uncommonlisp

A Scheme interpreter in one Python file, with a bytecode compiler.

λ> (define (fib n)
     (let loop ((a 0) (b 1) (i 0))
       (if (= i n) a (loop b (+ a b) (+ i 1)))))
λ> (map fib (iota 10))
(0 1 1 2 3 5 8 13 21 34)

Usage

python3 uncommonlisp.py                # interactive REPL
python3 uncommonlisp.py script.lsp     # run a file
python3 uncommonlisp.py -e '(+ 1 2)'  # eval an expression
python3 uncommonlisp.py --fast script.lsp  # auto-compile (7-19x faster)

Bytecode compiler

uncommonlisp includes a stack-based bytecode compiler and VM. Enable it with --fast or (auto-compile! #t):

python3 uncommonlisp.py --fast examples/fibonacci.lsp

(auto-compile! #t)
(define (ack m n)
  (cond ((= m 0) (+ n 1))
        ((= n 0) (ack (- m 1) 1))
        (else    (ack (- m 1) (ack m (- n 1))))))
(compiled? ack)  ; => #t
(ack 3 4)        ; => 125

The compiler handles: if, begin, and, or, when, unless, cond, define, set!, lambda, let, named-let, let*, letrec, do, call/cc, function calls with tail-call optimization. Macros are expanded at compile time. 20 specialized opcodes for hot builtins (+, -, *, =, <, car, cdr, cons, null?, etc.) avoid function call overhead.

Features:

• Explicit frame stack — compiled-to-compiled calls don't grow the Python stack
• Full continuations — call/cc supports upward continuations; generators work
• Constant folding — (+ 1 2) folds to 3 at compile time
• Peephole optimizer — eliminates dead code (VOID+POP, JUMP-to-next)
• (disassemble proc) — inspect generated bytecode

What's implemented

Core language

• Full lexical scoping and closures
• Tail-call optimization (TCO) — deep recursion never blows the stack
• Hygienic macros via syntax-rules with ellipsis (...) support
• define-macro / defmacro for procedural macros
• call/cc — full continuations (escape + upward) in compiled code
• values / call-with-values
• dynamic-wind, guard, with-exception-handler
• quasiquote / unquote / unquote-splicing with proper nesting
• R7RS internal defines with letrec* body semantics
• R7RS error objects
• Exact rational arithmetic — (/ 1 3) → 1/3, (+ 1/4 3/4) → 1
• String ports — open-input-string open-output-string read on ports
• Mutable strings — string-set! string-fill! string-copy!
• Module system — module / import with export lists
• define-record-type with (inherit parent) for single-inheritance
• Pretty-print — pp / pretty-print
• Tracing — (trace fn) / (untrace fn)

Special forms define set! lambda λ if cond case and or when unless begin let let* letrec letrec* named-let do quasiquote define-macro define-syntax syntax-rules let-syntax letrec-syntax apply eval values call/cc dynamic-wind guard parameterize load error module import define-record-type

Built-ins

• Arithmetic: + - * / quotient remainder modulo expt sqrt abs floor ceiling round truncate min max gcd lcm log exp trig functions, numerator denominator
• Rationals: (/ 1 3) → 1/3, literal 1/3 syntax, exact / inexact conversion
• Comparison: = < > <= >= zero? positive? negative? odd? even?
• Pairs & lists: cons car cdr list length append reverse map for-each filter fold-left fold-right reduce any every sort partition find take drop zip flatten and more
• SRFI-1: last first–fifth delete lset-union lset-intersection lset-difference unfold list-tabulate
• Strings: string-length string-ref string-set! substring string-append string-copy string-copy! string-fill! string->list string->number format and more
• Characters: char->integer integer->char char-alphabetic? char-upcase char-downcase
• Vectors: make-vector vector vector-ref vector-set! vector-copy vector-copy!
• Hash tables: make-hash-table hash-table-set! hash-table-ref hash-table-keys hash-table-values hash-table-walk and more
• I/O: display write newline read read-char read-line open-input-string open-output-string with-output-to-string
• File system: file-exists? delete-file rename-file directory-files current-directory
• System: command-line get-environment-variable current-time exit
• Python interop: py-eval py-exec py-import py-call py-attr
• Compiler: compile compiled? disassemble auto-compile!

Standard library (stdlib.lsp) Additional macros, string/list/numeric/tree utilities, alist/hash helpers, simple object system, SRFI-2/8/64 test framework.

Examples

python3 uncommonlisp.py --fast examples/fibonacci.lsp
python3 uncommonlisp.py --fast examples/generator.lsp
python3 uncommonlisp.py --fast examples/mergesort.lsp
python3 uncommonlisp.py examples/objects.lsp

;; Generator using full continuations
(auto-compile! #t)
(define (make-gen thunk)
  (let ((k #f) (done #f))
    (lambda ()
      (if done 'done
          (call/cc (lambda (return)
            (if k (k return)
                (begin (thunk (lambda (val)
                         (call/cc (lambda (next)
                           (set! k next) (return val)))))
                       (set! done #t) (return 'done)))))))))

(define counter (make-gen (lambda (yield)
  (let loop ((i 0)) (yield i) (loop (+ i 1))))))
(counter) ; => 0
(counter) ; => 1
(counter) ; => 2

Running tests & benchmarks

make test          # run 529 tests
make test-verbose  # verbose output
make bench         # compare interpreter vs bytecode vs CPython
make lint          # syntax check all Python files

File layout

uncommonlisp.py   interpreter + bytecode compiler (one file, ~2600 lines)
stdlib.lsp        extended standard library
tests.py          test suite (529 tests)
bench.py          benchmarks vs CPython
examples/         example programs
Makefile          make test / make bench / make repl