Fix

packages/effect-fc/src/Lens.ts

@@ -156,6 +156,31 @@ export const mapField: {
     (a, b) => Object.setPrototypeOf({ ...a, [key]: b }, Object.getPrototypeOf(a)),
 ))
 
+export declare namespace mapMutableField {
+    type IfEquals<X, Y, A = X, B = never> = (<T>() => T extends X ? 1 : 2) extends (<T>() => T extends Y ? 1 : 2) ? A : B
+    export type WritableKeys<T> = {
+        [K in keyof T]-?: IfEquals<
+            { [P in K]: T[K] },
+            { -readonly [P in K]: T[K] },
+            K,
+            never
+        >
+    }[keyof T]
+}
+
+export const mapMutableField: {
+    <A extends object, K extends mapMutableField.WritableKeys<A>, ER, EW, RR, RW>(
+        self: Lens<A, ER, EW, RR, RW>,
+        key: K,
+    ): Lens<A[K], ER, EW, RR, RW>
+    <A extends object, K extends mapMutableField.WritableKeys<A>, ER, EW, RR, RW>(
+        key: K,
+    ): (self: Lens<A, ER, EW, RR, RW>) => Lens<A[K], ER, EW, RR, RW>
+} = Function.dual(2, <A extends object, K extends mapMutableField.WritableKeys<A>, ER, EW, RR, RW>(
+    self: Lens<A, ER, EW, RR, RW>,
+    key: K,
+): Lens<A[K], ER, EW, RR, RW> => map(self, a => a[key], (a, b) => { a[key] = b; return a }))
+
 export const mapArrayAt: {
     <A extends readonly any[], ER, EW, RR, RW>(
         self: Lens<A, ER, EW, RR, RW>,
@@ -287,3 +312,9 @@ export const updateAndGetEffect: {
         next => Effect.succeed([next, next] as const),
     )),
 )
+
+
+declare const lens: Lens<{ readonly a: string, b: number }>
+const t = lens.pipe(
+    mapMutableField("b")
+)