Show HN: Match(it): A C++17 pattern-matching library with lots of good stuffs

[amazing42]

A lightweight single-header pattern-matching library for C++17 with macro-free APIs.

Try it at https://godbolt.org/z/8YMr8Kz8j

[dymk]

What a neat concept. It never ceases to amaze me what people can arm-bar C++ into doing through typesystem, metaprogramming, and operator overloading shenanigans.

I hope I never see this in a production codebase, though!

On a kind of related note, I want to see an alternate history where C++ had support for Rust-ish proc macros, or Racket-ish macros. What would that language even look like? There'd probably be no need to hack existing C++ language semantics to add new features, you'd "just" generate the code that the meta-language lowers to.

[Longhanks]

> I hope I never see this in a production codebase, though!

Why? Looks completely readable, easy to understand to me.

Do you also hope to never see the STL in production? Because internally, that thing is high level unreadable C++.

[gpderetta]

That alternative history is currently being implemented by the Circle compiler.

[olvy0]

Interesting - in the godbolt.org link, looks like both GCC and Clang optimize the code to run at compile time, so the resulting function is just "mov eax, 24".

But MSVC latest instead generates the full runtime code and function call.

[phoe-krk]

Strangely, clang 13+ seems to have regressed - it emits runtime code as well, whereas clang 12 has just "mov eax, 24".

Should this be reported anywhere?

[fooker]

Wow, I didn't know you could #include a remote file in compiler explorer.

[amazing42]

Yeah, that is super cool feature of compiler explorer compared to other online compiler.

[hkalbasi]

Not super useful without ADTs, but nice! I hope someday ADT and pattern matching find their way to the c++ language.

[amazing42]

Sadly lvariant did not get into C++ language. The library supports pattern matching against std::variant/std::any and class inheritance as a replacement.

[jokoon]

I hope this will become part of C++ one day.

Although if the C++ committee manages to add this without deprecating other things, I would be impressed.

[corysama]

There has been a proposal to add pattern matching to C++ for nearly a decade now https://www.stroustrup.com/OpenPatternMatching.pdf Shame it never got in. With language support for matching, sum types and product types it would be a completely different language.

[synergy20]

Thought it's about regex matching for switch-case, turns out it's very different, interesting though, is this an idea borrowed from Rust? Not sure if I'm going to use it or not.

[klyrs]

Pattern matching has quite a long history[1]; it's been around in some form since 1957. It's been enjoying a bit of a renaissance lately; I suspect that's a result of Haskell's popularity but I don't really know.

[1] https://en.wikipedia.org/wiki/Pattern_matching

[fouronnes3]

Very nice looking. Can you pattern match std::variant?

[amazing42]

Yeah. That is supported.

[bhedgeoser]

Rust version:

const fn factorial(n: u128) -> u128 { match n { 0 => 1, _ => n * factorial(n-1) } }

fn main() { dbg!(factorial(20)); }

[amazing42]

The godbolt link is only a simple sample.

Rust does not support view pattern (called in Haskell) or app pattern (called in Racket). And that has been implemented in this library.

[rowanG077]

Please, please, please don't make fetching a library part of your cmake include process.

[amazing42]

Thanks for bringing this up. Now you can use cmake find_package with it.