StardustDL/scheme-from-python

An experimental scheme interpreter in Python.

scheme-from-python is an experimental scheme interpreter in Python.

demo.mov

• Scheme core
  • define
  • if
  • lambda (with closures)
  • macro (without capturing context)
• Builtin functions
  • Boolean operators
  • Arithmetic operators (integer, float, complex)
  • Comparing operators
  • Input / Output functions
• Interoperation
  • Auto signature inferring from Python to Scheme
  • Lazy or non-lazy functions
  • Capturing the evaluation context (symbol table)
  • Call pure Python functions
• Interpreter
  • File / command-line expression
  • Auto-indented multi-line
  • Auto fixing missing right parentheses

For sequence expression, the final value is the value of the last sub-expression, e.g., 1 2 3 = 3.

Python Interoperation

Most static Python functions can be accessed in sfpy, by the qualified name :builtinAttribute, module:, module:attribue or module:class.attribue, e.g., :dir, os:getcwd, os.path:split.

sfpy tries to wrap the Python function to adapt to the scheme environment. Have fun to try this experimental feature!

> sys:version
3.10.4 | packaged by conda-forge | (main, Mar 30 2022, 08:38:02) [MSC v.1916 64 bit (AMD64)]
> os:getcwd
(lambda () (...))
> (os:getcwd)
D:\scheme-from-python\src
> (:dir math:)
Object(['__doc__', '__loader__', '__name__', '__package__', '__spec__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'comb', 'copysign', 'cos', 'cosh', 'degrees', 'dist', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'gcd', 'hypot', 'inf', 'isclose', 'isfinite', 'isinf', 'isnan', 'isqrt', 'lcm', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'log2', 'modf', 'nan', 'nextafter', 'perm', 'pi', 'pow', 'prod', 'radians', 'remainder', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'tau', 'trunc', 'ulp'])

Values

• Boolean: #t true, #f false
• Integer: 0, -1, 2, ...
• Float: 0.1, 1.5, ...
• Complex: 1+2j, 3+4j, ...
• Symbol: a, b, c, +, -, ...
• String: a, b, c, +, -, ...
• Function: (lambda (x) (+ x 1)), ...
• Empty: <empty> only as return value

Builtins

Core

(define symbol_name value_expression)

Define a symbol with exact value, and return the value, def for a short alternative.

(if predicate_expression true_expression false_expresion)

Branch expression, if predicate_expression is not #f, then evaluate and return the value of true_expression, otherwise evaluate and return the value of false_expression.

(lambda (p1 p2 ... pn) body_expression

Lambda expresion, define an anonymous function (closure) with the parameters named p1, p2, ..., pn (can be empty, i.e., ()), and the function body, lam for a short alternative.

The lambda function creates a sub-symbol-table, so define in a lambda function can only write current symbols, and cannot write upstream symbols (can hidden them by define symbols in the same name). Here is an example code snippet.

>>> ((lambda () (define x 1)))
x
>>> x
Undefined symbol: x

(macro (p1 p2 ... pn) body_expression)

Macro expression, define a macro, for text replacement with the parameters named p1, p2, ..., pn (can be empty, i.e., ()), and the macro body, mac for a short alternative, no capturing context.

Boolean

(not b)

$$=\neg b$$

(and b1 b2 ... bn) without short circuit

$$=\bigwedge_{i=1}^n b_i$$

(or b1 b2 ... bn) without short circuit

$$=\bigvee_{i=1}^n b_i$$

Arithmetic

The operands must be integers.

(+ v1 v2 ... vn)

$$=\sum_{i=1}^n v_i$$

(- v1 v2)

$$=v_1 - v_2$$

(* v1 v2 ... vn)

$$=\prod_{i=1}^n v_i$$

(// v1 v2)

$$=\lfloor v_1 / v_2 \rfloor$$

(/ v1 v2)

$$=v_1 / v_2$$

(% v1 v2)

$$=v_1 % v_2$$

(^ v1 v2)

$$=v_1^{v_2}$$

(max v1 v2 ... vn)

$$=\max_{i=1}^n v_i$$

(min v1 v2 ... vn)

$$=\min_{i=1}^n v_i$$

Comparing

(< v1 v2)

$$=\textbf{boolean}(v_1 < v_2)$$

(<= v1 v2)

$$=\textbf{boolean}(v_1 \le v_2)$$

(> v1 v2)

$$=\textbf{boolean}(v_1 > v_2)$$

(>= v1 v2)

$$=\textbf{boolean}(v_1 \ge v_2)$$

(= v1 v2)

$$=\textbf{boolean}(v_1 = v_2)$$

(!= v1 v2)

$$=\textbf{boolean}(v_1 \ne v_2)$$

Input / Output

(print v)

Print the value and return empty.

(from filePath)

Interprete a source file and return the evaluated value.

(symbols)

Return all symbols in the current context, syms for a short alternative.

Install

conda create -n sfpy python=3.10
conda activate sfpy
cd src
pip install -r requirements.txt

Usage

sfpy supports interactor (for user input) and interpreter (for file input) mode.

Interactor Mode

python -m sfpy

• Use _ to access last value.

Interpreter Mode

python -m sfpy -f your_scheme_file

python -m sfpy -e "your scheme expression"

python -m sfpy -f ./demo/arithmetic.scm
python -m sfpy -f ./demo/lambda.scm
python -m sfpy -f ./demo/factorial.scm