Mal – Make a Lisp

- There are several reasons for the weird way that the output and expected messages are printed. One of the main reasons is that in order to enable more flexible test case results, the expected output is a regex rather than a plain string, while the output is just a plain string. There is probably a way this could be made clearer. I'll add it to my TODO list to look at.

- Do you have some specific examples? The test cases are marked as either deferrable or optional. You shouldn't ever have to implement optional (and if so that's a bug in the tests or the guide). I think the deferrable items are marked pretty clearly in the guide where they become mandatory in later steps. If it's not clear, then that's a bug.

- This is one of the tensions that exists with trying to make the guide incremental; later features may require re-work of earlier functionality. I do try and minimize that as much as possible although I've found it can really vary depending on the nature of the target language. Note that the primary goal of mal/make-a-lisp is pedagogical (as opposed to say "the easiest way to make your own Lisp"). So sometimes the need to go back and re-work something is in line with that goal.

If you have any concrete guide text or test driver improvements (especially that further the pedagogic goals of mal), I'm always happy to review pull requests! :-)

I liked MAL similar to Linux from scratch (LFS).

I also could not get the regex to work, so I ended up writing a custom tokenizer. It ended up being like 30 lines of code, and unlike the regex I could understand how it works

In C#, I ended up with

   static public List<string> Tokenizer(string source)
   {
      // Initialise the token list.
      List<string> tokens = new List<string>();

      // Define a regex pattern whose groups match the MAL syntax.
      string pattern = @"[\s ,]*(~@|[\[\]{}()'`~@]|""(?:[\\].|[^\\""])*""|;.*|[^\s \[\]{}()'""`~@,;]*)";
      //                 empty  ~@ | specials     |   double quotes      |;  | non-specials

      // Break the input string into its constituent tokens.
      string[] result = Regex.Split(source, pattern);

This took a while to understand and get going but it really improved my understanding of regex.

Regex to parse lisp expressions?

If you are using erlang you should be using pattern matching not regex. Erlang/Elixir are one of the easiest languages to build parsers in with their binary string pattern matching.

Brown University PLT textbook used lisp/scheme and their first paragraph was something like "nobody cares about parsing, let's get down to business in sexps"

I like parsing, I like regexes but I agree it's often a waste of time :)

If I recall correctly, the project started as "Make Lisp" (a lisp made with Make), and later changed to "Make A Lisp".

Also COBOL and VHDL. I don't know how a hardware description language could implement a Lisp variant btw.

I have a Dyalog APL implementation since 1 year or more on my fork, but still haven't released it because for reasons related to the input/output handling it cannot pass the self hosting tests :( To make the other tests pass, i wrapped the interpreter in a shell scripts that merges stdout and street, and that did the job for all bit the last step (self hosting). The self hosting interpreter (albeit very slow), appears to be working correctly when used interactively

I was a bit surprised nobody’s tried a “verified” implementation in something like Coq/Idris/Agda (unless i missed one - I wasn’t familiar with all the languages in the list).

Might be a fun project!

What was your approach to running the tests under Windows?

I wouldn't say it was entirely unintentional :-). I was definitely aware that "mal" could mean "evil" when I named it. It was a bit more apropos when mal only had a single implementation in GNU Make macro language.

In Spanish it means "badly", which is an encouraging goal to give the project a try :)