Python interpreter implementation in scheme
This project is valuable for several reasons:
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It will make you more familiar with the structure of the metacircular evaluator, because you will have to understand which procedure to modify in order to change some aspect of its operation.
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Working with another language may help overcome some of the confusion that students often have about working in two different versions of Scheme at once. In this project, it will be quite obvious when you are talking to Scheme and when you are talking to Python.
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This project will encourage you to think about the design of a programming language. Why did Scheme's designers and Python's designers make different choices?
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We hope that this project will encourage you to create your own language with its own syntax! One of the beautiful aspects of computer science is that if there isn't a tool or a language that satisfies your requirements, you can make your own!
The version of Python most widely used is written in C, but there are other popular versions written in Java, in C#, in Chinese (!), and yes, in Python itself. Our version will be written in a language that you already know (and maybe love): Scheme.
Getting Started:
You should have the following files to run and code Schython:
obj.scm
parser.scm
py-meta.scm
py-primitives.scm
primitives.py
start.scm
A note about the syntax of this specification: all capitalized words are either new terms or Scheme procedures and all words between single-quotes are to be typed in Python. Assume correct syntax; you are welcome to throw errors using the PY-ERROR procedure if you would like, but it is not required.
To start Schython, you would load the "start" Scheme file into the STk interpreter: (If you're using emacs, make sure to read the comments inside start.scm to get it working when running the file from within emacs).
STk> (load "start")
>>>
The Python interpreter prompt should show up. The INITIALIZE-PYTHON procedure sets up the global environment and the primitives necessary for a basic version of Schython to run. You should be able to perform basic arithmetic and simple assignments.
At the top of the "py-meta" Scheme file, we have included a DEBUGGING flag. If it is set to true, then all errors will dump you back into Scheme; if it is set to false, then all errors will be hidden, but PY-ERROR invocations will still print an error message and return you back to Python. The error-handling in Schython has a few rough edges: though we have tried to catch most errors in your Schython instructions and print appropriate Python-like errors, some errors could potentially dump you back into Scheme. If this happens, type
STk> (driver-loop)
to return to Schython. You should only use (initialize-python) once, or else you will lose any Schython variables or procedures you have defined.
NOTE TO MACINTOSH GAMBIT USERS: Before running this project you must tell Gambit to read a line, not a Scheme expression, in response to the ENTER key. To do this, look in the Edit menu and select Window Styles. Near the bottom right corner of the window that will appear are three check boxes; the middle one is labelled "Enter = Send Line". Check that box (so that you see an X in the box), then click OK.
NOTE TO WINDOWS USERS: If the INITIALIZE-PYTHON procedure causes an infinite loop, then your computer does not understand UNIX-based newline characters. We don't see any workaround for this problem, so we suggest that you work on the lab computers, such as either STAR or NOVA.