MalO

A small Hy-like Lisp interpreter that runs on Python 3.13+. Programs are indented forms with nested parentheses for arguments, plus Python interop, macros, and keyword arguments.

Requirements

• Python 3.13+
• uv

Install and run

uv sync
uv run malo path/to/file.malo

python -m malo path/to/file.malo does the same thing. Passing no file prints usage and exits with status 2.

Try the bundled example:

uv run malo examples/main.malo

Syntax

Each non-empty line is a form (a call or special form). Nested arguments on the same line use (...), [...], or {key val ...}. Comments start with ; and run to the end of the line.

Block structure uses indentation: one tab or four spaces per level. Dedenting closes the block. Do not wrap a block in parentheses.

while (< i 10)
	print i
	update i + 1

Reader macros:

Language

Values and builtins

True, False, and None are bound. Arithmetic and comparison are Python-backed: +, -, *, /, =, <, <=, >, >=. Also available: print, list, range, first, rest, len.

Bindings and functions

setv x 100
def y 1

defn greet [name &optional [suffix "!"]]
	print "Hello," name suffix

greet "MalO"
greet :name "MalO"

let [a 1 b 2]
	print (+ a b)

setv and def are aliases. fn is an anonymous function. update applies a function to the current value of a symbol and writes it back: update x + 100 sets x to (+ x 100).

Tail calls to defn / fn reuse the current stack frame. A recursive call that is the whole result of if, do, let, return, or the last body form will not grow the Python stack. Nested calls such as (+ "1" (additor (- x 1))) are not tail calls.

Parameter lists are [required... &optional opt... &rest name? &keys name?]:

• &optional name or &optional [name default]
• bare &rest binds extra positionals to args
• bare &keys binds leftover keywords to keys

Calls take positional args and :key value pairs. Keywords may fill named parameters (:name "MalO") or go into &keys.

Control flow

if test then else?
do body...
while test body...
for [pat iterable] body...
break
continue
return value?
assert test msg?

for can destructure:

for [[k v] (.items d)]
	print k v

break and continue only work inside while / for. return exits a defn / fn immediately; a bare return yields None. It is an error outside a function.

Python interop

import math
import math :as mt
from math :import sin cos
from math :import sqrt :as square_root
from math :import *
from builtins :import str type

print (math.sqrt 2)
print (.format "Hello, {}!" "world")

Dotted names resolve as attribute chains (math.sqrt). (.method obj args...) calls a method. (. obj attr) and (. obj (method args...)) do the same with an explicit object first.

Macros and quoting

quote form
quasiquote form

defmacro setv-str [var val]
	setv var (str val)

Macros substitute parameter forms into a template (they do not evaluate the body as Python). Use ` / ~ / ~@ for quasiquote, unquote, and splice.

Project layout

src/malo/          interpreter (tokenizer, parser, AST, eval, CLI)
examples/main.malo sample program