Add snapshot subtree selectors

.changeset/wise-snapshots-query.md

@@ -0,0 +1,7 @@
+---
+"@typeonce/effect-machine": minor
+---
+
+Allow state query helpers to inspect extracted snapshot subtrees, and add
+equality-aware `AtomMachine.selectSnapshot` and `selectSnapshotChild`
+combinators.

README.md

@@ -204,9 +204,10 @@ Binding a shared runtime once is the canonical form for service-backed
 applications. Service-free machines can use `AtomMachine.make(Counter)`.
 
 The bridge exposes `ref`, `snapshot`, `state`, fail-aware `result`, writable
-`send` and `stop` atoms, and `child(descriptor)`. Use `AtomMachine.select` and
-`AtomMachine.matches` for typed, equality-aware derivations. React applications
-using `@effect/atom-react` need a `RegistryProvider`.
+`send` and `stop` atoms, and `child(descriptor)`. Use `AtomMachine.select`,
+`AtomMachine.selectSnapshot`, and `AtomMachine.matches` for typed,
+equality-aware derivations. React applications using `@effect/atom-react` need
+a `RegistryProvider`.
 
 ## Persistence
 

docs/agent-guide.md

@@ -357,6 +357,15 @@ States.getSnapshot(snapshot, "Route.Ready")
 States.matches(snapshot, "Route.Ready.Saving")
 ```
 
+Snapshots returned by `getSnapshot` can be queried again with `get`,
+`getSnapshot`, or `matches`. Paths remain absolute and are restricted to the
+extracted snapshot and its descendants:
+
+```ts
+const ready = Option.getOrThrow(States.getSnapshot(snapshot, "Route.Ready"))
+States.matches(ready, "Route.Ready.Saving")
+```
+
 All paths are checked against the definition. `context.parent` is the immediate
 typed parent (`undefined` at a root). Use `parents` when another ancestor is
 needed:
@@ -642,11 +651,16 @@ the `DefinedStates` object:
 
