diff --git a/core/index.ts b/core/index.ts
new file mode 100644
index 0000000..ecdfd6a
--- /dev/null
+++ b/core/index.ts
@@ -0,0 +1,9 @@
+export type {
+    Observable,
+    Observer,
+    Subscription,
+    RatyStore,
+    RatyStoreOptions,
+} from "./store"
+export { create } from "./store"
+export * as Raty from './store'
\ No newline at end of file
diff --git a/core/store.ts b/core/store.ts
new file mode 100644
index 0000000..4b16d98
--- /dev/null
+++ b/core/store.ts
@@ -0,0 +1,64 @@
+export interface Observable<A> {
+    subscribe(observer: Observer<A>): Subscription
+}
+
+export interface Observer<A> {
+    (value: A): void
+}
+
+export interface Subscription {
+    unsubscribe(): void
+}
+
+export interface RatyStore extends Observable<number> {
+    get(): number
+    set(value: number): void
+    set(modifier: ()=> number): void
+    cancel(): void
+}
+
+export interface RatyStoreOptions {
+    value: (() => number) | number
+}
+
+export function create(options: RatyStoreOptions): RatyStore {
+    const initialValue = typeof options.value === 'function' ? options.value() : options.value
+    let value = initialValue
+
+    const listeners = new Set<Observer<number>>()
+
+    const notify = () => {
+        for (const listener of listeners) {
+            listener(value)
+        }
+    }
+
+    const set = (fa: (() => number) | number) => {
+        const newValue = typeof fa === 'function' ? fa() : value
+
+        if (newValue !== value) {
+            value = newValue
+
+            notify()
+        }
+    }
+
+    const get = () => value
+
+    const subscribe = (observer: Observer<number>): Subscription => {
+        listeners.add(observer)
+
+        return {
+            unsubscribe: () => listeners.delete(observer)
+        }       
+    }
+
+    const cancel = () => set(initialValue)
+
+    return {
+        get,
+        set,
+        subscribe,
+        cancel,
+    }
+}
\ No newline at end of file