如何使用 van Laarhoven 镜头删除聚焦的属性对象？

Question

通过我简单的镜头实现，我可以执行通常的修改、设置、获取和删除操作：

// Lens type

const Lens = f => ({runLens: f, [Symbol.toStringTag]: "Lens"});

const objLens = map => k =>
  Lens(f => o =>
    map(x => Object.assign({}, o, {[k]: x})) (f(o[k]))); // object lens

// Id type

const Id = x => ({runId: x, [Symbol.toStringTag]: "Id"});
const idMap = f => tx => Id(f(tx.runId)); // functor

// Const type

const Const = x => ({runConst: x, [Symbol.toStringTag]: "Const"});
const constMap = f => tx => Const(tx.runConst); // functor

// auxiliary function

const _const = x => y => x;

// MAIN

const o = {foo: "abc", bar: 123};

const get = objLens(constMap) ("foo").runLens(x => Const(x)) (o),
  set = objLens(idMap) ("bat").runLens(_const(Id(true))) (o),
  mod = objLens(idMap) ("foo").runLens(s => Id(s.toUpperCase())) (o),
  del = objLens(idMap) ("foo").runLens(_const(Id(null))) (o); //*

console.log("get", get.runConst);
console.log("set", set.runId);
console.log("mod", mod.runId);
console.log("del", del.runId);

然而，delete 并不令人满意，因为我想删除整个属性而不是仅仅用空值替换值。

我怎样才能做到这一点？

*请注意，我通常会使用适当的 Option 类型来表示没有值。

Answer 1

一种可能的方法是定义一个表示删除的特殊类型：

const Lens = f => ({runLens: f, [Symbol.toStringTag]: "Lens"});

const objLens = map => k =>
  Lens(f => o => map(x => {
    if (x[Symbol.toStringTag] === "Deleter") {
      const p = Object.assign({}, o);
      delete p[k];
      return p;
    }

    else
      return Object.assign({}, o, {[k]: x});
    }) (f(o[k])));

const Id = x => ({runId: x, [Symbol.toStringTag]: "Id"});
const idMap = f => tx => Id(f(tx.runId)); // functor

const _const = x => y => x;

// deletion type

const Deleter =
  ({get runDeleter() {return Deleter}, [Symbol.toStringTag]: "Deleter"});

// MAIN

const o = {foo: "abc", bar: 123};

const del = objLens(idMap) ("foo").runLens(_const(Id(Deleter))) (o);

console.log("del", del.runId);

但是，_const(Id(Deleter)) 不是特别直观，而且看起来很老套。希望有更好的方法。

Answer 2

这就是我要做的：

// type Lens a b = forall f. Functor f => (b -> f b) -> a -> f a

// newtype Const b a = Const { getConst :: b } deriving Functor
const Const = getConst => ({ getConst, map: _ => Const(getConst) });

// newtype Identity a = Identity { runIdentity :: a } deriving Functor
const Identity = runIdentity => ({ runIdentity, map: f => Identity(f(runIdentity)) });

// remove :: String -> Object -> Object
const remove = key => ({ [key]: _, ...rest }) => rest;

// prop :: String -> Lens Object (Maybe Value)
const prop = key => fun => obj =>
    fun(obj.hasOwnProperty(key) ? { fromJust: obj[key] } : null)
        .map(data => Object.assign(remove(key)(obj), data && { [key]: data.fromJust }));

// get :: Lens a b -> a -> b
const get = lens => data => lens(Const)(data).getConst;

// modify :: Lens a b -> (b -> b) -> a -> a
const modify = lens => fun => data => lens(x => Identity(fun(x)))(data).runIdentity;

// set :: Lens a b -> b -> a -> a
const set = lens => value => modify(lens)(_ => value);

// del :: Lens a (Maybe b) -> a -> a
const del = lens => set(lens)(null);

// foo :: Lens Object (Maybe Value)
const foo = prop("foo");

console.log(get(foo)({ foo: 10, bar: 20 })); // { fromJust: 10 }
console.log(del(foo)({ foo: 10, bar: 20 })); // { bar: 20 }

如图所示，属性镜头的类型签名如 foo 是 Lens Object (Maybe Value)。这是有道理的，因为如果你尝试 get(foo)({ bar: 20 }) 你应该什么也得不到。 del 函数适用于任何关注可能值的镜头，并将其值设置为空（即 null）。

功劳归功于 Bergi，因为可以对计算属性进行模式匹配。

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