[Edu-sig] pyscheme

[Danny Yoo]

> > I've been tinkering with Scheme and Python for a while now, and I've
> > cooked up a translation of the interpreter from the textbook "The 
> > Structure and Interpretation of Computer Programs" into Python:
>
> 
> That's impressive.
> 
> I fed it the quine
> 
> ((lambda (x) (list x (list (quote quote) x))) (quote (lambda (x) (list
> x (list (quote quote) x)))))
> 
> and was impressed that I got back "the same" expression.
> Unfortunately, the notation is different, using Python lists and
> Python quotation instead of Scheme lists and Scheme quotation.  The
> result, since the interpreter won't accept Python lists as input, is
> that you can't _actually_ write a program in a simple way which prints
> itself out character-for-character.


Ah!  Ok, here's a function that translates Python structures into
scheme-looking strings:

###
def expressionToString(expr):
    if not isList(expr):
        return str(expr)
    if isCompoundProcedure(expr):
        return expressionToString([Symbol('compound-procedure'),
                                   procedureParameters(expr),
                                   procedureBody(expr)])
    return '(' + string.join(map(expressionToString, expr), ' ') + ')'
###

This function also summarises compound procedures so thtat only their
parameters and body will show, and does the work that USER-PRINT did in
the original metacircular evaluator.


I'll sandwich expressionToString() between output to the user, and that
should allow quine-expressions to work properly.  With the modifications,
the self-quining code now does this:

###
[PyScheme] >>> ((lambda (x) (list x (list (quote quote) x))) (quote
(lambda (x) (list x (list (quote quote) x)))))
((LAMBDA (X) (LIST X (LIST (QUOTE QUOTE) X))) (QUOTE (LAMBDA (X) (LIST X
(LIST (QUOTE QUOTE) X))))
###



The updated code is at:

    http://hkn.eecs.berkeley.edu/~dyoo/python/pyscheme/pyscheme-0.6.tar.gz


Thanks for the suggestion!