packages/effect-lens/src/Lens.ts

@@ -149,7 +149,7 @@ export class LensLazyImpl<
 >
 extends LensImpl<A, B, ER, ESR, EW, ESW, RR, RSR, RW, RSW> {
     constructor(
-        readonly source: LensLazyImpl.Source<B, ESW, ESR, RSR, RSW>
+        readonly source: LensLazyImpl.Source<B, ESW, ESR, RSR, RSW>,
     ) {
         super()
     }
@@ -174,6 +174,36 @@ export const makeLazy = <B, ESW, ESR, RSR, RSW>(
     source: LensLazyImpl.Source<B, ESW, ESR, RSR, RSW>
 ): Lens<B, ESW, ESR, RSR, RSW> => new LensLazyImpl(source)
 
+/**
+ * Derives a new `Lens` by immutably appending a step to an existing `LensImpl`.
+ */
+export const derive: {
+    <A, B, ER, ESR, EW, ESW, RR, RSR, RW, RSW, C, ER2, EW2, RR2, RW2>(
+        self: LensImpl<A, B, ER, ESR, EW, ESW, RR, RSR, RW, RSW>,
+        step: LensStep<C, A, ER2, ER, EW2, EW, RR2, RR, RW2, RW>,
+    ): Lens<C, ER2, EW2, RR2, RW2>
+    <A, ER, EW, RR, RW, C, ER2, EW2, RR2, RW2>(
+        step: LensStep<C, A, ER2, ER, EW2, EW, RR2, RR, RW2, RW>,
+    ): <B, ESR, ESW, RSR, RSW>(
+        self: LensImpl<A, B, ER, ESR, EW, ESW, RR, RSR, RW, RSW>
+    ) => Lens<C, ER2, EW2, RR2, RW2>
+} = Function.dual(2, <A, B, ER, ESR, EW, ESW, RR, RSR, RW, RSW, C, ER2, EW2, RR2, RW2>(
+    self: LensImpl<A, B, ER, ESR, EW, ESW, RR, RSR, RW, RSW>,
+    step: LensStep<C, A, ER2, ER, EW2, EW, RR2, RR, RW2, RW>,
+): Lens<C, ER2, EW2, RR2, RW2> => Object.defineProperty(
+    Object.defineProperties(
+        Object.create(Object.getPrototypeOf(self)),
+        Object.getOwnPropertyDescriptors(self),
+    ),
+    "steps",
+    {
+        configurable: true,
+        enumerable: true,
+        value: [...self.steps, step as LensStep<unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown, unknown>],
+        writable: false,
+    },
+) as Lens<C, ER2, EW2, RR2, RW2>)
+
 
 export declare namespace SynchronizedRefLensImpl {
     export interface SynchronizedRefWithInternals<in out A>