Reduce.md

<CppML/Algorithm/Reduce.hpp>

Reduce

template <typename F, typename Pipe = ml::Identity>
struct Reduce {
  template <typename Init, typename ...Ts>
  using f = /* .... */;
};

Reduce<F, Pipe>

Reduce<F, Pipe> is a metafunction that passes to Pipe the generalized sum (with F taking the role of the binary operator) of Ts..., initialized by Init. Pipe defaults to ml::Identity.

f:: Init, Ts... -> F( ... F(F(Init, T0), T1), ... Tn) >-> Pipe

F

F is to be a metafunction from two types, to one.

f:: Init, T -> Init'

Example

We implement ml::CountIf<ml::IsClass> using Reduce.

using T = ml::f<
    ml::Reduce<
      ml::ProductMap<                     // Construct binary function F:
          ml::Identity,                   // Map first element (Init in F) by identity
          ml::IsClass<>,                  // Map second element (T in F) by IsClass     
          ml::Add<>>>,                    // And pipe to Add
    ml::Int<0>,                           // Init
    int, std::string, bool, std::string>; // Ts...

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

NOTE that

ml::ProductMap<    // Construct binary function F:
    ml::Identity,  // Map first element (Init in F) by identity
    ml::IsClass<>, // Map second element (T in F) by IsClass     
    ml::Add<>>>,   // And pipe to Add

is a metafunction of type

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

which conforms to F. See ProductMap.