Hierarchical Checkout

A multi-step checkout flow covering cart, shipping, and payment. The payment step is complex enough to warrant its own sub-states — making this a natural fit for a nested submachine. The flattened variant shows what the same flow looks like when every state is promoted to the top level.

Nested (submachine)Flattened

open in viewer → PreviewCode

machine.ts

import {
  createMachine,
  defineStates,
  effect,
  setup,
  withReset,
  matchina,
} from "matchina";
import { submachine, nestedHsmRoot } from "matchina/hsm";

// Hierarchical checkout: main flow contains a payment submachine
export const paymentStates = defineStates({
  MethodEntry: undefined,
  Authorizing: undefined,
  AuthChallenge: undefined,
  AuthorizationError: undefined,
  Authorized: { final: true },
});

// Create payment machine factory
function createPayment() {
  const m = matchina(
    paymentStates,
    {
      MethodEntry: {
        authorize: "Authorizing",
        exit: "MethodEntry", // Exit resets to initial state
      },
      Authorizing: {
        authRequired: "AuthChallenge",
        authSucceeded: "Authorized",
        authFailed: "AuthorizationError",
        exit: "MethodEntry", // Exit from any payment state goes back to MethodEntry
      },
      AuthChallenge: {
        authSucceeded: "Authorized",
        authFailed: "AuthorizationError",
        exit: "MethodEntry",
      },
      AuthorizationError: {
        retry: "MethodEntry",
        exit: "MethodEntry",
      },
      Authorized: {
        exit: "MethodEntry",
      },
    },
    paymentStates.MethodEntry()
  );

  return withReset(nestedHsmRoot(m), paymentStates.MethodEntry());
}

const paymentFactory = submachine(createPayment, { id: "payment" });

const checkoutStates = defineStates({
  Cart: undefined,
  Shipping: undefined,
  ShippingPaid: undefined,
  Payment: paymentFactory,
  Review: undefined,
  Confirmation: undefined,
});

export function createCheckoutMachine() {
  const checkout = createMachine(
    checkoutStates,
    {
      Cart: { proceed: "Shipping" },
      Shipping: {
        back: "Cart",
        proceed: "Payment",
      },
      Payment: {
        back: "Shipping",
        exit: "Shipping",
        "child.exit": "Review",
      },
      Review: {
        back: "ShippingPaid",
        changePayment: "Payment",
        submitOrder: "Confirmation",
      },
      ShippingPaid: {
        back: "Cart",
        proceed: "Review",
        changePayment: "Payment",
      },
      Confirmation: { restart: "Cart" },
    },
    "Cart"
  );

  const hierarchical = nestedHsmRoot(checkout);

  // Get payment machine from state to wire up reset effect
  const getPayment = () => {
    const state = hierarchical.getState();
    return state.is("Payment") ? state.data.machine : null;
  };

  setup(hierarchical)(
    effect((ev) => {
      if (ev.type === "restart") {
        const payment = getPayment();
        if (payment) {
          payment.reset!();
        }
        return true;
      }
    })
  );

  return hierarchical;
}

// Type export for payment machine (used by CheckoutViewNested context)
export type PaymentMachine = ReturnType<typeof createPayment>;

machine-flat.ts

import { createHSM } from "matchina/hsm";

export function createFlatCheckoutMachine() {
  return createHSM({
    initial: "Cart",
    states: {
      Cart: {
        on: { proceed: "Shipping" }
      },
      Shipping: {
        on: {
          back: "Cart",
          proceed: "Payment"
        }
      },
      Payment: {
        initial: "MethodEntry",
        states: {
          MethodEntry: {
            on: { authorize: "Authorizing" }
          },
          Authorizing: {
            on: {
              authRequired: "AuthChallenge",
              authSucceeded: "Authorized",
              authFailed: "AuthorizationError"
            }
          },
          AuthChallenge: {
            on: {
              authSucceeded: "Authorized",
              authFailed: "AuthorizationError"
            }
          },
          AuthorizationError: {
            on: { retry: "MethodEntry" }
          },
          Authorized: {
            // Final payment state - child.exit automatically triggered
          }
        },
        on: {
          back: "Shipping",
          "child.exit": "Review"
        }
      },
      Review: {
        on: {
          back: "ShippingPaid",
          changePayment: "Payment",
          submitOrder: "Confirmation"
        }
      },
      ShippingPaid: {
        on: {
          back: "Cart",
          proceed: "Review",
          changePayment: "Payment"
        }
      },
      Confirmation: {
        on: { restart: "Cart" }
      }
    }
  });
}

