The unification of templates, concepts, and placeholders goes on. This time, I will look closely 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 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 at the center of lazy evaluation and the center 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 challenging, 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 more comfortable and powerful.

With fold expressions, you can implement Haskell 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.

Pure functions are pretty similar to mathematical functions. They are the reason that Haskell is called a pure functional programming language.