth-standard.org/standard/core/ALLOT

But how is that then used? If I maintain pointers for each part of a data structure on my stack, won't I have to linearly go through those pointers, to use them to lookup what they are pointing to? And surely it is not ergonomic or feasible to create a new word for something like each element of a 1000 by 1000 matrix?

You can put pointers inside data structures on the heap, similarly to what you would do in C. The functions that process those data structures only need local variables (or stack slots) for a few pointers, and with those pointers they can fetch more pointers from the data structures on the heap. In principle the entire program can be structured pretty much identically to C, although in Forth you would typically split the code into smaller functions, and often use the stack instead of local variables.

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