CheckoutViewNested.tsx

import React, { useContext } from "react";
import { useMachine } from "matchina/react";
import { eventApi } from "matchina";
import type { FactoryMachine } from "matchina";
import { createCheckoutMachine, type PaymentMachine } from "./machine";
import { CartItems, ShippingForm, CardForm, StepIndicator, Confirmation, STATE_TO_STEP } from "./ui";

type CheckoutMachine = FactoryMachine<ReturnType<typeof createCheckoutMachine>>;
type CheckoutActions = ReturnType<typeof eventApi<CheckoutMachine>>;

const CheckoutContext = React.createContext<{
  machine: CheckoutMachine;
  paymentMachine?: PaymentMachine;
  actions: CheckoutActions;
} | null>(null);

function useCheckoutContext() {
  const ctx = useContext(CheckoutContext);
  if (!ctx) throw new Error("useCheckoutContext must be used within CheckoutContext.Provider");
  return ctx;
}

interface CheckoutViewNestedProps {
  machine: FactoryMachine<ReturnType<typeof createCheckoutMachine>>;
}

export function CheckoutViewNested({ machine }: CheckoutViewNestedProps) {
  useMachine(machine);
  const state = machine.getState() as { key: string; data?: any };
  const parent = state.key;

  let paymentMachine: PaymentMachine | undefined;
  if (state.data?.machine) paymentMachine = state.data.machine;

  const actions = eventApi(machine);
  const currentStep = STATE_TO_STEP[parent] ?? "Cart";

  return (
    <CheckoutContext.Provider value={{ machine, paymentMachine, actions }}>
      <div className="max-w-xs mx-auto bg-card rounded-2xl border border-border p-5">
        <StepIndicator currentStep={currentStep} />
        <div className="space-y-5">
          <MainContent parentState={parent} />
          {parent === "Payment" && paymentMachine && (
            <PaymentSection paymentMachine={paymentMachine} />
          )}
          <CheckoutControls parentState={parent} />
        </div>
      </div>
    </CheckoutContext.Provider>
  );
}

function MainContent({ parentState }: { parentState: string }) {
  switch (parentState) {
    case "Cart":
      return <CartItems />;
    case "Shipping":
      return <ShippingForm />;
    case "Review":
    case "ShippingPaid":
      return (
        <div>
          <p className="text-[9px] font-mono uppercase tracking-widest text-muted-foreground mb-2">Order Summary</p>
          <CartItems readOnly />
        </div>
      );
    case "Confirmation":
      return <Confirmation />;
    default:
      return null;
  }
}

function PaymentSection({ paymentMachine }: { paymentMachine: PaymentMachine }) {
  useMachine(paymentMachine);
  const paymentActions = eventApi(paymentMachine);
  const pState = paymentMachine.getState();

