Reshaped and simplified Workflow definition

Author: oskardudycz

src/packages/emmett/src/index.ts

@@ -14,3 +14,4 @@ export * from './testing';
 export * from './typing';
 export * from './utils';
 export * from './validation';
+export * from './workflows';

src/packages/emmett/src/typing/index.ts

@@ -6,7 +6,6 @@ export * from './message';
 export * from './messageHandling';
 
 export * from './decider';
-export * from './workflow';
 
 export type Brand<K, T> = K & { readonly __brand: T };
 export type Flavour<K, T> = K & { readonly __brand?: T };

src/packages/emmett/src/typing/workflow.ts

@@ -0,0 +1,2 @@
+export * from './workflow';
+export * from './workflowProcessor';

src/packages/emmett/src/workflows/index.ts

@@ -0,0 +1,39 @@
+import type { EmmettError } from '../errors';
+import type { AnyCommand } from '../typing/command';
+import type { AnyEvent } from '../typing/event';
+
+/// Inspired by https://blog.bittacklr.be/the-workflow-pattern.html
+
+export type Workflow<
+  Input extends AnyEvent | AnyCommand,
+  State,
+  Output extends AnyEvent | AnyCommand,
+> = {
+  name?: string;
+  decide: (command: Input, state: State) => WorkflowOutput<Output>;
+  evolve: (currentState: State, event: WorkflowEvent<Input | Output>) => State;
+  initialState: () => State;
+};
+
+export type WorkflowEvent<Output extends AnyEvent | AnyCommand> = Extract<
+  Output,
+  { kind?: 'Event' }
+>;
+
+export type WorkflowCommand<Output extends AnyEvent | AnyCommand> = Extract<
+  Output,
+  { kind?: 'Command' }
+>;
+
+export type WorkflowOutput<Output extends AnyEvent | AnyCommand | EmmettError> =
+  Output | Output[];
+
+export const Workflow = <
+  Input extends AnyEvent | AnyCommand,
+  State,
+  Output extends AnyEvent | AnyCommand,
+>(
+  workflow: Workflow<Input, State, Output>,
+): Workflow<Input, State, Output> => {
+  return workflow;
+};