Domain.ExternalSystem reference

ExternalSystem

Source: packages/domain/src/ExternalSystem.ts

An outside system your model calls, such as a payment provider or a fraud scorer, declared as the operations the model uses. It’s an Effect service: policies yield* it, an adapter implements it with toLayer, and stories answer its calls themselves, so the model depends on what it asks for and never on a vendor.

ExternalSystem: Declaring an external system and an adapter

import * as Domain from "@alchemy.run/domain";
import * as Effect from "effect/Effect";
import * as Schema from "effect/Schema";

export class FraudCheck extends Domain.ExternalSystem("FraudCheck", {
  score: { args: { accountId: Schema.String, amount: Schema.Int }, success: { risk: Schema.Number } },
}) {}

export const SiftFraudCheck = FraudCheck.toLayer(
  Effect.succeed({
    score: ({ amount }) => Effect.succeed({ risk: amount >= 80_000 ? 0.95 : 0.05 }),
  }),
);