  return (
    <div className="space-y-3">
      {pState.match({
        MethodEntry: () => (
          <div className="space-y-3">
            <CardForm />
            <button onClick={() => paymentActions.authorize()} className="btn btn-primary w-full">
              Authorize Payment
            </button>
          </div>
        ),
        Authorizing: () => (
          <div className="space-y-3">
            <div className="flex items-center gap-2 text-sm text-muted-foreground">
              <div className="w-3.5 h-3.5 border-2 border-border border-t-primary rounded-full animate-spin" />
              Authorizing payment
            </div>
            <div className="border border-border rounded-xl p-3 space-y-2">
              <p className="text-[8px] font-mono uppercase tracking-widest text-muted-foreground">Simulate outcome</p>
              <div className="flex gap-2">
                <button onClick={() => paymentActions.authSucceeded()} className="btn btn-outline btn-sm flex-1">Approve</button>
                <button onClick={() => paymentActions.authRequired()} className="btn btn-outline btn-sm flex-1">Challenge</button>
                <button onClick={() => paymentActions.authFailed()} className="btn btn-destructive btn-sm flex-1">Fail</button>
              </div>
            </div>
          </div>
        ),
        AuthChallenge: () => (
          <div className="space-y-3">
            <div className="bg-muted rounded-xl p-3">
              <p className="text-sm font-medium mb-0.5">Authentication required</p>
              <p className="text-xs text-muted-foreground">Your bank is requesting additional verification.</p>
            </div>
            <div className="flex gap-2">
              <button onClick={() => paymentActions.authSucceeded()} className="btn btn-primary btn-sm flex-1">Approve</button>
              <button onClick={() => paymentActions.authFailed()} className="btn btn-destructive btn-sm flex-1">Deny</button>
            </div>
          </div>
        ),
        AuthorizationError: () => (
          <div className="space-y-3">
            <div className="bg-destructive/10 border border-destructive/20 rounded-xl p-3">
              <p className="text-sm text-destructive font-medium">Authorization failed</p>
              <p className="text-xs text-muted-foreground mt-0.5">Your payment could not be authorized.</p>
            </div>
            <div className="flex gap-2">
              <button onClick={() => paymentActions.retry()} className="btn btn-outline btn-sm flex-1">Try Again</button>
              <button onClick={() => paymentActions.exit()} className="btn btn-ghost btn-sm flex-1">Cancel</button>
            </div>
          </div>
        ),
        Authorized: () => (
          <div className="flex items-center gap-2 text-[oklch(0.55_0.16_142)] text-sm font-medium bg-[oklch(0.55_0.16_142)]/10 rounded-xl px-3 py-2.5">
            <span>✓</span>
            <span>Payment authorized</span>
          </div>
        ),
      })}
    </div>
  );
}

function CheckoutControls({ parentState }: { parentState: string }) {
  const { actions } = useCheckoutContext();

  switch (parentState) {
    case "Cart":
      return (
        <button onClick={() => actions.proceed()} className="btn btn-primary w-full">
          Continue to Shipping
        </button>
      );
    case "Shipping":
      return (
        <div className="flex gap-2">
          <button onClick={() => actions.back()} className="btn btn-outline flex-1">Back</button>
          <button onClick={() => actions.proceed()} className="btn btn-primary flex-1">Continue to Payment</button>
        </div>
      );
    case "Payment":
      return (
        <button onClick={() => actions.back()} className="btn btn-outline w-full">Back to Shipping</button>
      );
    case "ShippingPaid":
      return (
        <div className="space-y-2">
          <button onClick={() => actions.proceed()} className="btn btn-primary w-full">Continue to Review</button>
          <div className="flex gap-2">
            <button onClick={() => actions.back()} className="btn btn-outline btn-sm flex-1">Back to Cart</button>
            <button onClick={() => actions.changePayment()} className="btn btn-outline btn-sm flex-1">Change Payment</button>
          </div>
        </div>
      );
    case "Review":
      return (
        <div className="space-y-2">
          <button onClick={() => actions.submitOrder()} className="btn btn-primary w-full">Place Order</button>
          <div className="flex gap-2">
            <button onClick={() => actions.back()} className="btn btn-outline btn-sm flex-1">Back</button>
            <button onClick={() => actions.changePayment()} className="btn btn-outline btn-sm flex-1">Change Payment</button>
          </div>
        </div>
      );
    case "Confirmation":
      return (
        <button onClick={() => actions.restart()} className="btn btn-outline w-full">Start New Order</button>
      );
    default:
      return null;
  }
}

