Mochi is a small, statically typed programming language
4 days ago
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Mochi is a small, statically typed programming language built for clarity, safety, and expressiveness — whether you're writing tools, processing real-time data, or powering intelligent agents.
Mochi is:
Agent-friendly: structured, safe, and embeddable
Declarative and functional, with clean, expressive syntax
Fast and portable: zero-dependency single binary
Testable by design with built-in test and expect blocks
Simple enough to explore in minutes. Powerful enough to build something real.
To run Mochi in a container, you’ll need to have Docker installed.
Make sure Docker is running before using any container-based commands.
Also, to use Mochi inside tools like Claude or VS Code Agent Mode, you may need a local LLM (like llama.cpp).
chmod +x mochi
mochi run examples/hello.mochi
mochi cheatsheet
It’s a single binary — no dependencies, no setup.
Use Docker to run Mochi without installing anything:
docker run -i --rm ghcr.io/mochilang/mochi run examples/hello.mochi
docker run -i --rm ghcr.io/mochilang/mochi serve
Want to use it like a native CLI? Set an alias:
alias mochi="docker run -i --rm -v $PWD:/app -w /app ghcr.io/mochilang/mochi"
Then use it anywhere:
mochi run examples/hello.mochi
mochi test examples/math.mochi
mochi build examples/hello.mochi -o hello
mochi cheatsheet
To hack on the language or contribute:
git clone https://github.com/mochilang/mochi
cd mochi
make install # install Deno for TypeScript tests
make build
make test
This installs mochi into ~/bin and runs the full test suite.
Usage with Visual Studio Code
There is a VS Code extension under tools/vscode that bundles the Mochi language syntax. Run npm install and npm run package in that folder to build mochi.vsix for local installation.
You can also run Mochi as an MCP server inside VS Code’s agent mode.
The easiest way is to use a .vscode/mcp.json config file in your project:
Usage: mochi [--version] <command> [args]
Commands:
run Run a Mochi source file
test Run test blocks
build Compile to binary or other languages
init Initialize a new module
get Download module dependencies
repl Start interactive REPL
llm Send prompt to LLM
infer Infer externs from a package
serve Start MCP server
cheatsheet Print language reference
Examples:
mochi run examples/hello.mochi
mochi test examples/math.mochi
mochi build examples/hello.mochi -o hello
mochi build --target py examples/hello.mochi -o hello.py
mochi init mymodule
mochi get
mochi llm "hello"
mochi infer go fmt
mochi cheatsheet
Send this to Claude or run it in your shell:
let π = 3.14
fun area(r: float): float {
return π * r * r
}
print(area(10.0))
test "π" {
expect π == 3.14
expect area(10.0) == 314.0
}
let 🍡 = "🍡૮₍ ˃ ⤙ ˂ ₎ა"
print(🍡)
It will output:
Mochi is expressive, safe, and joyful.
let name = "Mochi"
var count = 1
Immutable by default. Use var for mutable bindings.
if count > 0 {
print("positive")
} else {
print("non-positive")
}
for i in 0..3 {
print(i)
}
var j = 0
while j < 3 {
print(j)
j = j + 1
}
Use for to iterate over ranges or collections. A while loop continues
running until its condition becomes false.
fun double(x: int): int {
return x * 2
}
let square = fun(x: int): int => x * x
let user = {"name": "Ana", "age": 22}
let tags = {"a", "b", "c"}
let nums = [1, 2, 3]
print(user["name"])
print(tags)
print(nums[1])
Strings behave like read‐only lists of characters. They can be indexed
and iterated just like a list:
let text = "hello"
print(text[1]) // "e"
for ch in text {
print(ch)
}
Query lists with SQL-like syntax using from, where, sort by, skip, take and select.
Datasets can also be loaded from external files.
type Person {
name: string
age: int
}
let people = load "people.yaml" as Person
let adults = from p in people
where p.age >= 18
select { name: p.name, age: p.age }
for a in adults {
print(a.name, "is", a.age)
}
save adults to "adults.json"
Combine records from multiple lists using different join types.
