Question

There are a lot of impressive Boost libraries such as Boost.Lambda or Boost.Phoenix which go a long way towards making C++ into a truly functional language. But is there a straightforward way to create a composite function from any 2 or more arbitrary functions or functors?

If I have: int f(int x) and int g(int x), I want to do something like f . g which would statically generate a new function object equivalent to f(g(x)).

This seems to be possible through various techniques, such as those discussed here. Certainly, you can chain calls to boost::lambda::bind to create a composite functor. But is there anything in Boost which easily allows you to take any 2 or more functions or function objects and combine them to create a single composite functor, similar to how you would do it in a language like Haskell?

Answer 1

I don’t know of anything that supports the syntax you wish for currently. However, it would be a simple matter to create one. Simply override * for functors (boost::function<> for example) so that it returns a composite functor.

template < typename R1, typename R2, typename T1, typename T2 >
boost::function<R1(T2)> operator * (boost::function<R1(T2)> const& f, boost::function<R2(T2)> const& g)
{
  return boost::bind(f, boost::bind(g, _1));
}

Untested, but I suspect it’s close if it doesn’t work out of the box.