Slang: Review of 2012

Already the first quarter of 2013 is behind us, but I wanted to take a look whether I accomplished the goals I set for Slang development in 2012. I’m not going to reiterate the goals here, but almost none of them were completed.

I wanted to bring the language into a usable form, but so far there is still a lot of work to do. I wanted to make the compiler open source, but I’m still not sure at which point I would like to do that. I think I will postpone that quite a bit, because I don’t have the time to run an open source project at the moment (and parts of the compiler are still full of horrible hacks). Actually, lack of development time was one of the things that prevented me from fulfilling the goals. I worked on Slang mostly during vacations and when I found a drive to start work during other times I would continue for a couple of weeks and then take a break for a few months.

I am suspecting that development will continue at a similar pace. I have made some progress, though, sporadically during 2012 and more continuously during the last few weeks.

New Parser

I think I spent most of my Slang development time last year on the new parser, using the mixfix parser combinators I blogged about (sorry, I kind of forgot to post the third part of that). I have improved the parser framework a lot since then, but it does have a downside of being quite slow for large programs, even with Packrat parsers. So I have some further work to do here, like maybe replacing the whole parser again, but I will probably leave that to some unknown future date.

Custom Operator Precedence Rules

The new mixfix parser theoretically enabled a modular grammar: the default grammar can be relatively small, but programs could define their own prefix, postfix, infix and closed operators and their precedence rules. And recently, I made that actually work. You still can’t quite define a grammar per Slang source file, but you can add operator graph (.ops) files to the list of compiled files and together with the standard rules, they will define the grammar for that compiler run. Some examples.

New Compiler Infrastructure

I just finished the initial work on this, a step towards separate compilation: instead of including library code into the source file being compiled, now the new Slang compiler driver can compile multiple source files at the same time, but still requires that the library sources are available. The end result is linked together with the LLVM IR linker.

Slang IDE

I started work on an Eclipse-based Slang IDE. Currently it includes syntax highlighting, error reporting, a few quick fixes, and quick-assist for rewriting expressions to use different aliases of methods. E.g. if you write x.times(y) you can then click Ctrl+1 and select “Use alias: *”. Then the expression will be rewritten to x * y. This is much more useful if you have Unicode operators that can’t be easily typed. Another example: you write array(1), Ctrl+1 suggests “Use alias: subscript” and selecting it turns the expression into array₁

The IDE also shows warnings and errors with squiggly lines, and I added a few more useful warnings to the compiler and improved the messages.

Debugging

This is still in progress, but for some programs, Slang compiler can now emit DWARF debug info, and the programs can be debugged using the GNU Debugger. I was quite pleased to get that working, and together with the IDE, and new compiler driver, it has made Slang much more usable for myself.

Next Steps

This time, I don’t want to give any deadlines, but the next steps are to make the tooling more usable, improve error messages, add some of the language features I didn’t get to add in 2012, etc. I think I might even release a build in the form of the Slang IDE with the compiler embedded, even if it will not be open source.

I have various language features in mind that I want to explore, such as proper generic types, some form of dependent types, Strings, IO, better interoperability with C, JVM back-end. Strings and IO are pretty much given, and generics as well. Not sure how soon I’ll get to the other things.

I think I will pretty much be doing pain-driven development: what is currently giving me most pain will get fixed first. I think actually generics might be the thing, because adding collection methods for only a specific type of arrays is no fun.

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