feat(collect): generate a lens if possible (#34)

Co-authored-by: Flavio Corpa <[email protected]>

Author: serras

README.md

@@ -189,6 +189,13 @@ Generates a new object whose keys are based on the given optics. Depending on th
 collect({ edad: optic('age') }).view({ name: 'Alex', age: 32 }) // { edad: 32 }
 ```
 
+In addition, if *every* optic is a lens (or can be turned into one), then the result is also a lens. This makes `over` able to change part of a data structure depending on the value of other elements.
+
+```js
+collect({ month: optic('birthmonth'), age: optic('age') })
+  .over(x => { ...x, age: x.month === 'april' ? x.age + 1 : x.age }, person)
+```
+
 #### `transform : (s -> a) -> Getter s a`
 
 Applies a transformation to the values targetted up to then. Since the transformation may not be reversible, after composing with `transform` you lose the ability to set or modify the value.

__tests__/operations.test.js

@@ -130,4 +130,16 @@ describe('Operations over Optics', () => {
     const obj = { one: 1, two: 2 }
     expect(view(o, obj)).toBe(3)
   })
+
+  test('collect + over', () => {
+    const o = optic(collect({ a: optic('one'), b: optic('two') }))
+    const obj = { one: 1, two: 2 }
+    expect(set(o, { a: 2, b: 3 }, obj)).toStrictEqual({ one: 2, two: 3 })
+  })
+
+  test('collect + over', () => {
+    const o = optic(collect({ a: optic('one'), b: optic('two') }))
+    const obj = { one: 1, two: 2 }
+    expect(over(o, ({ a, b }) => ({ a, b: a + b }), obj)).toStrictEqual({ one: 1, two: 3 })
+  })
 })

src/operations.js

@@ -185,10 +185,12 @@ export const firstOf = (...optics) => {
   }
 }
 
-export const collect = template =>
-  getter(obj =>
+export const collect = template => {
+  const t = Object.entries(template)
+
+  const computeGetter = () => obj =>
     Object.fromEntries(
-      Object.entries(template).map(([k, o]) => {
+      t.map(([k, o]) => {
         if (o.asGetter) {
           return [k, view(o, obj)]
         } else if (o.asPartialGetter) {
@@ -199,8 +201,13 @@ export const collect = template =>
           throw new OpticComposeError('collect', o.constructor.name, 'non-getter optic')
         }
       }),
-    ),
-  )
+    )
+
+  const computeSetter = () => (newVal, obj) =>
+    t.reduce((acc, [k, o]) => set(o, newVal[k], acc), obj)
+
+  return t.every(([, o]) => o.asLens) ? lens(computeGetter(), computeSetter()) : getter(computeGetter())
+}
 
 export const transform = getter