CheckoutViewFlat.tsx

import React, { useContext } from "react";
import { useMachine } from "matchina/react";
import { eventApi } from "matchina";
import { parseFlatStateKey } from "matchina/hsm";
import { createFlatCheckoutMachine } from "./machine-flat";
import { CartItems, ShippingForm, CardForm, StepIndicator, Confirmation, STATE_TO_STEP } from "./ui";

type CheckoutMachine = ReturnType<typeof createFlatCheckoutMachine>;
type CheckoutActions = ReturnType<typeof eventApi<CheckoutMachine>>;

const CheckoutContext = React.createContext<{
  machine: CheckoutMachine;
  actions: CheckoutActions;
} | null>(null);

function useCheckoutContext() {
  const ctx = useContext(CheckoutContext);
  if (!ctx) throw new Error("useCheckoutContext must be used within CheckoutContext.Provider");
  return ctx;
}

interface CheckoutViewFlatProps {
  machine: CheckoutMachine;
}

export function CheckoutViewFlat({ machine }: CheckoutViewFlatProps) {
  useMachine(machine);
  const state = machine.getState() as { key: string; data?: any };
  const parsed = parseFlatStateKey(state.key);
  const parent = parsed.parent;
  const child = parsed.child;
  const actions = eventApi(machine);
  const currentStep = STATE_TO_STEP[parent] ?? "Cart";

  return (
    <CheckoutContext.Provider value={{ machine, actions }}>
      <div className="max-w-xs mx-auto bg-card rounded-2xl border border-border p-5">
        <StepIndicator currentStep={currentStep} />
        <div className="space-y-5">
          <MainContent parent={parent} child={child} />
          <CheckoutControls parent={parent} />
        </div>
      </div>
    </CheckoutContext.Provider>
  );
}

function MainContent({ parent, child }: { parent: string; child: string | null }) {
  if (parent === "Cart") return <CartItems />;
  if (parent === "Shipping") return <ShippingForm />;
  if (parent === "Payment" && child) return <PaymentContent child={child} />;
  if (parent === "Review" || parent === "ShippingPaid") {
    return (
      <div>
        <p className="text-[9px] font-mono uppercase tracking-widest text-muted-foreground mb-2">Order Summary</p>
        <CartItems readOnly />
      </div>
    );
  }
  if (parent === "Confirmation") return <Confirmation />;
  return null;
}

function PaymentContent({ child }: { child: string }) {
  const { actions } = useCheckoutContext();

  switch (child) {
    case "MethodEntry":
      return (
        <div className="space-y-3">
          <CardForm />
          <button onClick={() => actions.authorize?.()} className="btn btn-primary w-full">
            Authorize Payment
          </button>
        </div>
      );
    case "Authorizing":
      return (
        <div className="space-y-3">
          <div className="flex items-center gap-2 text-sm text-muted-foreground">
            <div className="w-3.5 h-3.5 border-2 border-border border-t-primary rounded-full animate-spin" />
            Authorizing payment
          </div>
          <div className="border border-border rounded-xl p-3 space-y-2">
            <p className="text-[8px] font-mono uppercase tracking-widest text-muted-foreground">Simulate outcome</p>
            <div className="flex gap-2">
              <button onClick={() => actions.authSucceeded?.()} className="btn btn-outline btn-sm flex-1">Approve</button>
              <button onClick={() => actions.authRequired?.()} className="btn btn-outline btn-sm flex-1">Challenge</button>
              <button onClick={() => actions.authFailed?.()} className="btn btn-destructive btn-sm flex-1">Fail</button>
            </div>
          </div>
        </div>
      );
    case "AuthChallenge":
      return (
        <div className="space-y-3">
          <div className="bg-muted rounded-xl p-3">
            <p className="text-sm font-medium mb-0.5">Authentication required</p>
            <p className="text-xs text-muted-foreground">Your bank is requesting additional verification.</p>
          </div>
          <div className="flex gap-2">
            <button onClick={() => actions.authSucceeded?.()} className="btn btn-primary btn-sm flex-1">Approve</button>
            <button onClick={() => actions.authFailed?.()} className="btn btn-destructive btn-sm flex-1">Deny</button>
          </div>
        </div>
      );
    case "AuthorizationError":
      return (
        <div className="space-y-3">
          <div className="bg-destructive/10 border border-destructive/20 rounded-xl p-3">
            <p className="text-sm text-destructive font-medium">Authorization failed</p>
            <p className="text-xs text-muted-foreground mt-0.5">Your payment could not be authorized.</p>
          </div>
          <button onClick={() => actions.retry?.()} className="btn btn-outline btn-sm">Try Again</button>
        </div>
      );
    case "Authorized":
      return (
        <div className="flex items-center gap-2 text-[oklch(0.55_0.16_142)] text-sm font-medium bg-[oklch(0.55_0.16_142)]/10 rounded-xl px-3 py-2.5">
          <span>✓</span>
          <span>Payment authorized</span>
        </div>
      );
    default:
      return null;
  }
}

