README revision 4479
14479Sbinkertn@umich.eduInspired by a September 14, 2006 Salon article "Why Johnny Can't Code" by
24479Sbinkertn@umich.eduDavid Brin (http://www.salon.com/tech/feature/2006/09/14/basic/index.html),
34479Sbinkertn@umich.eduI thought that a fully working BASIC interpreter might be an interesting,
44479Sbinkertn@umich.eduif not questionable, PLY example.  Uh, okay, so maybe it's just a bad idea,
54479Sbinkertn@umich.edubut in any case, here it is.
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74479Sbinkertn@umich.eduIn this example, you'll find a rough implementation of 1964 Dartmouth BASIC
84479Sbinkertn@umich.eduas described in the manual at:
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104479Sbinkertn@umich.edu   http://www.bitsavers.org/pdf/dartmouth/BASIC_Oct64.pdf
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124479Sbinkertn@umich.eduSee also:
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144479Sbinkertn@umich.edu  http://en.wikipedia.org/wiki/Dartmouth_BASIC
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164479Sbinkertn@umich.eduThis dialect is downright primitive---there are no string variables
174479Sbinkertn@umich.eduand no facilities for interactive input. Moreover, subroutines and functions
184479Sbinkertn@umich.eduare brain-dead even more than they usually are for BASIC. Of course,
194479Sbinkertn@umich.eduthe GOTO statement is provided.
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214479Sbinkertn@umich.eduNevertheless, there are a few interesting aspects of this example:
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234479Sbinkertn@umich.edu  - It illustrates a fully working interpreter including lexing, parsing,
244479Sbinkertn@umich.edu    and interpretation of instructions.
254479Sbinkertn@umich.edu 
264479Sbinkertn@umich.edu  - The parser shows how to catch and report various kinds of parsing
274479Sbinkertn@umich.edu    errors in a more graceful way.
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294479Sbinkertn@umich.edu  - The example both parses files (supplied on command line) and
304479Sbinkertn@umich.edu    interactive input entered line by line.
314479Sbinkertn@umich.edu
324479Sbinkertn@umich.edu  - It shows how you might represent parsed information.  In this case,
334479Sbinkertn@umich.edu    each BASIC statement is encoded into a Python tuple containing the
344479Sbinkertn@umich.edu    statement type and parameters.  These tuples are then stored in
354479Sbinkertn@umich.edu    a dictionary indexed by program line numbers.
364479Sbinkertn@umich.edu
374479Sbinkertn@umich.edu  - Even though it's just BASIC, the parser contains more than 80
384479Sbinkertn@umich.edu    rules and 150 parsing states. Thus, it's a little more meaty than
394479Sbinkertn@umich.edu    the calculator example.
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414479Sbinkertn@umich.eduTo use the example, run it as follows:
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434479Sbinkertn@umich.edu   % python basic.py hello.bas
444479Sbinkertn@umich.edu   HELLO WORLD
454479Sbinkertn@umich.edu   %
464479Sbinkertn@umich.edu
474479Sbinkertn@umich.eduor use it interactively:
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494479Sbinkertn@umich.edu   % python basic.py
504479Sbinkertn@umich.edu   [BASIC] 10 PRINT "HELLO WORLD"
514479Sbinkertn@umich.edu   [BASIC] 20 END
524479Sbinkertn@umich.edu   [BASIC] RUN
534479Sbinkertn@umich.edu   HELLO WORLD
544479Sbinkertn@umich.edu   [BASIC]
554479Sbinkertn@umich.edu
564479Sbinkertn@umich.eduThe following files are defined:
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584479Sbinkertn@umich.edu   basic.py         - High level script that controls everything
594479Sbinkertn@umich.edu   basiclex.py      - BASIC tokenizer
604479Sbinkertn@umich.edu   basparse.py      - BASIC parser
614479Sbinkertn@umich.edu   basinterp.py     - BASIC interpreter that runs parsed programs.
624479Sbinkertn@umich.edu
634479Sbinkertn@umich.eduIn addition, a number of sample BASIC programs (.bas suffix) are
644479Sbinkertn@umich.eduprovided.  These were taken out of the Dartmouth manual.
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664479Sbinkertn@umich.eduDisclaimer: I haven't spent a ton of time testing this and it's likely that
674479Sbinkertn@umich.eduI've skimped here and there on a few finer details (e.g., strictly enforcing
684479Sbinkertn@umich.eduvariable naming rules).  However, the interpreter seems to be able to run
694479Sbinkertn@umich.eduthe examples in the BASIC manual.
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714479Sbinkertn@umich.eduHave fun!
724479Sbinkertn@umich.edu
734479Sbinkertn@umich.edu-Dave
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