 ```ts
 AtomMachine.select(machineAtom, "Ready")
+AtomMachine.selectSnapshot(machineAtom, "Ready")
 AtomMachine.matches(machineAtom, "Ready.Saving")
 AtomMachine.selectChild(childAtom, "Editing")
+AtomMachine.selectSnapshotChild(childAtom, "Editing")
 AtomMachine.matchesChild(childAtom, "Editing")
 ```
 
+`select` returns only the decoded state value. Use `selectSnapshot` when a
+component needs the selected node's compound or parallel child topology.
+
 Like ordinary Effect Atom combinators, each selector call returns a derived
 atom. Define it at a stable composition boundary or memoize it when constructing
 it inside a component.

src/Machine.ts

@@ -2341,14 +2341,26 @@ export declare namespace Machine {
     readonly initial: InitialBuilder<States>
 
     /**
-     * Returns the decoded value for an active state path.
+     * Returns the decoded value for an active state path. The supplied
+     * snapshot may be a complete root snapshot or a snapshot previously
+     * extracted from this definition. Extracted snapshots accept only their
+     * own absolute path and descendant paths.
      *
      * @since 0.4.0
      */
-    readonly get: <Path extends StateIdentifier<States>>(
-      snapshot: Snapshot<States>,
-      path: Path
-    ) => Option.Option<StateByIdentifier<States, Path>>
+    readonly get: {
+      <Path extends StateIdentifier<States>>(
+        snapshot: Snapshot<States>,
+        path: Path
+      ): Option.Option<StateByIdentifier<States, Path>>
+      <
+        const From extends StateIdentifier<States>,
+        const Path extends StateIdentifier<States>
+      >(
+        snapshot: SnapshotByIdentifier<States, From>,
+        path: Path & (Path extends NoInfer<From> | `${NoInfer<From>}.${string}` ? unknown : never)
+      ): Option.Option<StateByIdentifier<States, Path>>
+    }
 
     /**
      * Returns the decoded value for an active state path together with all of
@@ -2366,24 +2378,48 @@ export declare namespace Machine {
     ) => Option.Option<StateWithParents<States, Path>>
 
     /**
-     * Returns the snapshot for an active state path.
+     * Returns the snapshot for an active state path. The supplied snapshot may
+     * be a complete root snapshot or a snapshot previously extracted from this
+     * definition. Extracted snapshots accept only their own absolute path and
+     * descendant paths.
      *
      * @since 0.4.0
      */
-    readonly getSnapshot: <Path extends StateIdentifier<States>>(
-      snapshot: Snapshot<States>,
-      path: Path
-    ) => Option.Option<SnapshotByIdentifier<States, Path>>
+    readonly getSnapshot: {
+      <Path extends StateIdentifier<States>>(
+        snapshot: Snapshot<States>,
+        path: Path
+      ): Option.Option<SnapshotByIdentifier<States, Path>>
+      <
+        const From extends StateIdentifier<States>,
+        const Path extends StateIdentifier<States>
+      >(
+        snapshot: SnapshotByIdentifier<States, From>,
+        path: Path & (Path extends NoInfer<From> | `${NoInfer<From>}.${string}` ? unknown : never)
+      ): Option.Option<SnapshotByIdentifier<States, Path>>
+    }
 
     /**
-     * Returns whether a state path is active in the snapshot.
+     * Returns whether a state path is active in the snapshot. The supplied
+     * snapshot may be a complete root snapshot or a snapshot previously
+     * extracted from this definition. Extracted snapshots accept only their
+     * own absolute path and descendant paths.
      *
      * @since 0.4.0
      */
-    readonly matches: <Path extends StateIdentifier<States>>(
-      snapshot: Snapshot<States>,
-      path: Path
-    ) => boolean
+    readonly matches: {
+      <Path extends StateIdentifier<States>>(
+        snapshot: Snapshot<States>,
+        path: Path
+      ): boolean
+      <
+        const From extends StateIdentifier<States>,
+        const Path extends StateIdentifier<States>
+      >(
+        snapshot: SnapshotByIdentifier<States, From>,
+        path: Path & (Path extends NoInfer<From> | `${NoInfer<From>}.${string}` ? unknown : never)
+      ): boolean
+    }
   }
 
