Provide lazy proxies for algorithms

[brunocodutra]

As shown by the example below, lazy proxies help reducing clutter when fancy composition is desired, the question is where to put them. One option would be within namespace metal::lazy, another would be to name them like shown below, so they could live at the root of namespace metal.

originally posted on cpplang.slack.com

using namespace metal;

template<typename... args>
using Transform = bind<lambda<transform>, args...>;

template<typename... args>
using If_ = bind<lambda<if_>, args...>;

template<typename... args>
using Invoke = bind<lambda<invoke>, args...>;

using lbd = Transform<If_<Invoke<_1, _2, _3>, _2, _3>, quote<l>>;

Given a predicate pred and unary lambdas f and g, you can then conditionally transform l with either f or g according to pred.

invoke<lbd, pred, f, g>;