struct: evolve to skip unspecified transformations

Author: imcotton

src/struct.ts

@@ -53,13 +53,14 @@ export const getAssignSemigroup = <A extends object = never>(): Semigroup<A> =>
  *
  * @since 2.11.0
  */
-export const evolve = <A, F extends { [K in keyof A]: (a: A[K]) => unknown }>(transformations: F) => (
+export const evolve = <A, F extends { [K in keyof A]?: (a: A[K]) => unknown }>(transformations: F) => (
   a: A
-): { [K in keyof F]: ReturnType<F[K]> } => {
+): { [K in keyof A]: F[K] extends (a: A[K]) => unknown ? ReturnType<F[K]> : A[K] } => {
   const out: Record<string, unknown> = {}
   for (const k in a) {
     if (_.has.call(a, k)) {
-      out[k] = transformations[k](a[k])
+      const fn = transformations[k]
+      out[k] = typeof fn === 'function' ? fn(a[k]) : a[k]
     }
   }
   return out as any

test/struct.ts

@@ -1,4 +1,4 @@
-import { pipe } from '../src/function'
+import { pipe, increment } from '../src/function'
 import * as _ from '../src/struct'
 import * as U from './util'
 
@@ -35,5 +35,7 @@ describe('struct', () => {
     const x: Record<'b', number> = Object.create({ a: 1 })
     x.b = 1
     U.deepStrictEqual(pipe(x, _.evolve({ b: (b) => b > 0 })), { b: true })
+    // does not invoke absent transformations
+    U.deepStrictEqual(pipe({ a: 1, b: 's' }, _.evolve({ a: increment })), { a: 2, b: 's' })
   })
 })