Crypto

Package: createCryptoPackage({ fetchPrice }). Opt-in, and not in the default engine: there is no free, keyless crypto price API to bundle, so you register it with your own fetch (see choosing packages).

crypto("BTC") looks up the current price of a coin. Like stocks, and for the same reason, the package is opt-in: there is no free crypto price API to ship, so you supply a fetchPrice backed by whichever provider and key you have.

import { ExpressionEngine } from "solve-engine";
import { BUILTIN_PACKAGES, createCryptoPackage } from "solve-engine/packages";

const crypto = createCryptoPackage({
  fetchPrice: async (coin, signal) => {
    const res = await fetch(`https://example.com/price/${coin}`, { signal });
    return { price: (await res.json()).usd, currency: "USD" };
  },
});

const engine = new ExpressionEngine({ packages: [...BUILTIN_PACKAGES, crypto] });

With that in place, a coin symbol resolves to its price, returned as ordinary money so the rest of the language does the arithmetic:

Expression	Result
crypto("BTC")	the current price of one coin
0.5 * crypto("BTC")	the value of half a coin
crypto("ETH") in GBP	the price converted to pounds

Because the price comes back as money, 0.5 * crypto("BTC") is the “half a Bitcoin in dollars” a reader wants, and in GBP converts through the currency package. The currency the answer is quoted in is whatever the provider returns, US dollars above.

Without a fetchPrice, crypto("BTC") returns a clearly named CRYPTO_NOT_CONFIGURED error rather than a faked or zero price: a number the caller did not provide is never guessed.