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 }),
  }),
);

ExternalSystem: Annotating external systems

The last argument is its annotations: data for what is built on the model, read back as X.annotations. Add your own keys by augmenting Domain.Annotations.

Nothing a client calls shows it; it is for the people and the tools that read the model, as X.annotations. annotations is the class’s own static, so no operation may take that name.

export class Payments extends Domain.ExternalSystem(
  "Payments",
  { refund: { args: { settlementId: Schema.String, amount: Cents }, success: { refundId: Schema.String } } },
  { description: "The payment provider that refunds settlements." },
) {}