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Inspired by a September 14, 2006 Salon article "Why Johnny Can't Code" by
David Brin (http://www.salon.com/tech/feature/2006/09/14/basic/index.html),
I thought that a fully working BASIC interpreter might be an interesting,
if not questionable, PLY example.  Uh, okay, so maybe it's just a bad idea,
but in any case, here it is.

In this example, you'll find a rough implementation of 1964 Dartmouth BASIC
as described in the manual at:

   http://www.bitsavers.org/pdf/dartmouth/BASIC_Oct64.pdf

See also:

  http://en.wikipedia.org/wiki/Dartmouth_BASIC

This dialect is downright primitive---there are no string variables
and no facilities for interactive input. Moreover, subroutines and functions
are brain-dead even more than they usually are for BASIC. Of course,
the GOTO statement is provided.

Nevertheless, there are a few interesting aspects of this example:

  - It illustrates a fully working interpreter including lexing, parsing,
    and interpretation of instructions.
 
  - The parser shows how to catch and report various kinds of parsing
    errors in a more graceful way.

  - The example both parses files (supplied on command line) and
    interactive input entered line by line.

  - It shows how you might represent parsed information.  In this case,
    each BASIC statement is encoded into a Python tuple containing the
    statement type and parameters.  These tuples are then stored in
    a dictionary indexed by program line numbers.

  - Even though it's just BASIC, the parser contains more than 80
    rules and 150 parsing states. Thus, it's a little more meaty than
    the calculator example.

To use the example, run it as follows:

   % python basic.py hello.bas
   HELLO WORLD
   %

or use it interactively:

   % python basic.py
   [BASIC] 10 PRINT "HELLO WORLD"
   [BASIC] 20 END
   [BASIC] RUN
   HELLO WORLD
   [BASIC]

The following files are defined:

   basic.py         - High level script that controls everything
   basiclex.py      - BASIC tokenizer
   basparse.py      - BASIC parser
   basinterp.py     - BASIC interpreter that runs parsed programs.

In addition, a number of sample BASIC programs (.bas suffix) are
provided.  These were taken out of the Dartmouth manual.

Disclaimer: I haven't spent a ton of time testing this and it's likely that
I've skimped here and there on a few finer details (e.g., strictly enforcing
variable naming rules).  However, the interpreter seems to be able to run
the examples in the BASIC manual.

Have fun!

-Dave