Build a polyglot app

This page builds one program that uses an npm package from JavaScript and a PyPI package from Python, in one process.

Needs	How
Python	`elide setup python`; `elide info` lists `python` under `Languages`
Cross-language calls	`--interop` on every run; there is no manifest setting
npm packages	`dependencies.npm` in `elide.pkl`, then `elide install --ecosystems=npm`
PyPI packages	A Python 3.13 virtual environment, selected with `VIRTUAL_ENV`

Set up the project

// elide.pkl
amends "elide:project.pkl"

name = "poly"

dependencies {
  npm { packages { "pretty-ms@9.3.1" } }
  pypi { packages { "humanize==4.12.3" } }
}

elide install --ecosystems=npm

A bare elide install installs Maven only. Elide does not install PyPI packages: elide install --ecosystems=pypi exits 2 with pypi installs are not available in this build. Keep the pypi block as the record, and install it with a Python 3.13 pip into a virtual environment:

python3.13 -m venv .venv
.venv/bin/pip install humanize==4.12.3
export VIRTUAL_ENV=$PWD/.venv

Elide loads packages from $VIRTUAL_ENV/lib/python3.13/site-packages. Pure-Python packages work; some C extension modules do not load (see Python).

Write the program

# report.py
import humanize

def size(n):
    return humanize.naturalsize(n)

def summarize(files):
    total = sum(f.bytes for f in files)
    return {"count": len(files), "total": humanize.naturalsize(total)}

// app.mjs
import prettyMs from "pretty-ms";
import report from "./report.py";

const files = [
  { name: "build.log", bytes: 1_500_000, ageMs: 172_800_000 },
  { name: "test.log", bytes: 32_000, ageMs: 5_400_000 },
];

for (const f of files) {
  console.log(`${f.name}: ${report.size(f.bytes)}, ${prettyMs(f.ageMs)} old`);
}

const summary = report.summarize(files);
console.log(`${summary.count} files, ${summary.total}`);

elide --interop run app.mjs

build.log: 1.5 MB, 2d old
test.log: 32.0 kB, 1h 30m old
2 files, 1.5 MB

A Python file’s default export is its module; import it from an .mjs entry. Two common failures:

Missing	Error
`--interop`	`Error: Cannot import 'report.py': JavaScript entries run without Python. Re-run with --interop to use it.`
`VIRTUAL_ENV`	`ModuleNotFoundError: No module named 'humanize'`

Pass values

# shapes.py
def label(order):
    return f"{order.item} x{order.qty}"

def doubled(prices):
    return [p * 2 for p in prices]

def config():
    return {"region": "eu", "retries": 3}

def apply(fn, value):
    return fn(value)

def fail():
    raise ValueError("bad input")

// values.mjs
import shapes from "./shapes.py";

console.log(shapes.label({ item: "bolt", qty: 4 }));
console.log(shapes.doubled([1, 2.5, 3]).map((n) => n + 1));
const config = shapes.config();
console.log(config.region, config.get("retries"));
console.log(JSON.stringify(Object.fromEntries(config.items())));
console.log(shapes.apply((s) => s.toUpperCase(), "shout"));
console.log(shapes.apply((v) => v, null), shapes.apply((v) => v, undefined));

elide --interop run values.mjs

bolt x4
(3)[3, 6, 7]
eu 3
{"region":"eu","retries":3}
SHOUT
null undefined

Value	From JavaScript into Python	From Python into JavaScript
Numbers, strings, booleans	Native values	Native values
`null`, `undefined` / `None`	`None`, and back unchanged	`null`
Object / `dict`	Read fields as attributes (`order.item`); `order["item"]` raises `TypeError`	Read keys as `d.key`, `d["key"]` or `d.get("key")`; `Object.keys`, `in` and `JSON.stringify` see the dict’s methods, not its keys. Convert with `Object.fromEntries(d.items())`.
Array / `list`	Iterable, indexable, `len()` works	`Array.isArray` is `true`; `map`, spread and `Array.from` work
Function	Callable	Callable

Call JavaScript from Python

The elide module evaluates JavaScript and returns its value:

# calls.py
from elide import js

slug = js("(s) => s.trim().toLowerCase().replaceAll(' ', '-')")
print(slug("  Hello Polyglot World "))

point = js("({ x: 1, y: 2, tags: ['a', 'b'] })")
print(point.x + point.y, point.tags[0], len(point.tags))
print(sorted(js("[3, 1, 2]")))

try:
    js("JSON.parse('{oops')")
except RuntimeError as e:
    print("caught:", e)

js("Promise.resolve(42)").then(lambda v: print("resolved:", v))

elide --interop python calls.py

hello-polyglot-world
3 a 2
[1, 2, 3]
caught: JSException: SyntaxError: Unexpected token o in JSON at position 1
resolved: 42

Handle errors

A Python exception reaches JavaScript as an Error whose name is the exception class; a JavaScript error reaches Python as RuntimeError. An uncaught error prints one stack across both languages:

// errors.mjs
import shapes from "./shapes.py";

try {
  shapes.fail();
} catch (e) {
  console.log(e.name, "|", e.message);
}

shapes.apply((x) => {
  throw new RangeError(`bad value ${x}`);
}, 7);

elide --interop run errors.mjs

ValueError | ValueError: bad input
RangeError: bad value 7
    at :=> (errors.mjs:10:9)
    at apply (shapes.py:11:12)
    at :module:eval (errors.mjs:9:1)

The program exits 1.

Call async code

From	Works	Does not work
JavaScript calling Python	Call a sync wrapper that runs `asyncio.run(...)`	`await` on a Python coroutine returns the coroutine, unrun
Python holding a JavaScript promise	`promise.then(callback)`	`await promise` raises `TypeError: object polyglot.ForeignObject can't be used in 'await' expression`

# rates.py
import asyncio

async def fetch_rate(code):
    await asyncio.sleep(0.01)
    return {"EUR": 0.92, "GBP": 0.79}[code]

def rate(code):
    return asyncio.run(fetch_rate(code))

def when_ready(promise, done):
    promise.then(lambda value: done(f"python saw {value}"))

// rates.mjs
import rates from "./rates.py";

console.log("rate:", rates.rate("EUR"));

const total = new Promise((resolve) => setTimeout(() => resolve(7), 10));
rates.when_ready(total, (msg) => console.log(msg));
await total;

elide --interop run rates.mjs

rate: 0.92
python saw 7

asyncio.run blocks the JavaScript event loop until the coroutine finishes. Python threads are not available under --interop.

Java and Kotlin

elide run Hello.kt runs Kotlin on its own; JavaScript and Python cannot call it. Importing it fails:

import hello from "./Hello.kt";

TypeError: Unsupported file extension: 'file:/path/to/Hello.kt'