[destructionator]

Just a personal anecdote, Walter Bright's Digital Mars C++ compiler also had the contracts (D started life almost literally as recycled code from Mr. Bright's other compilers - he wrote a native Java compiler, a Javascript 1.3 stdlib, and a C++ compiler with a bunch of extensions.... smash those together and you have the early D releases!).

Anyway, I used the DM C++ compiler originally because it was the only one I could download to the high school computers without filling out a form, and pimply-face youth me saw "DESIGN BY CONTRACT" at the top of the website and got kinda excited thinking it was a way to make some easy money coding online.

Imagine my disappointment when I saw it was just in/out/invariant/assert features. (I'm pretty sure D had just come out when I saw that, but I saw `import` instead of `#include` and dismissed it as a weenie language. Came back a couple years later and cursed my younger self for being a fool! lol)

[WalterBright]

The in/out features come into their own when inheritance is in play, i.e. for member functions of classes and interfaces. See https://dlang.org/spec/function.html#in_out_inheritance

`import` is so cool we extended it to be able to import .c files! The D compiler internally translates them to D so they can be used. When this was initially proposed, the reaction was "what's that good for?" It turned out to be incredibly useful and a huge time saver.

The concept is sort of like C++ being a superset of C and so being able to incorporate C code, except unlike C++, the C syntax can be left behind. After all, don't we get tired of:

    struct Tag { ... } Tag;

?

[1718627440]

> struct Tag { ... } Tag;

What's the thing with the syntax? If you don't intend to use the type elsewhere don't give it a tag, if you want, you have to give it a name. (Assuming you are annoyed by the duplicate Tag)

[WalterBright]

Which would you prefer:

    struct Tag { ... }

or:

    typedef struct Tag { ... } Tag;

? It's just simpler and easier to write code in D than in C/C++. For another example, in C/C++:

    int foo();
    int bar() { return foo(); }
    int foo() { return 3; }

The D equivalent:

    int bar() { return foo(); }
    int foo() { return 3; }

[1718627440]

If the user of Tag is supposed to know how the internal details:

    struct Tag { ... };

if it needs to rely on the internals, but the user shouldn't care:

    typedef struct { ... } Tag;

if it can be opaque (what I would default to):

    typedef struct {} Tag;

I also think that is a good feature to separate specification from implementation, I like being forced to declare first what I want to implement. Funnily in your special case you wouldn't need the declaration. (But of course it's a bad idea to rely on this "feature")

[WalterBright]

My C++ compiler also implemented contracts back in the 90s: https://www.digitalmars.com/ctg/contract.html

Modern C++ is slowly adopting D features, many of which came from extensions I added to my C++ compiler.