ProductMap.md

<CppML/Functional/ProductMap.hpp>

ProductMap

template <typename ...Fs, typename Pipe>
struct ProductMap {
  template <typename ...Ts>
  using f = /* .... */;
};

ProductMap<Fs..., Pipe>

ProductMap<Fs..., Pipe> is a metafunction that passes to Pipe the parameter pack Us..., which is generated by invoking each F in the parameter pack Fs... on the corresponding element of the parameter pack Ts....

f:: Ts... -> Fs(Ts).. >-> Pipe

Args...

The parameter pack of the ProductMap<Args...> will be treated as Pipe, and must always be provided (there is no default).

Example

We will write the ml::CountIf<ml::IsClass<> using ml::Reduce. For that, we need the accumulator function

f:: Init, T -> Init, IsClass(T) >-> Add

which we will make using ml::ProductMap. Note that the above is equivalent to

f:: Init, T -> Identity(Init), IsClass(T) >-> Add

Thus,

template <typename Predicate>
using MakeCounter = ml::ProductMap<ml::Identity<>, Predicate, ml::Add<>>;

using T = ml::f<
    ml::Reduce<
               MakeCounter<ml::CountIf<>>>,
    ml::Int<0>,                           // Init
    int, std::string, bool, std::string>; // Ts...

static_assert(
              std::is_same_v<
                  T,
                  ml::Int<2>>);