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.

Each price records the provider it came from and when it was fetched, and every line computed from it carries that record; pass provider with your service’s name so the record can name it. To keep a price from moving, end the line with frozen; see frozen answers.