   /**

src/internal/machine/atom.ts

@@ -435,9 +435,13 @@ type SnapshotValueByIdentifier<State, Path extends SnapshotIdentifier<State>> =
   Node extends { readonly path: Path; readonly value: infer Value } ? Value : never
   : never
 
+type SnapshotByIdentifier<State, Path extends SnapshotIdentifier<State>> = SnapshotNode<State> extends infer Node ?
+  Node extends { readonly path: Path } ? Node : never
+  : never
+
 type ChildState<Child extends Machine.ChildMachine.Any> = RefState<Machine.ChildMachine.Ref<Child>>
 
-const selectSnapshot = <
+const selectValueByPath = <
   State extends Machine.Machine.AtomicSnapshot<string, unknown>,
   Path extends SnapshotIdentifier<State>
 >(
@@ -448,6 +452,15 @@ const selectSnapshot = <
     Option.map((snapshot) => snapshot.value)
   ) as Option.Option<SnapshotValueByIdentifier<State, Path>>
 
+const selectSnapshotByPath = <
+  State extends Machine.Machine.AtomicSnapshot<string, unknown>,
+  Path extends SnapshotIdentifier<State>
+>(
+  snapshot: State,
+  path: Path
+): Option.Option<SnapshotByIdentifier<State, Path>> =>
+  Topology.getSnapshotByPath(snapshot, path) as Option.Option<SnapshotByIdentifier<State, Path>>
+
 export const select = <
   State extends Machine.Machine.AtomicSnapshot<string, unknown>,
   Event,
@@ -461,7 +474,24 @@ export const select = <
 ): Atom.Atom<
   AsyncResult.AsyncResult<Option.Option<SnapshotValueByIdentifier<State, Path>>, StartError | Error>
 > =>
-  Atom.mapResult(self.result, (snapshot) => selectSnapshot(snapshot, path)).pipe(
+  Atom.mapResult(self.result, (snapshot) => selectValueByPath(snapshot, path)).pipe(
+    Atom.withEquality(Equal.equals)
+  )
+
+export const selectSnapshot = <
+  State extends Machine.Machine.AtomicSnapshot<string, unknown>,
+  Event,
+  Error,
+  Output,
+  StartError,
+  const Path extends SnapshotIdentifier<State>
+>(
+  self: MachineAtom<State, Event, Error, Output, StartError>,
+  path: Path
+): Atom.Atom<
+  AsyncResult.AsyncResult<Option.Option<SnapshotByIdentifier<State, Path>>, StartError | Error>
+> =>
+  Atom.mapResult(self.result, (snapshot) => selectSnapshotByPath(snapshot, path)).pipe(
     Atom.withEquality(Equal.equals)
   )
 
@@ -480,7 +510,25 @@ export const selectChild = <
 > =>
   Atom.mapResult(
     self.result,
-    Option.flatMap((snapshot) => selectSnapshot(snapshot, path))
+    Option.flatMap((snapshot) => selectValueByPath(snapshot, path))
+  ).pipe(Atom.withEquality(Equal.equals))
+
+export const selectSnapshotChild = <
+  Child extends Machine.ChildMachine.Any,
+  StartError,
+  const Path extends SnapshotIdentifier<ChildState<Child>>
+>(
+  self: ChildMachineAtom<Child, StartError>,
+  path: Path
+): Atom.Atom<
+  AsyncResult.AsyncResult<
+    Option.Option<SnapshotByIdentifier<ChildState<Child>, Path>>,
+    StartError | RefError<Machine.ChildMachine.Ref<Child>>
+  >
+> =>
+  Atom.mapResult(
+    self.result,
+    Option.flatMap((snapshot) => selectSnapshotByPath(snapshot, path))
   ).pipe(Atom.withEquality(Equal.equals))
 
 export const matches = <

src/internal/machine/machine.ts

@@ -713,18 +713,21 @@ export const defineStates: DefineStates = (<const States extends Machine.StateSc
   return {
     states,
     initial: makeSnapshotBuilder(states, { mode: "initial", prefix: "" }) as Machine.InitialBuilder<States>,
-    get: ((snapshot, path) =>
-      Topology.getSnapshotByPath(snapshot, path).pipe(
-        Option.map((snapshot) => snapshot.value)
-      )) as Machine.DefinedStates<States>["get"],
+    get:
+      ((snapshot: Machine.AtomicSnapshot<string, unknown>, path: string) =>
+        Topology.getSnapshotByPath(snapshot, path).pipe(
+          Option.map((snapshot) => snapshot.value)
+        )) as Machine.DefinedStates<States>["get"],
     getWithParents: ((snapshot, path) => {
       const parents: Record<string, unknown> = {}
       return Topology.getSnapshotByPath(snapshot, path, parents).pipe(
         Option.map((snapshot) => ({ value: snapshot.value, parents }))
       )
     }) as Machine.DefinedStates<States>["getWithParents"],
     getSnapshot: Topology.getSnapshotByPath as unknown as Machine.DefinedStates<States>["getSnapshot"],
