Hacker News

This is really cool. Is the source available?

C++ lambdas are just structs that overload operator(); once you realize that they become a lot less mysterious. But it's a great example of how this tool is useful to learn about those things (of course you could also read the C++ standard but who has time for that?)

Here's an example: https://cppinsights.io/s/f04b6896

(That also shows how C++'s range-based for-loops work, which is similarly some syntactic suger for a fairly mundane for-loop, but details are interesting: unlike a typical for-loop, the loop variable is declared inside the loop body.)

You can see it by typing in the sample code in the tool here - https://news.ycombinator.com/item?id=49928362

I second this.

Lambdas in D are just nested functions:

    int foo(int i) {
        int add(int x) { return i + x; } // nested function
        return add(3);
    }

    int bar(int i {
        alias lambda = (x) => x + i;  // lambda
        return lambda(3);
    }
    }

You have misunderstood the language and the tool.

Compilers for all languages have the concept of "lowering" the level of source-language abstractions from high-level to low-level viz. Written Source -> Simpler Source -> AST -> IR -> Machine Code. One type is "desugaring" i.e. transforming complex syntax to simpler ones so that the following stages of compilation have to operate only on a smaller and hence more manageable language subset.

This tool shows both desugaring and other lowering (eg. implicit transformations, type deductions etc.) within the C++ source language by regenerating the source from the AST thus giving you full visibility until that stage in the compilation process.

PS: Explanatory video presentation by the author - https://www.youtube.com/watch?v=VJ6ZvDRYzNE&t=558s

Which language do you think doesn't need it? For instance for me I wish I had such a tool whenever I used any interpreted or JIT language, to understand clearly to which semantics whatever is happening maps and how their respective front-end optimizes. Things as simple as "a" + "b" in JavaScript: are you sure you know how every js runtime maps it? Do they fold the strings as an optimization during parsing ? Do they preallocate a larger native string type and push? Do they just do something that ends up an equivalent of std::u16string("a") + std:: u16string("b")?

No a language doesn’t need it. It’s an educational tool for those new to the language or new to a particular language feature. With practice language features just become automatic so you don’t think about them when thinking about a problem.

In the README example, the C++ practitioner just doesn’t think about those automatically synthesized copy constructors and such for POD types, nor do they think about the fact that assignment is desugared to operator= or that an assignment can involve a static_cast.

In what language is compiling simple? Do you not like compiled languages? I'm not sure what you're complaining about here.

I don't think it's only C++ though, Python decorators, Rust macros, almost every language's compiler write code you never see. C++ just does more of it and makes it harder to guess. Being able to look at the expanded version is useful in any language that does this.

Good for C++'s coroutines too

This is very different and doesn’t relate to what the linked repo does.