Last weekend I published basic101, a BASIC interpreter in Ruby. I am fascinated by what’s inside a compiler or interpreter. Now, BASIC isn’t that exciting a language, but it’s a good one to start with before moving on to more difficult things. Even though basic101 is just a tutorial project, it is reasonably polished. has decent error detection, runs a fair subset of 1980’s BASIC, and the code is well organized. Even (especially?) a tutorial project should be good, clean code.

Parslet

I wanted to get some solid experience with Parslet before moving on to other things. Parslet is an easy to use Parsing Expression Grammar (PEG), with a twist: It has a wonderful pattern matching DSL for turning a parse tree into an abstract syntax tree. Another plus is that it is an internal DSL rather than external. Not having a separate compilation step is nice, but that’s not the important thing. What’s important is that since Parslet’s DSL is all ruby, meta is possible. For example, suppose you are writing a parser for a language that must ignore spaces in keywords (FORTRAN). In treetop, this rule parses the keyword STOP:

    rule keyword_stop
      'S' spaces 'T' spaces 'O' spaces 'P'
    end
    

(the spaces rule, not listed here, eats zero or more characters of white space). This is hard to read, repetitive, and litters the parse rules with a separate rule for each keyword.

With Parslet, you can just define a method like this (don’t worry about what’s in it. What’s important is that this method takes a string and returns a little Parslet parser that recognizes keywords which may have embedded spaces:

    def keyword(s)
      s.chars.map do |c|
        str(c)
      end.inject do |l, r|
        l >> spaces >> r
      end
    end
    

And then just use it:

    rule(:stop) do
      sym('STOP') >> line_break
    end
    

Parlet is nice like that, and in many other ways.

A BASIC gem?!

I also published it as a gem, even though nobody is likely to use it. Having it as a gem does make it easier for some lost soul to try it. And, for publishing a gem, there doesn’t seem to be any real bar to entry: A great many of them are, in one way or another, not really useful to anyone, by virtual of being undocumented, obsolete, or whatever. Hey, my gem has test coverage.