schemepy

Implementation of scheme in python supporting call/cc and hygienic macros

Using schemepy

There are 3 basic ways to use schemepy. As a stand-alone scheme interpreter:

$ /usr/bin/schemepy <script.scm>

As a stand-alone REPL:

$ /usr/bin/schemepy
schemepy> 

Or from inside a python script

import scheme
scheme.repl.repl()
schemepy> 

Or to run predefind strings from python

import scheme
scheme.eval.Eval(myString)
#or
scheme.eval.Eval(myFile)

Eval will only execute the first statement in its input, so if you want compound inputs, wrap them with

(begin )

The default environment setup is controlled in builtins.py, with additional definitions in builtins.scm (scheme/builtins.scm in the source, /usr/share/schemepy/stdlib/builtins.scm once installed).

Scheme is sandboxed away from python, so only functions provided into the global Environment (scheme.Globals.Globals) or some other scheme environment can be accessed. Note that by default, the interpreter is given access to the file system and other sensitive functions. If you want to use it as a sandbox for user code, you need to strip out anything you don't want called. Also, getattr and getitem are undefined in the default environment. If you are running trusted code, you can simply add the standard getattr to the global Environment. If you are running user code, and want to provide getattr, write one that only allows access to approved data types:

def safegetattr(obj, attr):
    if isinstance(obj, some_class):
        return getattr(obj, attr)
    raise TypeError("getattr only supports objects of type %r" % some_class)

or similar.

Differences from r5rs scheme

Macro expansion is mixed with code execution. Normally macro expansion would be done at compile time, but for simplicity each statement is expanded right before execution. By itself, the only effect this has is on performance.

Macros are first-class objects. Normally, macros and normal procedures are essentially the same, except that macros' names are listed in a MacroTable, while procedures are listed in the normal variable table. I don't maintain a separate list of macros, so an object being a macro is recorded on the object itself. Normally, this isn't noticable, as any code which wouldn't generate errors in racket should produce the same output (This is done by making define-syntax take either a macro or a procedure and wrap it in a macro, which calls the wrapped object with the syntax and expects the return type to be syntax), but it does open the door to some things which scheme users won't expect.

(define some-macro #f)
(define (somefun) 
    (define-syntax junk (lambda (x) #'(+ 1 2)))
    (set! some-macro junk)
)
(somefun) (some-macro)
;3

Tail recursion and general Tail-Call-Optimisation

Tail recursion is not handled differently from other tail calls; but, TCO is partially supported. Some calls recursively call process(), which breaks TCO, but most calls are properly TC optimised.

Booleans

Truth values follow python's convention rather than scheme's (0, False, None, (), and '' are false, or anything which provides a bool method which returns False). If you need scheme's behaviour, simply rewrite eq? and what not to check for identity against False.