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2 points by aw 70 days ago | link | parent

It's interesting to be taking an axiomatic approach. That is, in this case, to add to the language axioms that expressions can be labeled with their source file locations.

It might not work: it might turn out that the feature I want (to be able to track source locations through macro expansions) can't be expressed in terms of this particular set of axioms. Or, it might be that it can, but the result is a runtime too slow for me to want to use it.

But, if it does work, it has its own internal logic. What does (cdr x) mean when x has been labeled with source locations? Well, clearly, what it ought to mean is the tail of x, labeled with the source locations of the tail of x. Theorems such as (apply (fn args args) xs) ≡ xs should continue to work.

On the other end of the spectrum from an axiomatic approach is engineering. Have a list of features you want, and design a system that implements all of them. This too might fail sometimes (perhaps the features you want turn out to be incompatible, or you design yourself into a corner that's hard to get out of)... but most of the time it's more reliable, in the sense that usually we can come up with some design that implements all (or at least most!) of the features we want... even if maybe the result isn't very pretty.

The downside of engineering is design complexity. Complexity will probably at least scale linearly with the number of features, if not more likely by some power law. If we're lucky we may see some simplifications in the design along the way that we can refactor into, some axioms of the design that become apparent that we can incorporate... but most of the time, in practice, the design gets more and more complex as we add features.

Engineering is attractive because it gets things done. "I just want X, let's implement X". There are a lot of times when what I want is just to implement X, and I engineer a design, and it works out fine.

The axiomatic approach is more uncertain. Will it work? I don't know. It's also harder. Oops, ssyntax stopped working. Why? `some` stopped working. Why? `recstring` stopped working. Why? `+` stopped working. Why? Is it because my implementation of `apply` is broken, or because I broke the compiler and its now outputting broken code, or because my runtime is broken? It could be any of these. Another day, another week of debugging.

It's also more fun. There are many macro systems. Many of them are practical. Some have features I don't care about, some are more complicated than I like, but I don't have much interest myself in engineering yet another macro system. Axioms are more interesting. Perhaps it will turn out that for these particular set of axioms, it doesn't work out for this particular feature. But then at least I know why :-)



1 point by akkartik 70 days ago | link

You've got me curious now about how this relates to Amacx :)

I find that having tests allows me to start out in a sort of engineering mindset, in your terms, where I just get individual cases working one by one. But at the same time they keep me from growing too attached to a single implementation and leave me loose to try to think up more axiomatic generalizations over time. You can kinda see that in http://akkartik.name/post/list-comprehensions-in-anarki if you squint a little; I don't show my various working copies there, but the case analysis I describe does faithfully show how I started out thinking about the problem, before the clean general solution suddenly fell out in a flash of insight.

(Having tests doesn't preclude more systematic thinking about the space, and proving to myself that a program is correct. But even if I've proved a program correct to myself I still want to retain the tests. My proofs have been shown up too often in the past ^_^)

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2 points by aw 68 days ago | link

> You've got me curious now about how this relates to Amacx :)

Why, everything! :-) E.g. I start with: what if top level variables were implemented by an Arc table, and top level variable references were implemented by an Arc macro? That is, what if top level variables were built out of lower level language axioms, instead of being built in?

We end up with something that's kind of like modules, but doesn't do everything that we'd typically expect modules to do (though perhaps we could implement modules on top of them if we wanted to), and also does some things that modules don't do (for example we can load code into a different environment where the language primitive act differently).

To give a name to this thing that is kind of like modules but different, I called them "containers", because they're something you load code into.

Are containers useful? Well, I'm guessing it would depend on whether we'd ever want to want load code into different environments in our program. If we only want to load code once, and all we want is a module system, I imagine it'd probably be more straightforward to just implement a module system directly.

On the other hand, suppose we have a runtime that gives us some nifty features, but is slower than plain Arc. Now it seems like containers could turn out to be a useful idea. Perhaps I have some code that I want to load in plain Arc where it'll run fast, and other code that I want to run in the enhanced runtime and I don't mind that it's slower.

> I find that having tests allows me to start out in a sort of engineering mindset, in your terms, where I just get individual cases working one by one. But at the same time they keep me from growing too attached to a single implementation and leave me loose to try to think up more axiomatic generalizations over time.

Exactly!

This is the classic test driven development refactoring cycle: add features with tests, then refactor e.g. to remove duplicate code, and/or otherwise refactor to make the code more axiomatic.

Since "Any sufficiently complicated C or Fortran program contains an ad-hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp", one could, in theory, start with such a C or Fortran program and refactor towards an axiomatic approach until you had reinvented Lisp, with the program written in Lisp :-)

But in practice I think going the other way is sometimes necessary: that is, starting with some axioms, and seeing what can be implemented out of them.

I'm not sure why that is (why doesn't anyone keep refactoring a large C program and end up with Lisp?) but I suppose it might be because it's too cognitively difficult, or you end up at some kind of local maximum in your design, or something.

In any case, I find tests absolutely essential for working on Amacx... not just "nice to have" or "saves me a lot of time", but "impossible to do without them"!

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