Wednesday, April 25, 2007

Eating my own (DSL) dogfood

I'm still working on the Prolog extension, in the too rare occasions where I get some spare time. Since the problem by itself is of limited interest (I'm pretty sure SICP contains detailed instructions in Scheme, which would translate line-by-line in metalua), I use it as a testbed for design patterns in metalua programming:
  • Integrating a syntactically similar but semantically radically different DSL into metalua (i.e. use the normal expression/statement parsers, but give them a different meaning): that's more or less what's done by pattern matching and type extensions, but all interesting things occur at compile time. Prolog programs will, at least in a first step, be largely interpreted at runtime.
  • As mentioned in an earlier post, Lua's default syntax (and its use of mutable tables as primary data) tends to encourage procedural thinking, whereas the syntax sugar I use the most is intended to make it more functional. It's interesting to work on a typically functional problem, and check these extensions' suitability.
  • Using the types extension on a non-trivially-short problem, with some advanced data structures. I mean, types only help on rather long and/or tricky programs.
More specifically, I really enjoy the type annotations in this program: most of the problem's complexity can be embedded into the data structures, and as every ML programmer knows, shifting complexity from algorithms to data structures is almost always a good thing, since the later is much easier to debug. In ML, this is even truer, since the static type-checking and type-inference system is amazing at catching bugs during compilation. Here I'm just trying to offer an intermediate solution in a dynamic language. What I liked or missed so far:
  • Parametric types: I love being able to write such signatures as: function solve_all (qlist :: list(query), ss :: stream(sub)) :: stream(sub) This is extremely helpful, even if only as embedded documentation; the bonus is, I just have to turn type checking on to check whether documentation is up to date with code, something which can never be taken for granted in the real life.
  • I want to easily compose types with "and" and "or": therefore I updated the type builder to parse operators, and assigned "and" to * and "or" to +.
  • Still, I miss a handy way to define type synonyms, such as: newtype sub = table(string, ast) + false newtype rule = {left = query; right = list(query)} I'm going to add this very soon, that's mostly trivial. I would guess (untested code): mlp.lexer:add "newtype" mlp.stat:add{ "newtype",, "=", mlp.expr, ___builder = |n,t| +{stat: types[-{mlp.id2string(n)}] = -{process_type(t)}}} It wouldn't be much more difficult to support parametric type definitions.
  • I also miss a couple of very simple type constructors, such as one which would check that a table has a certain metatable (I'm thinking about the stream type here). That's something offered by Lua for userdata in its C API, and that's really handy.
Now what I'm really eager to see is how much debugging time is saved by advanced runtime types. Static typing in ML/Haskell in an incredible debug-time-saver, and often the main rationale for choosing such a language for a project; I'd love to get part of that productivity boost in a dynamic context...

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