Call Jacquard from your application

Jacquard Host lets Python, TypeScript and Go code call checked Jacquard functions. Jacquard owns the decision logic. Your application owns persistence, I/O and lifetimes, and implements each effect the logic declares with an ordinary service.

The effect row is the interface

This policy decides how many units to reorder. It cannot read stock levels itself; it performs on-hand, an operation of a Stock effect that the host must supply.

-- How many units to order. Stock levels come from the host.
once effect Stock where {
  on-hand : (sku: Text) -> Int
}

reorder : (Text, Int) ->{Stock} Int
reorder(sku, target) = {
  let have = on-hand(sku)
  if lt(have, target) then sub(target, have) else 0
}

$ jac check reorder.jac --print-sigs
reorder : (Text, Int) ->{Stock} Int

The signature ->{Stock} is everything the policy can ask its host for. A reviewer reads one line to learn what the application has to provide, and what the policy cannot reach.

Your service implements the effect

In Python, an object binds to an effect, and kebab-case operations select snake_case methods. The SDK prepares and checks the program once, then each call runs it in a fresh Core worker that asks the host for every on-hand.

from jacquard_host import Runtime


class Warehouse:
    def __init__(self, counts):
        self.counts = counts

    def on_hand(self, sku):
        return self.counts.get(sku, 0)


program = Runtime().prepare("reorder.jac")
reorder = program.function("reorder", effects={"Stock": Warehouse({"bolt": 12})})

print(reorder("bolt", 40))
print(reorder("nut", 40))

$ python app.py
28
40

A missing service is refused before anything runs

Every operation of every effect in the function's checked row must be bound. Leave one out and the SDK refuses while binding, before a worker starts or any outside action happens.

from jacquard_host import BindingError, Runtime

program = Runtime().prepare("reorder.jac")
try:
    program.function("reorder")
except BindingError as error:
    print(error)

$ python missing.py
reorder: missing handlers: stock.on_hand. Bind every operation in each declared effect before calling

Recorded with jacquard-host 48946f1 and Core 0.3.0, re-checked by scripts/verify_examples.sh.

Three SDKs, one protocol

• Python 3.11+ maps dataclasses, enums and optional fields, and binds synchronous services. See the SDK guide.
• TypeScript on Node.js 22+ supports async service methods. See the TypeScript walkthrough.
• Go 1.24+ maps native Go types and honours context cancellation. See the Go walkthrough.

None has third-party runtime dependencies. Each implements Core's jacquard-host-v0 protocol independently and replays the same pinned conformance kit. The adapters pin one Core release, 0.3.0, in core-pin.json, and the setup script installs it and verifies its checksum. Every invocation keeps its raw protocol evidence, which you can render or save for inspection.

Get started

git clone https://github.com/jbwinters/jacquard-host
cd jacquard-host
python3 -m venv .venv && . .venv/bin/activate
python -m pip install .
eval "$(python3 scripts/core_pin.py install)"
python examples/quickstart/run.py

The example projects go further, up to a fulfillment service with SQLite transactions, idempotency and explicit outbox recovery. Setup covers other platforms and building Core from source.

The boundary