-    matches: (snapshot, path) => Option.isSome(Topology.getSnapshotByPath(snapshot, path))
+    matches:
+      ((snapshot: Machine.AtomicSnapshot<string, unknown>, path: string) =>
+        Option.isSome(Topology.getSnapshotByPath(snapshot, path))) as Machine.DefinedStates<States>["matches"]
   }
 }) as DefineStates
 

src/unstable/reactivity/AtomMachine.ts

@@ -295,6 +295,10 @@ type SnapshotValueByIdentifier<State, Path extends SnapshotIdentifier<State>> =
   Node extends { readonly path: Path; readonly value: infer Value } ? Value : never
   : never
 
+type SnapshotByIdentifier<State, Path extends SnapshotIdentifier<State>> = SnapshotNode<State> extends infer Node ?
+  Node extends { readonly path: Path } ? Node : never
+  : never
+
 type ChildState<Child extends Machine.ChildMachine.Any> = RefState<Machine.ChildMachine.Ref<Child>>
 
 /**
@@ -339,6 +343,27 @@ export const select: <
   AsyncResult.AsyncResult<Option.Option<SnapshotValueByIdentifier<State, Path>>, StartError | Error>
 > = internal.select
 
+/**
+ * Selects the typed logical snapshot for an active state path.
+ *
+ * Unlike {@link select}, the selected value retains its child snapshot
+ * topology. The derived atom suppresses structurally equal updates. Keep the
+ * returned atom stable when constructing it inside a component.
+ *
+ * @category combinators
+ * @since 0.7.0
+ */
+export const selectSnapshot: <
+  State extends Machine.Machine.AtomicSnapshot<string, unknown>,
+  Event,
+  Error,
+  Output,
+  StartError,
+  const Path extends SnapshotIdentifier<State>
+>(self: MachineAtom<State, Event, Error, Output, StartError>, path: Path) => Atom.Atom<
+  AsyncResult.AsyncResult<Option.Option<SnapshotByIdentifier<State, Path>>, StartError | Error>
+> = internal.selectSnapshot
+
 /**
  * Selects the typed value for an active state path in an invoked child.
  *
@@ -367,6 +392,28 @@ export const selectChild: <
   >
 > = internal.selectChild
 
+/**
+ * Selects the typed logical snapshot for an active state path in an invoked
+ * child.
+ *
+ * An inactive child or state path produces `Option.none()`. Unlike
+ * {@link selectChild}, the selected value retains its child snapshot topology.
+ * The derived atom suppresses structurally equal updates.
+ *
+ * @category combinators
+ * @since 0.7.0
+ */
+export const selectSnapshotChild: <
+  Child extends Machine.ChildMachine.Any,
+  StartError,
+  const Path extends SnapshotIdentifier<ChildState<Child>>
+>(self: ChildMachineAtom<Child, StartError>, path: Path) => Atom.Atom<
+  AsyncResult.AsyncResult<
+    Option.Option<SnapshotByIdentifier<ChildState<Child>, Path>>,
+    StartError | RefError<Machine.ChildMachine.Ref<Child>>
+  >
+> = internal.selectSnapshotChild
+
 /**
  * Returns whether a state path is active.
  *

test/machine/Machine.test.ts

@@ -426,6 +426,24 @@ describe("Machine", () => {
     )
     assert.strictEqual(states.matches(snapshot, "fulfillment.shipping"), true)
     assert.strictEqual(states.matches(snapshot, "fulfillment.shipping.quoted"), false)
+
+    const fulfillmentSnapshot = Option.getOrThrow(states.getSnapshot(snapshot, "fulfillment"))
+    assert.deepStrictEqual(states.get(fulfillmentSnapshot, "fulfillment.inventory"), Option.some(inventory))
+    assert.deepStrictEqual(
+      states.getSnapshot(fulfillmentSnapshot, "fulfillment.shipping"),
+      Option.some(fulfillmentSnapshot.states.shipping)
+    )
+    assert.strictEqual(states.matches(fulfillmentSnapshot, "fulfillment.inventory.checking"), true)
+    assert.strictEqual(states.matches(fulfillmentSnapshot, "fulfillment.inventory.reserved"), false)
+
+    const inventorySnapshot = Option.getOrThrow(
+      states.getSnapshot(fulfillmentSnapshot, "fulfillment.inventory")
+    )
+    assert.deepStrictEqual(states.get(inventorySnapshot, "fulfillment.inventory"), Option.some(inventory))
+    assert.deepStrictEqual(
+      states.get(inventorySnapshot, "fulfillment.inventory.checking"),
+      Option.some(checking)
+    )
   })
 
   it.effect("initial builder constructs compound initial snapshots", () =>