A small Scheme written in Python.
Originally written as a warmup exercise to prepare for this class.
smallscheme
is not a complete R7RS Scheme (or R5RS for that matter)
-- it implements everything needed to follow along in
SICP
up to page 102 (however, see "Limitations," below). smallscheme
is very lightweight and can be installed on any Python 3 installation
as follows:
pip install smallscheme
See tests.scm for many examples of the language in action.
$ smallscheme
scheme> (define (fact n) (if (< n 2) n (* n (fact (- n 1)))))
scheme> (fact 50)
30414093201713378043612608166064768844377641568960512000000000000
scheme> ^D
$
The REPL comes with arrow-key history, line editing, and other features provided by the Python Prompt Toolkit.
Example:
$ cat fact.scm
(define (fact n)
(if (< n 2)
n
(* n (fact (- n 1)))))
(define f100 (fact 100))
(display f100)
(newline)
$ smallscheme fact.scm
933262154439441526816992388562667004907159682643816214685929638
952175999932299156089414639761565182862536979208272237582511852
10916864000000000000000000000000
$
smallscheme
implements the basics of Scheme required to follow
examples or work problems in SICP, including the following:
*
+
-
/
<
=
>
atan
car
cdr
cons
cos
display
newline
not
random
remainder
runtime
sin
There are also two simple functions used in the Scheme-language tests: is
and test
; test
currently behaves like a progn
or do
in other lisps, in that it collects multiple forms to be evaluated and returns the result of the last evaluation. is
is basically assert
.
and
cond
define
if
lambda
let
or
quote
For explanation of these, and of Scheme in general, I recommend reading Structure and Intepretation of Computer Programs, in print or (free!) online.
smallscheme
is easily extendable using Python functions. Anything
Python can do, your smallscheme
program can as well.
To add a new function to smallscheme
, create a function which
accepts a list of arguments args
. These will have to be converted
into Python objects via the helper functions in smallscheme.dtypes
.
For example, to create a function inc
which increments its argument,
from smallscheme.dtypes import *
def inc(args):
arg_number = args[0]
assert len(args) == 1, "inc only takes one argument"
assert typeof(arg_number) in ['int', 'float'], "inc expects a number"
num_value = value(arg_number)
return int_(num_value + 1)
Then the function should be "registered" so that smallscheme
knows about it:
from smallscheme.interop import register_fn
register_fn('inc', inc)
Your Python program should then execute Scheme code, either by executing one or more Scheme source files, or by launching a REPL:
import smallscheme
smallscheme.run_file("myprogram.scm")
# ... or ...:
smallscheme.repl()
For a full list of data type operators (for converting from Python to Scheme and vice-versa), look at dtypes.py. See also the sample program in the examples folder.
Not a production-ready language implementation -- error messages and performance in particular may not be the best. Features used in later parts of SICP may not be available yet. I have been implementing them roughly in the order they are introduced.
smallscheme
uses Python lists to represent Scheme lists. As such it
does not support dotted-pair notation or cons
ing onto a non-list.
This will require slight changes in a few of the examples in SICP.
The pair representation of rational numbers in Chapter 2, for example,
would need to be adapted to use lists of two numbers rather than a
single cons pair.
pip install -r requirements.txt
Python tests are run with pytest
.
Scheme tests are run with ./smallscheme/main.py -t tests.scm
. These
tests are particularly helpful in seeing what's been implemented so
far.
Some code fragments used in tests are copied directly from SICP, which is licensed with the Creative Commons "Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)" license.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.