I wrote a few posts about using concepts. Concepts are a named set of requirements. Let's define a few concepts in this post.

The unification of templates, concepts, and placeholders goes on. This time, I will have a closer look at constrained (concepts) and unconstrained (auto) placeholders in the context of templates.

C++11 has auto unconstrained placeholders. You can use concepts in C++20 as constrained placeholders. What seems at first glimpse not so thrilling is for me the decisive quantum leap. C++ templates will become an easy to use C++ feature.

Expression templates are "structures representing a computation at compile-time, which structures are evaluated only as needed to produce efficient code for the entire computation" (https://en.wikipedia.org/wiki/Expression_templates). As needed, now we are in the centre of lazy evaluation and in the centre of this post.

In my previous post Recursion, List Manipulation, and Lazy Evaluation, I wrote about the characteristics of functional programming:  The story about lazy evaluation in C++ is short. Sorry to say, but I have forgotten templates. The two advanced techniques CRTP and expression templates, are based on lazy evaluation.

Monads in C++? What a strange name for a post. But it's not so strange. With std::optional C++17 gets a monad. The ranges library from Eric Niebler and the extended futures are also monads. For both, we can hope for in C++20.

We stay in the year 2020. With high probability, we will get concepts. Of course, waterproof statements about the future are difficult but the statement is from Bjarne Stroustrup (Meeting C++ 2016 at Berlin).

A small-time jump and we are in the year 2020. C++ will get - as far as the future is predictable - the new ranges library. Thanks to Eric Nieblers library, working with the Standard Template Library (STL) will become much more comfortable and powerful.

With fold expressions, you can implement Haskell known functions foldl, foldr, foldl1 and foldr1 directly in C++. These four functions successively reduce a list to a single value.

The remaining three characteristics of functional programming are told quite quickly: Recursion, manipulation of lists and lazy evaluation.