function CheckoutControls({ parent }: { parent: string }) {
  const { actions } = useCheckoutContext();

  switch (parent) {
    case "Cart":
      return (
        <button onClick={() => actions.proceed?.()} className="btn btn-primary w-full">
          Continue to Shipping
        </button>
      );
    case "Shipping":
      return (
        <div className="flex gap-2">
          <button onClick={() => actions.back?.()} className="btn btn-outline flex-1">Back</button>
          <button onClick={() => actions.proceed?.()} className="btn btn-primary flex-1">Continue to Payment</button>
        </div>
      );
    case "Payment":
      return (
        <button onClick={() => actions.back?.()} className="btn btn-outline w-full">Back to Shipping</button>
      );
    case "ShippingPaid":
      return (
        <div className="space-y-2">
          <button onClick={() => actions.proceed?.()} className="btn btn-primary w-full">Continue to Review</button>
          <div className="flex gap-2">
            <button onClick={() => actions.back?.()} className="btn btn-outline btn-sm flex-1">Back to Cart</button>
            <button onClick={() => actions.changePayment?.()} className="btn btn-outline btn-sm flex-1">Change Payment</button>
          </div>
        </div>
      );
    case "Review":
      return (
        <div className="space-y-2">
          <button onClick={() => actions.submitOrder?.()} className="btn btn-primary w-full">Place Order</button>
          <div className="flex gap-2">
            <button onClick={() => actions.back?.()} className="btn btn-outline btn-sm flex-1">Back</button>
            <button onClick={() => actions.changePayment?.()} className="btn btn-outline btn-sm flex-1">Change Payment</button>
          </div>
        </div>
      );
    case "Confirmation":
      return (
        <button onClick={() => actions.restart?.()} className="btn btn-outline w-full">Start New Order</button>
      );
    default:
      return null;
  }
}

index.tsx

import { useMemo, useState } from "react";
import { createFlatCheckoutMachine } from "./machine-flat";
import { createCheckoutMachine } from "./machine";
import { CheckoutViewFlat } from "./CheckoutViewFlat";
import { CheckoutViewNested } from "./CheckoutViewNested";
import { MachineVisualizer } from "@components/MachineVisualizer";

type Mode = "flat" | "nested";

export default function HSMCheckoutIndex() {
  const [mode, setMode] = useState<Mode>("flat");

  // Create separate machines for each mode
  const flatMachine = useMemo(() => createFlatCheckoutMachine(), []);
  const nestedMachine = useMemo(() => createCheckoutMachine(), []);
  const machine = mode === "flat" ? flatMachine : nestedMachine;

  return (
    <div className="space-y-6">
      {/* Mode Toggle - Sticky below header */}
      <div className="flex justify-center mb-6 sticky top-16 z-10 py-2 bg-white/80 dark:bg-gray-900/80 backdrop-blur-sm -mx-4 px-4">
        <div className="inline-flex bg-gray-100 dark:bg-gray-800 p-1 rounded-lg">
          <button
            onClick={() => setMode("flat")}
            className={`px-4 py-2 rounded-md text-sm font-medium transition-colors ${
              mode === "flat"
                ? "bg-white dark:bg-gray-700 text-blue-600 dark:text-blue-400 shadow-sm"
                : "text-gray-500 hover:text-gray-700 dark:text-gray-400 dark:hover:text-gray-200"
            }`}
          >
            Flattened
          </button>
          <button
            onClick={() => setMode("nested")}
            className={`px-4 py-2 rounded text-sm font-medium transition-colors ${
              mode === "nested"
                ? "bg-white dark:bg-gray-700 text-blue-600 dark:text-blue-400 shadow-sm"
                : "text-gray-500 hover:text-gray-700 dark:text-gray-400 dark:hover:text-gray-200"
            }`}
          >
            Nested (Hierarchical)
          </button>
        </div>
      </div>

