mo-language 0.4.7

Mo - Modern, Expressive, Fast Programming Language

Mo Language

Mo is a lightweight, dynamically-typed scripting language with a clean syntax, a bytecode compiler, and a growing standard library.

Installation

pip install mo-language

After installing, use the mo command:

mo program.mo

Quick Example

fn greet(name) {
    print("Hello, " + name + "!")
}

greet("world")

pip install mo-language

After installing, use the mo command:

mo program.mo

Windows users: If mo is not found after install, add Python's Scripts folder to your PATH.
Mac/Linux users: If mo is not found, run the command the installer prints.

Language Guide

Variables

let x = 42
let name = "Mo"
let flag = true
let nothing = null

Reassign with = (no let):

x = x + 1

Types

Type	Example
Number	3.14, 100
String	"hello"
Bool	true, false
Null	null
List	[1, 2, 3]
Dict	{"key": "value"}

Operators

1 + 2        # arithmetic: + - * /
x == y       # equality: == !=
x < y        # comparison: < > <= >=
true && false  # logical: && ||
!true          # not

String concatenation uses +:

let msg = "Hello, " + "world"

Control Flow

if x > 0 {
    print("positive")
} else if x < 0 {
    print("negative")
} else {
    print("zero")
}

let i = 0
while i < 5 {
    print(i)
    i = i + 1
}

for item in [1, 2, 3] {
    print(item)
}

Functions

fn add(a, b) {
    return a + b
}

let result = add(3, 4)   # 7

Functions are first-class values:

let double = fn(x) { return x * 2 }

Lists

let nums = [10, 20, 30]
print(nums[0])          # 10
push(nums, 40)          # append
let last = pop(nums)    # remove last
print(len(nums))        # 3

Dicts

let user = {"name": "Alice", "age": 30}
print(user["name"])     # Alice
user["age"] = 31

for k in keys(user) {
    print(k + ": " + str(user[k]))
}

Dict keys can also be accessed with .:

print(user.name)
user.age = 32

Classes

class Animal {
    fn init(name) {
        self.name = name
    }
    fn speak() {
        print(self.name + " makes a sound")
    }
}

class Dog extends Animal {
    fn speak() {
        print(self.name + " barks")
    }
}

let d = Dog("Rex")
d.speak()   # Rex barks

Error Handling

try {
    throw "something went wrong"
} catch e {
    print("Caught: " + e)
}

Imports

import "std/math"
print(math_sqrt(16))   # 4.0

---

Standard Library

All stdlib modules are imported with import "std/<name>".

Core Modules

Module	Description
std/math	Math functions (sqrt, pow, abs, sin, cos, etc.)
std/random	Random numbers (int, float, choice)
std/time	Time functions (now, sleep, format)
std/json	JSON parse/stringify
std/file	File operations (read, write, append, exists, delete)
std/os	OS operations (getEnv, cwd, exit)
std/path	Path manipulation (join, dir, base, ext, abs)
std/string	String operations (toUpper, toLower, trim, split, replace)

Data Formats

Module	Description
std/yaml	YAML parse/stringify
std/xml	XML processing
std/csv	CSV parse/stringify
std/base64	Base64 encoding/decoding

Databases & Cache

Module	Description
std/sqlite	SQLite database
std/redis	Redis client
std/cache	LRU cache

Networking

Module	Description
std/http	HTTP server and client
std/net	TCP/UDP networking
std/url	URL parsing
std/websocket	WebSocket client

Security & Validation

Module	Description
std/crypto	SHA256, HMAC
std/jwt	JWT authentication
std/validator	Email, URL, phone validation

GUI & Media

Module	Description
std/gui	Tkinter GUI
std/audio	Audio playback

Text Processing

Module	Description
std/regex	Regular expressions
std/template	Template engine
std/fmt	String formatting

Utilities

Module	Description
std/datetime	Date/time handling
std/logging	Logging system
std/uuid	UUID generation
std/gzip	Compression
std/decimal	High precision decimals
std/config	Config file loader (TOML/YAML/JSON)
std/cli	CLI argument parsing
std/test	Unit testing framework

Data Structures

Module	Description
std/list	List operations (map, filter, reduce)
std/collections	Stack, queue, set, heap
std/concurrency	Threads and locks

System

Module	Description
std/process	Process management
std/email	SMTP email sending

Note: Built-in functions like print, len, str, input, range can be used directly without import.

