On modern C++ and PLT, part 1

Howdy, It's been a while :-). I've been largely immersed in modern C++ for the last few years—particularly C++20 and C++23, which have some groundbreaking changes if you haven't kept up.

I'll get to the point and let it speak for itself:

export module palindrome;
import <ranges>;
import <locale>;
export import <string_view>;
 
export bool constexpr is_palindrome(std::string_view str) {
    auto letters_only{str
        | std::views::filter([](char c) { return std::isalnum(c, std::locale{}); })
        | std::views::transform([](char c) { return std::tolower(c, std::locale{}); })
    };
    auto reversed{letters_only | std::views::reverse};
    
    return std::ranges::equal(letters_only, reversed);
}

Wow, right? That's a lot to unpack - especially if you haven't looked at C++ in a while. For starters, C++20 has a new modules feature akin to import/export in other languages. Second, C++20 has what's known as "ranges" that work on "views":

std::vector<std::string> strings{"string", "one", "two", "testing" };

auto sizes = strings
	| std::ranges::views::transform([](auto&& str) { return str.size(); })
	| std::ranges::views::filter([](auto size) { return size > 3; });

The astute programmer (or PLT enthusiast) will notice that ranges are higher-order operators and views are an algebraic type abstraction that satisfies constant time copy, move, and assignment operators.

This is profound because it's very close to monadic composition. Not even Rust has higher-kinded polymorphism or free monadic composition.

This is just the beginning. C++23 takes the next big step — behold — the Maybe monad:

using namespace std::literals;

const std::vector<std::optional<std::string>> v
{
    "1234", "15 foo", "bar", "42", "5000000000", " 5", std::nullopt, "-43"
};

for (auto&& x : v | std::views::transform(
    [](auto&& o)
    {
        // debug print the content of input optional<string>
        std::cout << std::left << std::setw(13)
                  << std::quoted(o.value_or("nullopt")) << " -> ";

        return o
            // if optional is nullopt convert it to optional with "" string
            .or_else([]{ return std::optional{""s}; })
            // flatmap from strings to ints (making empty optionals where it fails)
            .and_then(to_int)
            // map int to int + 1
            .transform([](int n) { return n + 1; })
            // convert back to strings
            .transform([](int n) { return std::to_string(n); })
            // replace all empty optionals that were left by
            // and_then and ignored by transforms with "NaN"
            .value_or("NaN"s);
    }))
    std::cout << x << '\n';

Moreover, flat_map was added as a free standard header for container types. I can't put the language down right now.

As a personal researcher in PLT and computational linguistics, C++ is hitting all the right spots, and I expect this to continue to be the case for quite some time. There are enough toys in it to keep me busy for many years. On a side note, as far as PLT goes, I am not particularly interested in Rust: the syntax is offputting, and it can't do anything that grabs my attention.

I'd like to end this post with one last gem, C++23 has a new class templatestd::expected - functional programmers should quickly catch that this is the try monad for error handling. :-)

Catch you next time!