      {mode === "flat" ? (
        <CheckoutViewFlat machine={flatMachine} />
      ) : (
        <CheckoutViewNested machine={nestedMachine} />
      )}

      <MachineVisualizer
        key={mode} // Force re-mount of visualizer when mode changes
        machine={machine}
        title={`Hierarchical Traffic Light (${mode === "flat" ? "Flattened" : "Nested"})`}
        defaultViz="sketch"
        interactive={true}
        layout="stacked"
      />
    </div>
  );
}

open in viewer → PreviewCode

machine-flat.ts

import { createHSM } from "matchina/hsm";

export function createFlatCheckoutMachine() {
  return createHSM({
    initial: "Cart",
    states: {
      Cart: {
        on: { proceed: "Shipping" }
      },
      Shipping: {
        on: {
          back: "Cart",
          proceed: "Payment"
        }
      },
      Payment: {
        initial: "MethodEntry",
        states: {
          MethodEntry: {
            on: { authorize: "Authorizing" }
          },
          Authorizing: {
            on: {
              authRequired: "AuthChallenge",
              authSucceeded: "Authorized",
              authFailed: "AuthorizationError"
            }
          },
          AuthChallenge: {
            on: {
              authSucceeded: "Authorized",
              authFailed: "AuthorizationError"
            }
          },
          AuthorizationError: {
            on: { retry: "MethodEntry" }
          },
          Authorized: {
            // Final payment state - child.exit automatically triggered
          }
        },
        on: {
          back: "Shipping",
          "child.exit": "Review"
        }
      },
      Review: {
        on: {
          back: "ShippingPaid",
          changePayment: "Payment",
          submitOrder: "Confirmation"
        }
      },
      ShippingPaid: {
        on: {
          back: "Cart",
          proceed: "Review",
          changePayment: "Payment"
        }
      },
      Confirmation: {
        on: { restart: "Cart" }
      }
    }
  });
}

CheckoutViewFlat.tsx

import React, { useContext } from "react";
import { useMachine } from "matchina/react";
import { eventApi } from "matchina";
import { parseFlatStateKey } from "matchina/hsm";
import { createFlatCheckoutMachine } from "./machine-flat";
import { CartItems, ShippingForm, CardForm, StepIndicator, Confirmation, STATE_TO_STEP } from "./ui";

type CheckoutMachine = ReturnType<typeof createFlatCheckoutMachine>;
type CheckoutActions = ReturnType<typeof eventApi<CheckoutMachine>>;

const CheckoutContext = React.createContext<{
  machine: CheckoutMachine;
  actions: CheckoutActions;
} | null>(null);

function useCheckoutContext() {
  const ctx = useContext(CheckoutContext);
  if (!ctx) throw new Error("useCheckoutContext must be used within CheckoutContext.Provider");
  return ctx;
}

interface CheckoutViewFlatProps {
  machine: CheckoutMachine;
}

export function CheckoutViewFlat({ machine }: CheckoutViewFlatProps) {
  useMachine(machine);
  const state = machine.getState() as { key: string; data?: any };
  const parsed = parseFlatStateKey(state.key);
  const parent = parsed.parent;
  const child = parsed.child;
  const actions = eventApi(machine);
  const currentStep = STATE_TO_STEP[parent] ?? "Cart";

  return (
    <CheckoutContext.Provider value={{ machine, actions }}>
      <div className="max-w-xs mx-auto bg-card rounded-2xl border border-border p-5">
        <StepIndicator currentStep={currentStep} />
        <div className="space-y-5">
          <MainContent parent={parent} child={child} />
          <CheckoutControls parent={parent} />
        </div>
      </div>
    </CheckoutContext.Provider>
  );
}

