smarm/examples/statem_macro.rs

//! RFC 017 — the **same** machine as `examples/statem_fused.rs`, written through
//! the `gen_statem!` macro. Diff this file against that one to see exactly what
//! the macro buys: every `// ===` section there that was boilerplate (the `Ev`
//! enum, the `DoorSm` struct, `start`, the whole `Machine` impl, the `enter`
//! dispatch, the stay/transition apply-tail) collapses into the invocation
//! below. What stays hand-written is what carries meaning: the four types, the
//! per-state successor enum, and the handler fns.
//!
//! The point of the exercise is that the macro is *pure sugar*: the four
//! compile-time guarantees the fused spike demonstrates are properties of the
//! emitted code, not of the macro, so they survive expansion unchanged. The
//! BREAK-CASE MENU at the bottom is the same four cases, re-expressed against
//! the macro surface — flip any one on and the compiler fires identically.
//!
//! Run: `cargo run --example statem_macro`

#![deny(dead_code)] // guarantee #3 (orphan handlers); the macro denies the
                    // dispatch's own unreachable_patterns internally.

use smarm::gen_statem;
use smarm::run;
use smarm::gen_statem::Reply;

// === user types (identical to statem_fused.rs) =============================

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
enum Door {
    Open,
    Closed,
    Locked,
}

struct Data {
    enters: u32, // total state entries (incl. initial)
    pushes: u32, // times a push closed the door
}

enum Cast {
    Push,
    Pull,
    Lock,
    Unlock(u32), // carries a key
}

enum Call {
    GetState(Reply<Door>),
    GetEnters(Reply<u32>),
    GetPushes(Reply<u32>),
}

const CODE: u32 = 1234;

// === per-row successor enum for the one branching row ======================
// `Locked + Unlock` may end in Closed (right key) or Locked (wrong key) — and
// nothing else. This type IS that declared set; `on_unlock` cannot name Open.
enum UnlockOutcome {
    Closed,
    Locked,
}

impl From<UnlockOutcome> for Door {
    fn from(o: UnlockOutcome) -> Door {
        match o {
            UnlockOutcome::Closed => Door::Closed,
            UnlockOutcome::Locked => Door::Locked,
        }
    }
}

// === module-private handlers (side effects / branch choice only) ===========
// Reachable solely through the table below. Orphan one and it is dead_code.

fn on_push(data: &mut Data) {
    data.pushes += 1;
}

fn on_unlock(key: u32) -> UnlockOutcome {
    if key == CODE {
        UnlockOutcome::Closed
    } else {
        UnlockOutcome::Locked // wrong key: caller will see this == current -> stay
    }
}

// === the machine ===========================================================
// Everything below — Ev, DoorSm, start, Machine, enter — is generated.

gen_statem! {
    machine: DoorSm { state: Door, data: Data };
    event:   Ev { cast: Cast, call: Call };

    // You name the bindings the bodies use; the macro can't lend you its own
    // `self`/`cx` across macro hygiene. `data` = &mut Data, `prev` = current
    // state tag, `cx` = context handle (unused in chunk 1).
    context(data, prev, cx);

    enter {
        _ => data.enters += 1,
    }

    on Door::Open => {
        cast Cast::Push                            => { on_push(data); Door::Closed },
        cast Cast::Pull | Cast::Lock | Cast::Unlock(_) => unhandled,
    }
    on Door::Closed => {
        cast Cast::Pull                            => Door::Open,
        cast Cast::Lock                            => Door::Locked,
        cast Cast::Push | Cast::Unlock(_)          => unhandled,
    }
    on Door::Locked => {
        cast Cast::Unlock(key)                     => on_unlock(key), // branch -> UnlockOutcome
        cast Cast::Push | Cast::Pull | Cast::Lock  => unhandled,
    }

    // state-independent queries (reply, then stay via `prev`)
    on _ => {
        call Call::GetState(r)  => { r.reply(prev); prev },
        call Call::GetEnters(r) => { r.reply(data.enters); prev },
        call Call::GetPushes(r) => { r.reply(data.pushes); prev },
    }
}

fn main() {
    run(|| {
        let door = DoorSm::start(Door::Closed, Data { enters: 0, pushes: 0 });

        door.send(Ev::Cast(Cast::Lock)).unwrap(); // Closed -> Locked
        door.send(Ev::Cast(Cast::Push)).unwrap(); // Locked: Push invalid -> Unhandled
        door.send(Ev::Cast(Cast::Unlock(0))).unwrap(); // Locked: wrong key -> stay
        door.send(Ev::Cast(Cast::Unlock(CODE))).unwrap(); // Locked -> Closed
        door.send(Ev::Cast(Cast::Pull)).unwrap(); // Closed -> Open
        door.send(Ev::Cast(Cast::Push)).unwrap(); // Open  -> Closed (pushes=1)

        let st = door.call(|r| Ev::Call(Call::GetState(r))).unwrap();
        let enters = door.call(|r| Ev::Call(Call::GetEnters(r))).unwrap();
        let pushes = door.call(|r| Ev::Call(Call::GetPushes(r))).unwrap();

        println!("state={st:?} enters={enters} pushes={pushes}");
        assert_eq!(st, Door::Closed);
        assert_eq!(enters, 5); // Closed(start) + Locked + Closed + Open + Closed
        assert_eq!(pushes, 1);
        println!("ok");
    });
}

// ===========================================================================
// BREAK-CASE MENU — the four guarantees, through the macro. Each fires exactly
// as it does in the hand-written statem_fused.rs.
//
// 1. ORPHAN HANDLER (dead_code -> error):
//      add `fn on_slam(_d: &mut Data) {}` and don't reference it.
//      => error: function `on_slam` is never used
//
// 2. CONFLICTING ROW (unreachable_patterns):
//      duplicate a row, e.g. add a second
//      `cast Cast::Push => unhandled,` under `on Door::Open`.
//      NOTE: this example is a separate crate from `smarm`, so rustc's
//      in_external_macro rule SILENCES this lint here even though the macro
//      denies it — the duplicate compiles. The guarantee is real only for
//      machines defined inside the smarm crate (see the unit test in
//      src/statem.rs). This is the documented macro_rules! limitation.
//
// 3. MISSING PAIR (non-exhaustive match, E0004):
//      delete the `cast Cast::Push | Cast::Pull | Cast::Lock => unhandled,`
//      row under `on Door::Locked`.
//      => error[E0004]: non-exhaustive patterns: ... not covered
//
// 4. OUT-OF-SET TARGET (E0599):
//      in `on_unlock`, return `UnlockOutcome::Open`.
//      => error[E0599]: no variant ... named `Open` found for enum `UnlockOutcome`
// ===========================================================================