immuts的等价物是什么，它有一个名称吗？
我不知道是否有一个通用的子状态操作名称。但在Haskell中，有一个Lens提供了您所需的内容。
在C ++中，您可以将lens视为一对getter和setter函数，两个函数都仅关注其直接子部分。然后有一个组合器将两个镜头组合在一起，以便聚焦于结构的更深层次的子部分。
template<class D, class A>
using get = std::function<A const &(D const &)>;
template<class D, class A>
using set = std::function<D(D const &, A)>;
template<class D, class A>
using lens = std::pair<get<D, A>, set<D, A>>;
template<class D, class A, class B>
lens<D, B>
lens_composer(lens<D, A> da, lens<A, B> ab) {
auto abgetter = ab.first;
auto absetter = ab.second;
auto dagetter = da.first;
auto dasetter = da.second;
get<D, B> getter = [abgetter, dagetter]
(D const &d) -> B const&
{
return abgetter(dagetter(d));
};
set<D, B> setter = [dagetter, absetter, dasetter]
(D const &d, B newb) -> D
{
A const &a = dagetter(d);
A newa = absetter(a, newb);
return dasetter(d, newa);
};
return {getter, setter};
};
您可以像这样编写基本镜头：
struct Bob {
immut<Alice> alice;
immut<Anna> anna;
};
auto bob_alice_lens
= lens<Bob, Alice> {
[] (Bob const& b) -> Alice const & {
return b.alice.get();
},
[] (Bob const& b, Alice newAlice) -> Bob {
return { immut{newAlice}, b.anna };
}
};
注意，这个过程可以通过宏自动化。
然后，如果
Bob包含
immut<Alice>，
Alice包含
immut<Charlie>，你可以编写2个镜头（从
Bob到
Alice和从
Alice到
Charlie），
Bob到
Charlie的镜头可以组合成：
auto bob_charlie_lens =
lens_composer(bob_alice_lens, alice_charlie_lens);
实时演示
注意：
下面提供了一个更完整的示例，其深度相关的内存增长是线性的，没有类型抹除开销（std::function），并且具有完全的编译时类型检查。它还使用宏生成基本的镜头。
#include <type_traits>
#include <utility>
template<class T>
using GetFromType = typename T::FromType;
template<class T>
using GetToType = typename T::ToType;
template<class Derived>
struct LensMixin {
Derived &self() { return static_cast<Derived&>(*this); }
template<class D, class Mutation>
auto modify(D const &d, Mutation f)
{
auto a = self().get(d);
f(a);
return self().set(d, a);
}
};
template<
class Getter, class Setter,
class D, class A
>
struct SimpleLens : LensMixin<SimpleLens<Getter, Setter, D, A>> {
static_assert(std::is_same<
std::invoke_result_t<Getter, const D&>, const A&>{},
"Getter should return const A& for (const D&)");
static_assert(std::is_same<
std::invoke_result_t<Setter, const D&, A>, D>{},
"Setter should return D for (const D&, A)");
using FromType = D;
using ToType = A;
SimpleLens(Getter getter, Setter setter)
: getter(getter)
, setter(setter)
{}
A const &get(D const &d) { return getter(d); }
D set(D const &d, A newa) { return setter(d, newa); }
private:
Getter getter;
Setter setter;
};
template<
class LensDA, class LensAB
>
struct ComposedLens : LensMixin<ComposedLens<LensDA, LensAB>> {
static_assert(std::is_same<
GetToType<LensDA>, GetFromType<LensAB>
>{}, "Cannot compose two Lens with wrong intermediate type");
using FromType = GetFromType<LensDA>;
using ToType = GetToType<LensAB>;
private:
using intermediateType = GetToType<LensDA>;
using D = FromType;
using B = ToType;
LensDA da;
LensAB ab;
public:
ComposedLens(LensDA da, LensAB ab) : da(da), ab(ab) {}
B const &get(D const &d) { return ab.get(da.get(d)); }
D set(D const &d, B newb) {
const auto &a = da.get(d);
auto newa = ab.set(a, newb);
return da.set(d, newa);
}
};
namespace detail {
template<class LensDA, class LensAB>
auto MakeComposedLens(LensDA da, LensAB ab) {
return ComposedLens<LensDA, LensAB> { da, ab };
}
template<class D, class A, class Getter, class Setter>
auto MakeSimpleLens(Getter getter, Setter setter)
{
return SimpleLens<Getter, Setter, D, A> {
getter, setter
};
}
}
template<class LensDA, class LensAB>
auto lens_composer(LensDA da, LensAB ab) {
return detail::MakeComposedLens (da, ab);
}
#include <memory>
template<class T>
struct immut {
T const& get() const {return *state;}
immut(T in):state(std::make_shared<T>(std::move(in))){}
private:
std::shared_ptr<T const> state;
};
#define MAKE_SIMPLE_LENS(D, A, Aname) \
detail::MakeSimpleLens<D, A>( \
+[] (D const &d) -> A const & { \
return d . Aname . get(); \
}, \
+[] (D const &d, A newa) -> D { \
D newd = d; \
newd . Aname = newa; \
return newd; \
})
struct Charlie {
int id = 0;
};
struct Alice{
immut<Charlie> charlie;
};
struct Anna {};
struct Bob {
immut<Alice> alice;
immut<Anna> anna;
};
auto alice_charlie_lens = MAKE_SIMPLE_LENS(Alice, Charlie, charlie);
auto bob_alice_lens = MAKE_SIMPLE_LENS(Bob, Alice, alice);
auto bob_charlie_lens = lens_composer(bob_alice_lens, alice_charlie_lens);
static_assert(std::is_same<GetFromType<decltype(bob_charlie_lens)>, Bob>{});
static_assert(std::is_same<GetToType<decltype(bob_charlie_lens)>, Charlie>{});
#include <iostream>
int main() {
immut<Charlie> charlie{Charlie{77}};
immut<Alice> alice{Alice{charlie}};
immut<Anna> anna{Anna{}};
Bob bob{alice, anna};
std::cout << "bob -> anna: " << static_cast<void const*>(&bob.anna.get()) << "\n";
std::cout << "bob -> charlie: " << bob_charlie_lens.get(bob).id << "\n";
Bob newbob = bob_charlie_lens.modify(bob, [] (auto &charlie) {
charlie.id += (148 - 77);
});
std::cout << "new bob -> anna: " << static_cast<void const*>(&bob.anna.get()) << "\n";
std::cout << "old bob -> charlie: " << bob_charlie_lens.get(bob).id << "\n";
std::cout << "new bob -> charlie: " << bob_charlie_lens.get(newbob).id << "\n";
}