Inner join keeps only matching pairs:
let result = from o in orders
join from c in customers on o.customerId == c.id
select { orderId: o.id, customer: c.name }
Cross join pairs every item from both lists:
let pairs = from o in orders
from c in customers
select { order: o.id, customer: c.name }
Left join keeps all left items even if the right side has no match:
let ordersWithCustomer = from o in orders
left join c in customers on o.customerId == c.id
select { orderId: o.id, customer: c }
Right join keeps all right items even if the left side has no match:
let customersWithOrder = from c in customers
right join o in orders on o.customerId == c.id
select { customerName: c.name, order: o }
test "math" {
expect 2 + 2 == 4
expect 5 > 3
}
Invoke large language models directly from Mochi using the generate block.
Models can be configured globally with a model block and referenced by name.
let poem = generate text {
prompt: "Write a haiku about spring"
}
print(poem)
model quick {
provider: "openai"
name: "gpt-3.5-turbo"
}
let fancy = generate text {
model: "quick"
prompt: "Write a haiku about spring"
}
print(fancy)
type Person {
name: string
age: int
email: string
}
let p = generate Person {
prompt: "Generate a fictional software engineer"
}
print(p.name)
let vec = generate embedding {
text: "hello world"
normalize: true
}
print(len(vec))
You can also expose Mochi functions as tools for the LLM to call:
fun getWeather(location: string): string {
if location == "Paris" {
return "sunny with a gentle breeze"
}
return "weather data unavailable"
}
fun calc(expression: string): string {
if expression == "2 + 2" {
return "4"
}
return "error"
}
let result = generate text {
prompt: "What's the weather like in Paris and what's 2 + 2?",
tools: [
getWeather {
description: "Returns the current weather for a given city"
},
calc {
description: "Evaluates a simple math expression"
}
]
}
print(result)
Retrieve JSON over HTTP with the new fetch expression. Results are automatically
decoded into the expected type. Use with to supply options such as the HTTP
method, headers, or request body.
type Todo {
userId: int
id: int
title: string
completed: bool
}
let todo = fetch "https://example.com/todos/1" as Todo
let created: Todo = fetch "https://example.com/todos" with {
method: "POST",
headers: {
"Content-Type": "application/json"
},
body: todo
}
Mochi now supports union types declared with the | syntax as well as inline
methods defined inside type blocks.
type Tree =
Leaf
| Node(left: Tree, value: int, right: Tree)
fun sum(t: Tree): int {
return match t {
Leaf => 0
Node(l, v, r) => sum(l) + v + sum(r)
}
}
type Circle {
radius: float
fun area(): float {
return 3.14 * radius * radius
}
}
Organize reusable code with packages. Each directory forms a package. Declare
the package name at the top of each file and mark exported names with export.
package mathutils
export fun add(a: int, b: int): int {
return a + b
}
Import packages in other files:
import "mathutils"
print(mathutils.add(2, 3))
import "mathutils" as mu
print(mu.add(2, 3))
Declare streams of structured events and handle them with agents. Use emit to
send events to a stream.
stream Sensor { id: string, temperature: float }
on Sensor as s {
print(s.id, s.temperature)
}
emit Sensor { id: "sensor-1", temperature: 22.5 }
Agents can also maintain persistent state and expose intent functions that
can be called like methods or via the MCP server.
agent Monitor {
var count: int = 0
on Sensor as s {
count = count + 1
}
intent status(): string {
return "events = " + str(count)
}
}
let m = Monitor {}
emit Sensor { id: "sensor-2", temperature: 30.0 }
print(m.status())
Foreign Function Interface
Use the import keyword to access libraries from other languages. Declare
extern variables and functions to call them directly. If an alias is not
specified with as, the final path component will be used as the module name.
import go "math" as math
extern fun math.Sqrt(x: float): float
print(math.Sqrt(16.0))
When running mochi serve, the following tools are available:
Tool
Description
mochi_eval
Run a program and return output
mochi_cheatsheet
Return language reference
These tools integrate with Claude, Open Interpreter, and others.
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