function MainContent({ parent, child }: { parent: string; child: string | null }) {
  if (parent === "Cart") return <CartItems />;
  if (parent === "Shipping") return <ShippingForm />;
  if (parent === "Payment" && child) return <PaymentContent child={child} />;
  if (parent === "Review" || parent === "ShippingPaid") {
    return (
      <div>
        <p className="text-[9px] font-mono uppercase tracking-widest text-muted-foreground mb-2">Order Summary</p>
        <CartItems readOnly />
      </div>
    );
  }
  if (parent === "Confirmation") return <Confirmation />;
  return null;
}

function PaymentContent({ child }: { child: string }) {
  const { actions } = useCheckoutContext();

  switch (child) {
    case "MethodEntry":
      return (
        <div className="space-y-3">
          <CardForm />
          <button onClick={() => actions.authorize?.()} className="btn btn-primary w-full">
            Authorize Payment
          </button>
        </div>
      );
    case "Authorizing":
      return (
        <div className="space-y-3">
          <div className="flex items-center gap-2 text-sm text-muted-foreground">
            <div className="w-3.5 h-3.5 border-2 border-border border-t-primary rounded-full animate-spin" />
            Authorizing payment
          </div>
          <div className="border border-border rounded-xl p-3 space-y-2">
            <p className="text-[8px] font-mono uppercase tracking-widest text-muted-foreground">Simulate outcome</p>
            <div className="flex gap-2">
              <button onClick={() => actions.authSucceeded?.()} className="btn btn-outline btn-sm flex-1">Approve</button>
              <button onClick={() => actions.authRequired?.()} className="btn btn-outline btn-sm flex-1">Challenge</button>
              <button onClick={() => actions.authFailed?.()} className="btn btn-destructive btn-sm flex-1">Fail</button>
            </div>
          </div>
        </div>
      );
    case "AuthChallenge":
      return (
        <div className="space-y-3">
          <div className="bg-muted rounded-xl p-3">
            <p className="text-sm font-medium mb-0.5">Authentication required</p>
            <p className="text-xs text-muted-foreground">Your bank is requesting additional verification.</p>
          </div>
          <div className="flex gap-2">
            <button onClick={() => actions.authSucceeded?.()} className="btn btn-primary btn-sm flex-1">Approve</button>
            <button onClick={() => actions.authFailed?.()} className="btn btn-destructive btn-sm flex-1">Deny</button>
          </div>
        </div>
      );
    case "AuthorizationError":
      return (
        <div className="space-y-3">
          <div className="bg-destructive/10 border border-destructive/20 rounded-xl p-3">
            <p className="text-sm text-destructive font-medium">Authorization failed</p>
            <p className="text-xs text-muted-foreground mt-0.5">Your payment could not be authorized.</p>
          </div>
          <button onClick={() => actions.retry?.()} className="btn btn-outline btn-sm">Try Again</button>
        </div>
      );
    case "Authorized":
      return (
        <div className="flex items-center gap-2 text-[oklch(0.55_0.16_142)] text-sm font-medium bg-[oklch(0.55_0.16_142)]/10 rounded-xl px-3 py-2.5">
          <span>✓</span>
          <span>Payment authorized</span>
        </div>
      );
    default:
      return null;
  }
}

function CheckoutControls({ parent }: { parent: string }) {
  const { actions } = useCheckoutContext();

  switch (parent) {
    case "Cart":
      return (
        <button onClick={() => actions.proceed?.()} className="btn btn-primary w-full">
          Continue to Shipping
        </button>
      );
    case "Shipping":
      return (
        <div className="flex gap-2">
          <button onClick={() => actions.back?.()} className="btn btn-outline flex-1">Back</button>
          <button onClick={() => actions.proceed?.()} className="btn btn-primary flex-1">Continue to Payment</button>
        </div>
      );
    case "Payment":
      return (
        <button onClick={() => actions.back?.()} className="btn btn-outline w-full">Back to Shipping</button>
      );
    case "ShippingPaid":
      return (
        <div className="space-y-2">
          <button onClick={() => actions.proceed?.()} className="btn btn-primary w-full">Continue to Review</button>
          <div className="flex gap-2">
            <button onClick={() => actions.back?.()} className="btn btn-outline btn-sm flex-1">Back to Cart</button>
            <button onClick={() => actions.changePayment?.()} className="btn btn-outline btn-sm flex-1">Change Payment</button>
          </div>
        </div>
      );
    case "Review":
      return (
        <div className="space-y-2">
          <button onClick={() => actions.submitOrder?.()} className="btn btn-primary w-full">Place Order</button>
          <div className="flex gap-2">
            <button onClick={() => actions.back?.()} className="btn btn-outline btn-sm flex-1">Back</button>
            <button onClick={() => actions.changePayment?.()} className="btn btn-outline btn-sm flex-1">Change Payment</button>
          </div>
        </div>
      );
    case "Confirmation":
      return (
        <button onClick={() => actions.restart?.()} className="btn btn-outline w-full">Start New Order</button>
      );
    default:
      return null;
  }
}

