packages/effect-fc/src/Lens.ts

@@ -1,4 +1,4 @@
-import { Array, Chunk, Effect, Function, Pipeable, Predicate, Readable, Stream, Subscribable, type SubscriptionRef } from "effect"
+import { Array, Chunk, Effect, Function, Pipeable, Predicate, Readable, type Ref, Stream, Subscribable, type SubscriptionRef } from "effect"
 import type { NoSuchElementException } from "effect/Cause"
 
 
@@ -91,6 +91,28 @@ export const fromSubscriptionRef = <A>(
     ) => ref.modifyEffect(f),
 })
 
+/**
+ * Creates a `Lens` that proxies a `Ref`.
+ *
+ * Note: since `Ref` does not provide any kind of reactivity mechanism, the produced `Lens` will be non-reactive.
+ * This means its `changes` stream will only emit the current value once when evaluated and nothing else.
+ */
+export const fromRef = <A>(
+    ref: Ref.Ref<A>
+): Lens<A, never, never, never, never> => make({
+    get get() { return ref.get },
+    get changes() { return Stream.unwrap(Effect.map(ref.get, Stream.make)) },
+    modify: <B, E1 = never, R1 = never>(
+        f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
+    ) => Effect.flatMap(ref.get, a => Effect.flatMap(
+        f(a),
+        ([b, next]) => Effect.flatMap(
+            ref.set(next),
+            () => Effect.succeed(b),
+        ),
+    )),
+})
+
 /**
  * Flattens an effectful `Lens`.
  */