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 with 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).
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