80 lines
2.5 KiB
Text
80 lines
2.5 KiB
Text
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Inspired by a September 14, 2006 Salon article "Why Johnny Can't Code" by
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David Brin (http://www.salon.com/tech/feature/2006/09/14/basic/index.html),
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I thought that a fully working BASIC interpreter might be an interesting,
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if not questionable, PLY example. Uh, okay, so maybe it's just a bad idea,
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but in any case, here it is.
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In this example, you'll find a rough implementation of 1964 Dartmouth BASIC
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as described in the manual at:
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http://www.bitsavers.org/pdf/dartmouth/BASIC_Oct64.pdf
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See also:
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http://en.wikipedia.org/wiki/Dartmouth_BASIC
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This dialect is downright primitive---there are no string variables
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and no facilities for interactive input. Moreover, subroutines and functions
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are brain-dead even more than they usually are for BASIC. Of course,
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the GOTO statement is provided.
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Nevertheless, there are a few interesting aspects of this example:
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- It illustrates a fully working interpreter including lexing, parsing,
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and interpretation of instructions.
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- The parser shows how to catch and report various kinds of parsing
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errors in a more graceful way.
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- The example both parses files (supplied on command line) and
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interactive input entered line by line.
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- It shows how you might represent parsed information. In this case,
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each BASIC statement is encoded into a Python tuple containing the
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statement type and parameters. These tuples are then stored in
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a dictionary indexed by program line numbers.
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- Even though it's just BASIC, the parser contains more than 80
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rules and 150 parsing states. Thus, it's a little more meaty than
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the calculator example.
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To use the example, run it as follows:
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% python basic.py hello.bas
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HELLO WORLD
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%
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or use it interactively:
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% python basic.py
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[BASIC] 10 PRINT "HELLO WORLD"
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[BASIC] 20 END
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[BASIC] RUN
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HELLO WORLD
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[BASIC]
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The following files are defined:
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basic.py - High level script that controls everything
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basiclex.py - BASIC tokenizer
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basparse.py - BASIC parser
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basinterp.py - BASIC interpreter that runs parsed programs.
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In addition, a number of sample BASIC programs (.bas suffix) are
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provided. These were taken out of the Dartmouth manual.
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Disclaimer: I haven't spent a ton of time testing this and it's likely that
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I've skimped here and there on a few finer details (e.g., strictly enforcing
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variable naming rules). However, the interpreter seems to be able to run
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the examples in the BASIC manual.
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Have fun!
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-Dave
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