index.tsx

import { useMemo, useState } from "react";
import { createFlatCheckoutMachine } from "./machine-flat";
import { createCheckoutMachine } from "./machine";
import { CheckoutViewFlat } from "./CheckoutViewFlat";
import { CheckoutViewNested } from "./CheckoutViewNested";
import { MachineVisualizer } from "@components/MachineVisualizer";

type Mode = "flat" | "nested";

export default function HSMCheckoutIndex() {
  const [mode, setMode] = useState<Mode>("flat");

  // Create separate machines for each mode
  const flatMachine = useMemo(() => createFlatCheckoutMachine(), []);
  const nestedMachine = useMemo(() => createCheckoutMachine(), []);
  const machine = mode === "flat" ? flatMachine : nestedMachine;

  return (
    <div className="space-y-6">
      {/* Mode Toggle - Sticky below header */}
      <div className="flex justify-center mb-6 sticky top-16 z-10 py-2 bg-white/80 dark:bg-gray-900/80 backdrop-blur-sm -mx-4 px-4">
        <div className="inline-flex bg-gray-100 dark:bg-gray-800 p-1 rounded-lg">
          <button
            onClick={() => setMode("flat")}
            className={`px-4 py-2 rounded-md text-sm font-medium transition-colors ${
              mode === "flat"
                ? "bg-white dark:bg-gray-700 text-blue-600 dark:text-blue-400 shadow-sm"
                : "text-gray-500 hover:text-gray-700 dark:text-gray-400 dark:hover:text-gray-200"
            }`}
          >
            Flattened
          </button>
          <button
            onClick={() => setMode("nested")}
            className={`px-4 py-2 rounded text-sm font-medium transition-colors ${
              mode === "nested"
                ? "bg-white dark:bg-gray-700 text-blue-600 dark:text-blue-400 shadow-sm"
                : "text-gray-500 hover:text-gray-700 dark:text-gray-400 dark:hover:text-gray-200"
            }`}
          >
            Nested (Hierarchical)
          </button>
        </div>
      </div>

      {mode === "flat" ? (
        <CheckoutViewFlat machine={flatMachine} />
      ) : (
        <CheckoutViewNested machine={nestedMachine} />
      )}

      <MachineVisualizer
        key={mode} // Force re-mount of visualizer when mode changes
        machine={machine}
        title={`Hierarchical Traffic Light (${mode === "flat" ? "Flattened" : "Nested"})`}
        defaultViz="sketch"
        interactive={true}
        layout="stacked"
      />
    </div>
  );
}

Nested vs flattened

In the nested variant, the Payment state contains a submachine (CardEntry → Processing → Confirmed / Failed). The outer machine delegates control to the submachine while payment is in progress and resumes once it reaches a terminal state. This keeps the top-level machine focused on the checkout stages and isolates payment complexity.

In the flattened variant, all payment sub-states are peers at the top level alongside Cart, Shipping, and OrderComplete. Every state is explicit and directly traversable, at the cost of a larger flat list of states and repeated transitions.

Submachines shine when a phase of your flow is complex enough to warrant local state management — and when you want to reuse or test that phase in isolation.

Next steps

• Hierarchical Traffic Light — simplest introduction to nesting
• Hierarchical Combobox — nesting for UI keyboard interactions
• Hierarchical Machines guide — full reference