Default action_kind to instant; document multiple action sets

- The #[action_kind(..)] attribute is now optional and defaults to instant
  (the most common kind), so bare enum variants are instant actions.
- Document the multiple-plugin pattern: add one ActionsPlugin::<T> per action
  enum (gameplay/menu/audio/debug) for separation of concerns, each with its
  own independent ActionState/InputMap/systems, plus the run_if(in_state(..))
  approach for mutually-exclusive sets.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/macros/src/lib.rs

@@ -12,16 +12,17 @@ use syn::{spanned::Spanned, Data, DeriveInput, Fields, Ident, Path};
 /// Derives `engine::GameAction` (and `engine::GameActionHandlers`) for a
 /// fieldless enum.
 ///
-/// Each variant must be annotated with `#[action_kind(...)]` — one of `instant`,
-/// `toggle`, `axis`, or `scalar` — and may carry any number of
-/// `#[action_handler(path::to::system)]` attributes:
+/// Each variant may carry `#[action_kind(...)]` — one of `instant`, `toggle`,
+/// `axis`, or `scalar` — and any number of `#[action_handler(path::to::system)]`
+/// attributes. `#[action_kind]` is optional and **defaults to `instant`**, so
+/// bare variants are instant actions:
 ///
 /// ```ignore
 /// #[derive(Copy, Clone, Eq, PartialEq, Hash, GameAction)]
 /// enum Action {
-///     #[action_kind(instant)] #[action_handler(jump)]
-///     Jump,
-///     #[action_kind(axis)]    #[action_handler(move_player)] // fn(In<Vec2>, ..)
+///     #[action_handler(jump)]
+///     Jump,                                                   // instant (default)
+///     #[action_kind(axis)] #[action_handler(move_player)]     // fn(In<Vec2>, ..)
 ///     Move,
 /// }
 /// ```
@@ -163,7 +164,8 @@ fn gen_handler(
     }
 }
 
-/// Parses the single required `#[action_kind(...)]` attribute on a variant.
+/// Parses the `#[action_kind(...)]` attribute on a variant, defaulting to
+/// [`Kind::Instant`] when it is omitted (the common case).
 fn parse_kind(variant: &syn::Variant) -> syn::Result<Kind> {
     let mut kind: Option<Kind> = None;
 
@@ -207,16 +209,9 @@ fn parse_kind(variant: &syn::Variant) -> syn::Result<Kind> {
         });
     }
 
-    kind.ok_or_else(|| {
-        syn::Error::new(
-            variant.ident.span(),
-            format!(
-                "action variant `{0}` is missing its kind; add one of \
-                 `#[action_kind(instant|toggle|axis|scalar)]`, e.g. `#[action_kind(instant)] {0}`",
-                variant.ident
-            ),
-        )
-    })
+    // A bare variant is an instant action — the most common kind — so the
+    // attribute is optional and only needed to pick a non-default kind.
+    Ok(kind.unwrap_or(Kind::Instant))
 }
 
 /// Parses every `#[action_handler(path)]` attribute on a variant (zero or more).

engine/src/actions.rs

@@ -8,11 +8,11 @@
 //! use engine::actions::*;
 //! use engine::*; // bevy prelude
 //!
-//! // 1. Your game's actions, with each variant's kind declared inline.
+//! // 1. Your game's actions. `#[action_kind]` defaults to `instant`, so a bare
+//! //    variant is an instant action; annotate only the others.
 //! #[derive(Copy, Clone, Eq, PartialEq, Hash, GameAction)]
 //! enum Action {
-//!     #[action_kind(instant)] // fires for one frame
-//!     Jump,
+//!     Jump,                   // instant (the default — no attribute needed)
 //!     #[action_kind(toggle)]  // stays on/off
 //!     Grid,
 //!     #[action_kind(axis)]    // carries a Vec2
@@ -105,6 +105,32 @@
 //!   axis/scalar. [`ActionsPlugin`] registers them when you add it.
 //! * **[`AppActionExt::on_action`]** — `app.on_action(Update, Action::Jump, jump)`,
 //!   the explicit, no-macro form that keeps registration visible in `main`.
+//!
+//! ## Multiple action sets
+//!
+//! The whole API is generic over the action type, so a game is **not** limited to
+//! one enum. Add an [`ActionsPlugin<T>`] per set to separate concerns — gameplay,
+//! menu, audio, debug — each with its own bindings and handlers:
+//!
+//! ```rust,ignore
+//! app.add_plugins(ActionsPlugin::<GameplayAction>::new().bind(/* .. */))
+//!    .add_plugins(ActionsPlugin::<MenuAction>::new().bind(/* .. */))
+//!    .add_plugins(ActionsPlugin::<AudioAction>::new().bind(/* .. */));
+//! ```
+//!
+//! Each set gets its **own** independent [`ActionState<T>`], [`InputMap<T>`],
+//! fan-in systems, and observers; [`on_action`], [`action_value`], and
+//! [`ActionSource<T>`] are all keyed by the set's type, so sets never collide.
+//! (Deriving [`GameAction`](macro@GameAction) generates a distinct set of impls
+//! per enum, and each `ActionsPlugin::<T>` is a distinct plugin type, so adding
+//! several is fine.)
+//!
+//! Bindings are read **independently** per set — there is no cross-set input
+//! consumption or priority — which is exactly right for concerns that are
+//! genuinely separate (audio vs debug vs gameplay). To make two sets *mutually
+//! exclusive* (menu vs gameplay), gate their handlers with `run_if(in_state(..))`:
+//! Bevy states are already exclusive, so only the active set's handlers run, even
+//! when the same key is bound in both.
 
 use core::hash::Hash;
 use core::marker::PhantomData;
@@ -201,13 +227,14 @@ impl ActionValue {
 }
 
 /// Derive macro for [`GameAction`]: annotate each variant with
-/// `#[action_kind(...)]` — `instant`, `toggle`, `axis`, or `scalar`.
+/// `#[action_kind(...)]` — `instant`, `toggle`, `axis`, or `scalar`. The
+/// attribute is optional and defaults to `instant`, so bare variants are instant
+/// actions.
 ///
 /// ```rust,ignore
 /// #[derive(Copy, Clone, Eq, PartialEq, Hash, GameAction)]
 /// enum Action {
-///     #[action_kind(instant)]
-///     Jump,
+///     Jump,                  // instant (default)
 ///     #[action_kind(toggle)]
 ///     Grid,
 /// }