FizzBuzz

compose.hxx

#include <functional>
// TODO : monoid
template<typename... Fn> struct composer;
template<> struct composer<> {
  template<typename Data> Data operator()(Data d){ return d; }
};
template<typename F0, typename... Fn>
struct composer<F0, Fn...> : private composer<Fn...> {
  F0 f;
  composer(F0 f0, Fn const&... fn): composer<Fn...>(fn...), f(f0){}
  template<typename Data> auto operator()(Data d)->decltype(f(composer<Fn...>::operator()(d)))
  { return f(composer<Fn...>::operator()(d)); }
};

template <typename... Fn>
composer<Fn...> comp(Fn const&... fn)
{ return composer<Fn...>(fn...); }

template <typename F>
struct compose_functions {
  F operator()(F f1, F f2) const { return comp(f1, f2); }
};

fizz-buzz.cxx

#include <iostream>
#include <iterator>
#include <string>
#include <tuple>
#include <functional>
#include "transducers.hxx"
#include "compose.hxx"
#include "tuple_reader.hxx"

// usage: seq 1 20 |./fizz-buzz-args 3 Fizz 5 Buzz

int main(int argc, char* argv[]){
  typedef int data_t;
  typedef std::tuple<data_t, std::string> tuple_t;
  typedef std::function<tuple_t(tuple_t)> fun_t; // for type erasure & copy cstor of lambda
  typedef decltype(std::cout)& out_t;
  typedef std::istream_iterator<data_t> in_t;

  transduce(comp(map([](data_t v){return tuple_t(v, "");})
                 , map(transduce(comp(tuple_reader<tuple_t>()
                                      , map([](tuple_t f_n){ return [f_n](tuple_t v_n){
                                            return (std::get<0>(v_n) % std::get<0>(f_n))
                                              ? v_n : tuple_t(std::get<0>(v_n), std::get<1>(f_n) + std::get<1>(v_n));};}))
                                 , compose_functions<fun_t>{}
                                 , fun_t([](tuple_t t){ return t;})
                                 , argv+1, argv+argc))
                 , map([](tuple_t const& v_n){ return std::get<1>(v_n).empty() ? std::to_string(std::get<0>(v_n)) : std::get<1>(v_n);}))
            , [](out_t out, std::string const& s)->out_t { return out << s << ' ';}
            , std::ref(std::cout), in_t(std::cin), in_t()).get() << std::endl;

  return 0;
}

transducers.hxx

#include <iostream>
#include <numeric>
#include <algorithm>
#include <tuple>
#include <sstream>
#include <functional>
 /* const correctness and pass by value vs const ref, move semantics
   left as an exercise to the reader.
iterator range API prevents a flatmap as the flatmapped function
 could not either return a value or a range.
(would return the paste-the-end iterator of an output range)*/


template<typename Op, template<typename, typename> class Transducer >
struct transducer_helper {
  Op op;
  transducer_helper(Op o):op(o){}
  template <typename Reducer>
  Transducer<Op, Reducer> operator()(Reducer r)
  {return Transducer<Op, Reducer>(op, r);}
};

template <typename Op, typename Reducer>
struct transducer_base {
  Op op;
  Reducer reducer;
  transducer_base(Op o, Reducer r): op(o), reducer(r){}
};

template <typename Op, typename Reducer>
struct map_transducer : transducer_base<Op, Reducer> {
  using transducer_base<Op, Reducer>::transducer_base;
  template<typename Result, typename Input>
  Result operator()(Result r, Input data)
  { return this->reducer(r, this->op(data));}
};

template <typename Op>
transducer_helper<Op, map_transducer> map(Op const& op)
  { return transducer_helper<Op, map_transducer>(op); }

template <typename Op, typename Reducer>
struct filter_transducer : transducer_base<Op, Reducer>{
  using transducer_base<Op, Reducer>::transducer_base;
  template<typename Result, typename Input>
  Result operator()(Result r, Input data)
  { return this->op(data) ? this->reducer(r, data) : r ;}
};


template <typename Op>
transducer_helper<Op, filter_transducer> filter(Op const& op)
{ return transducer_helper<Op, filter_transducer>(op); }


// the flatmap Op takes two args, the data and an output iter
// (and returns the past-the-end output iter)
template <typename Op, typename Reducer>
struct flatmap_transducer : transducer_base<Op, Reducer>{
  using transducer_base<Op, Reducer>::transducer_base;
  template<typename Result, typename Input>
  Result operator()(Result r, Input data){
    return (this->op(data, make_reducing_output_iterator(this->reducer, r))).res;
  }
};

template <typename Op>
transducer_helper<Op, flatmap_transducer> flatmap(Op const& op)
{ return transducer_helper<Op, flatmap_transducer>(op); }

template < typename Transducer, typename Reducer, typename Init, typename In>
Init transduce (Transducer xform, Reducer f, Init res, In b, In e)
{ return std::accumulate(b, e, res, xform(f)); }

tuple_reader.hxx

#include <tuple>
#include <sstream>

struct lexical_reader {
  std::string data;
  lexical_reader(std::string str):data(str){}
  lexical_reader(char const* str):data(str){}
  template<typename V>
  lexical_reader(V d){ std::ostringstream oss; oss<<d; data= oss.str();}
  template<class E> void operator()(E& elt){
    std::istringstream iss(data);
    iss >> elt;
  }
};

template<int... Is> struct seq {};
template<int N, int... Is> struct gen_seq : gen_seq<N-1, N-1, Is...> {};
template<int... Is> struct gen_seq<0, Is...> : seq<Is...> {};

template<int N, class T, class F>
void apply_one(T& p, F func){ func( std::get<N>(p)); }

template<class T, class F, int... Is>
void apply(T& p, int index, F func, seq<Is...>){
  using FT = void(T&, F);
  static constexpr FT* arr[] = { &apply_one<Is, T, F>... };
  arr[index](p, func);
}

template<class T, class F>
void apply(T& p, int index, F func)
{ apply(p, index, func, gen_seq<std::tuple_size<T>::value>()); }


template<typename T> struct tuple_reader{
  template <typename Reducer> struct tuple_reader_transducer {
    T t;
    std::size_t i;
    Reducer reducer;
    tuple_reader_transducer(Reducer r):i(0), reducer(r){}

    template<typename Result, typename Input>
    Result operator()(Result r, Input data) {
      apply(t, i, lexical_reader(data)); ++i;
      if(i == std::tuple_size<T>::value ){ i= 0; }
      return i ? r : this->reducer(r, t) ;
    }
  };
  template <typename Reducer>
  tuple_reader_transducer<Reducer> operator()(Reducer r) const
  { return tuple_reader_transducer<Reducer>(r); }
};