---

Built-in Functions

Function	Description
print(x)	Print value
len(x)	Length of string/list/dict
str(x)	Convert to string
num(x)	Convert to number
type(x)	Type name string
range(n) / range(a,b)	List of integers
push(list, val)	Append to list
pop(list)	Remove and return last element
keys(dict)	List of dict keys
has(dict, key)	True if key exists
delete(dict, key)	Remove key from dict
input(prompt)	Read line from stdin
floor(x)	Floor
ceil(x)	Ceil
round(x)	Round
abs(x)	Absolute value

---

Testing

Run pytest Suite (Recommended)

pip install pytest
pytest tests/test_basic.py -v

The pytest suite covers core language features:

• Variables, operators, functions
• Lists, control flow (if/while/for)
• Exceptions (try/catch/throw)
• Built-in functions

Run Python Test Runner

python tests/test_runner.py          # run all tests
python tests/test_runner.py -v       # verbose mode
python tests/test_runner.py test.mo   # run specific file

The test runner discovers .mo test files in tests/, runs them with mo, and checks exit code.

Run Mo Test Files Directly

mo tests/run_tests.mo

Note: Some test files may have parser compatibility issues with newer language features.

Example test metadata file (tests/foo.mo.meta.json):

{
  "exit_code": 0,
  "output": "42"
}

---

CLI Reference

mo program.mo           # run with bytecode VM (default)
mo --interp program.mo  # run with tree-walk interpreter
mo --dis program.mo     # disassemble bytecode
mo                      # interactive REPL

---

Requirements

• Python 3.9+
• No external dependencies (stdlib only)

std/http fetch functions use urllib (built-in). For advanced HTTP needs, install requests separately.

---

Advanced Features

Type System

let x: int = 42          # type annotation
let y = 10  # int         # type comment

is_number(x)              # check type
is_string(x)
is_bool(x)
is_list(x)
is_dict(x)

cast(x, "string")         # type cast

std/regex

import "std/regex"

regex_match("^[a-z]+", "hello")   # true/false
regex_replace("cat", "the cat sat", "dog")  # "the dog sat"
regex_split("\\s+", "a b c")    # ["a", "b", "c"]

std/sqlite

import "std/sqlite"

let db = sqlite_open("data.db")
let rows = sqlite_exec(db, "SELECT * FROM users WHERE age > ?", [18])

for row in rows {
    print(row["name"])
}

sqlite_close(db)

std/yaml

import "std/yaml"

yaml_parse("key: value")       # string → dict
yaml_stringify({"key": "val"}) # dict → string

std/xml

import "std/xml"

xml_parse("<root><item>text</item></root>")  # string → element
xml_to_string(elem)                     # element → string

std/csv

import "std/csv"

csv_read("data.csv")                  # list of dicts
csv_write("output.csv", [{"a": "1"}])  # write list of dicts

std/net

import "std/net"

let sock = net_listen(8080)

while true {
    let conn = net_accept(sock)
    let data = net_recv(conn)
    net_send(conn, "HTTP/1.0 200 OK\r\n\r\nHello")
    net_close(conn)
}

std/gui

GUI module using Python's tkinter (requires pip install pillow for images).

import "std/gui"

let win = window("My Game", 800, 600)

rectangle(100, 100, 50, 50, "#00ff00")
circle(300, 200, 30, "#ff0000")
text(100, 160, "Player", 16, "#ffffff")
line(0, 0, 800, 600, "#888888")

on_click(fn(pos) {
    print("Clicked at: " + str(pos))
})

on_key(fn(key) {
    print("Pressed: " + key)
})

loop()

std/audio

import "std/audio"

audio_play("sound.mp3")

---

Package Manager

Mo has a package ecosystem for sharing reusable modules.

Search packages

mo search redis
mo search http

Install packages

mo install user/repo           # from GitHub
mo install user/repo@v1.0.0    # specific version
mo install https://example.com/foo.mo  # direct URL

List installed packages

mo list

Remove a package

mo remove <package-name>

Publish a package

mo publish

See mo publish for detailed publishing instructions.

---

LSP (Language Server Protocol)

Mo includes an LSP server for IDE editor support (hover, completion, diagnostics).

Start LSP Server

mo lsp                    # Start via stdio
mo lsp --stdio            # Explicit stdio mode
mo lsp --tcp --port 8765  # TCP mode on port 8765

VS Code Extension

A VS Code extension scaffold is provided in mo-vlang/:

• package.json - VS Code extension configuration
• syntaxes/mo.tmLanguage.json - Syntax highlighting
• src/extension.ts - LSP client

To install the extension:

cd mo-vlang
npm install
code .
# Press F5 to run the extension in development mode

Supported Features

• Hover: Hover over variables to see their type
• Completion: Auto-complete keywords and symbols
• Diagnostics: Real-time syntax error reporting