2019 might just be the year of Hel on the desktop. I mean, not really, but I would like to talk a bit about my prospects for Hel going forward.
For those who don't know, Hel (huangho's Experimental Language/Lisp) is my playground for experimenting with programming language design and implementation ideas. The Hel 0.1 compiler was a very crude translator from a simple Lisp-like language to C, similar in spirit to lows. Hel 0.2 was a bit of an improvement, in that it at least had the rudiments of an intermediate representation. The goal for Hel 0.2 was to be a superset of a subset of R5RS Scheme (i.e., Hel and R5RS would have a common subset), and the compiler itself would be written mostly in this subset; the idea was to eventually be able to compile the Hel compiler with either a Scheme implementation or with the Hel compiler itself, thus bootstrapping the language (i.e., being able to compile the compiler with itself).
My goals have changed a bit, though. First of all, I'm now interested in exploring more the language side rather than the implementation side of the project. I think an interpreter might serve my goals better than a compiler, since it is easier to change and test ideas. The compiler can come later.
Second, I'm not so keen anymore in having a large common subset with Scheme. Multi-shot continuations (call/cc) have always been out of the subset, but as of late I'm willing to question things as basic as cons cells. I may not get that far away from Scheme, but when exploring design options I definitely don't want to be constrained by compatibility. So bootstrappability can come later too.
Third, because I want to write an interpreter, but I don't want it to be terribly slow, I'll probably be switching implementation platform. So far I had been doing development on GNU Guile, but I'll probably switch to either Chez Scheme (a pretty fast R6RS Scheme implementation), or SBCL (a pretty fast Common Lisp implementation). SBCL has the advantage of having more libraries available (and Common Lisp itself is a larger language than that provided by Chez), while Chez has the advantage of being (in principle) closer to Hel (although that's kind of moot if bootstrapping is not an immediate goal anymore). I thought SBCL would also be faster than Chez, but in my highly scientific benchmark (running (fib 45) on each implementation), Chez is actually faster out of the box, though SBCL is faster if type declarations are provided.
So what are my goals for Hel 0.3? Well:
Implementation-wise, one of my major goals is debuggability. I want to have a Lisp which can give proper stack traces, with proper line numbers, in the presence of macros, and show function arguments in the stack trace.
Language-wise, I want:
Optional types, with a lightweight syntax for type declarations;
Explicit indication of mutability, with more things immutable by default (strings, lists). Mutability is an always-present part of APIs in Scheme and Common Lisp (both language standards are very explicit in describing data sharing between function arguments and results, which values can or cannot be mutated, etc.); I think this really ought to be more explicit in the data types themselves. I have described a proposal for handling mutability before, but nowadays I'm more inclined to simply have mutable and immutable variants of lists, dictionaries and such; it's less elegant but easier to implement and use efficiently. We'll see how this goes. The important thing is to make mutability explicit in the data types (and their printed representation).
A concise notation for declaring and using new data types, reading and printing them, doing pattern matching, reflection, etc., to reduce the temptation to Use Lists For Everything™.
There are other things (some of which I intend to write about in the future), but I think these are the most important ones.
Wish everyone a happy new year, and may our living-dead open source projects thrive in 2019.
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