map_reduce() [1/2]

template<concepts::product_type T, concepts::monoid M, typename Function>

auto kumi::map_reduce ( Function f, M && m, T && t )

inlinenodiscardconstexpr

Performs a tree-like reduction of all elements of a product type. The given map function is applied before excution the reduction to each element of the input.

On record types, this function operates on the underlying values, not on the fields themselves.

Note

For associative operations, this produces the same result as a left or right fold preceeded by map, but have different intermediate evaluation order.

Parameters

f	Mapping function to apply
m	Monoid callable function to apply
t	Product type to reduce

Returns

The result of reducing the elements of t by m after beeing processed by f, recursively combining elements in a tree structure.

Helper type

namespace kumi::result
{
  template<monoid M, product_type T> struct map_reduce;
 
  template<monoid M, product_type T>
  using map_reduce_t = typename map_reduce<M,T>::type;
}

Computes the return type of a call to kumi::map_reduce

Examples:

Tuple:

#include <kumi/kumi.hpp>
#include <iostream>
 
int main()
{
  auto t1 = kumi::tuple{2.,5,short{3},'\4'};
  auto t2 = kumi::tuple{2,1,short{55},' '};
 
  auto times_two = [&](auto e){ return 2*e; };
  std::cout << kumi::map_reduce(times_two, kumi::function::multiplies, t1) << "\n";
 
  auto is_pair = [&](auto e){ return e%2==0; };
  std::cout << kumi::map_reduce(is_pair, kumi::function::plus, t2)        << "\n";
}

Record:

#include <kumi/kumi.hpp>
#include <iostream>
 
int main()
{
  using namespace kumi::literals;
  auto r1 = kumi::record{"a"_id = 2.,"b"_id = 5,"c"_id = short{3},"d"_id = '\4'};
  auto r2 = kumi::record{"a"_id = 2 ,"b"_id = 1,"c"_id = short{55},"d"_id = ' '};
 
  auto times_two = [&](auto e){ return 2*e; };
  std::cout << kumi::map_reduce(times_two, kumi::function::multiplies, r1) << "\n";
 
  auto is_pair = [&](auto e){ return e%2==0; };
  std::cout << kumi::map_reduce(is_pair, kumi::function::plus, r2)        << "\n";
}