LW minimal interpreter 1998


… a small java interpreter for the didactic (toy) LW imperative language, used in some universities to teach the basics of imperative languages, and the concepts of static analysis, basic parsing, handling scope, what a state is, the difference between an expression and a command, and how to walk an AST tree to do static checks and to run the program.

I did this compact LW (and its older brother, LWPlus) interpreter at uni in 1998. Funnily enough, I couldn’t find recent references to LW, so actually I had to reverse engineer the semantics of the language (and some examples) reading my very old undocumented code.

The language supports one type only (ints), it supports C-like block scoping rules and while loops. Programs read data from a file, and print out the calculated output. There is no support function definition / invocation.

There are two neat things about this: how compact the whole thing is (4 pages of code), and that the grammar for the language is actually not baked in the interpreter. Rather, it’s read at runtime from a grammar file which is almost in BNF form, so one could tweak that a little, I thought that was quite cool, at the time :-). The semantics of the static checks and execution are baked in the interpreter though.

To give a taste of LWPlus, here is what a factorial looks like:

  with PARTIAL begin
    with COUNTER begin
      read COUNTER ;
      PARTIAL := 1 ;
      while COUNTER <> 1 do
        COUNTER := sum COUNTER -1
      endw ;
      write PARTIAL

The interpreter is on github.