Functional C++

C++ is a multiparadigm, many level of abstracions, and very little overhead - runtime cost.

Usually procedural, object-oriented, but functional style is also possible. Not strictly functional, but possible to use problem solving schemas and basic bulding blocks (working with values as opposed to identities)

C++ 11 standard expands this support with lambdas

immutable data structures that maintains the speed C++ is known for while providing the protection that functional languages

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Values vs. Identities

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Lambda (value vs reference)

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Function Equivalence
Haskell	C++
map	for_each
foldl	accumulate
foldr	accumulate
filter	copy_if
replicate	fill_n

map

template<class InputIterator, class Function>
  Function for_each(InputIterator first, InputIterator last, Function fn)
{
  while (first!=last) {
    fn (*first);
    ++first;
  }
  return move(fn);
}

template <class InputIterator, class OutputIterator, class UnaryOperator>
  OutputIterator transform (InputIterator first1, InputIterator last1,
                            OutputIterator result, UnaryOperator op)
{
  while (first1 != last1) {
    *result = op(*first1);  // or: *result=binary_op(*first1,*first2++);
    ++result; ++first1;
  }
  return result;
}

foldl

template <class InputIterator, class T>
   T accumulate (InputIterator first, InputIterator last, T init)
{
  while (first!=last) {
    init = init + *first;  // or: init=binary_op(init,*first) for the binary_op version
    ++first;
  }
  return init;
}

foldr

user rbegin and rend

template <class InputIterator, class T>
   T accumulate (InputIterator first, InputIterator last, T init)
{
  while (first!=last) {
    init = init + *first;  // or: init=binary_op(init,*first) for the binary_op version
    ++first;
  }
  return init;
}

filter

template <class InputIterator, class OutputIterator, class UnaryPredicate>
  OutputIterator copy_if (InputIterator first, InputIterator last,
                          OutputIterator result, UnaryPredicate pred)
{
  while (first!=last) {
    if (pred(*first)) {
      *result = *first;
      ++result;
    }
    ++first;
  }
  return result;
}

replicate

template <class OutputIterator, class Size, class T>
  OutputIterator fill_n (OutputIterator first, Size n, const T& val)
{
  while (n>0) {
    *first = val;
    ++first; --n;
  }
  return first;
}

template <class ForwardIterator, class T>
  void fill (ForwardIterator first, ForwardIterator last, const T& val)
{
  while (first != last) {
    *first = val;
    ++first;
  }
}