[nsajko]

Julia is not without warts, but this blog post is kinda rubbish. The post claims vague but scary "correctness issues", trying to support this with a collection of unrelated issue tickets from all across Julia and the Julia package ecosystem. Not all of which were even bugs in the first place, and many of which have long been resolved.

The fact that bugs happen in software should not surprise anyone. Even software of critical importance, such as GCC or LLVM, whose correctness is relied upon by the implementations of many programming languages (including C, C++ and Julia itself), are buggy.

Instead the post could have focused more on actual design issues, such as some of the Base interfaces being underspecified:

> the nature of many common implicit interfaces has not been made precise (for example, there is no agreement in the Julia community on what a number is)

The underspecified nature of Number (or Real, or IO) is an issue, albeit not related with the rest of the blog post. It does not excuse the scaremongering in the blog post, however.

[StefanKarpinski]

Number isn’t an interface—there are no operations common to all numbers. Subtyping Number is a way to opt into numeric promotion and a few other useful generic behaviors. That’s it. The fact that some abstract types are interfaces with expected behaviors, while others are dispatch points to opt into behaviors is a double edged sword: powerful and flexible, but only explicitly expressed/explained in documentation.

[nsajko]

> Number isn’t an interface—there are no operations common to all numbers.

When creating a new type, it should be more clear cut when is subtyping Number (or Real, etc.) valid. Should unitful quantities be numbers? Should intervals be numbers? Related: I think there are some attempts by Tim Holy and others to create/document "thick numbers".

Furthermore, I believe it might be good to align the Number type hierarchy with math/abstract algebra as much as possible without breaking backwards compatibility, which might making Number, or some subtypes of it, actual interfaces.

> Subtyping Number is a way to opt into numeric promotion and a few other useful generic behaviors. That’s it.

OK, but I think that's not documented either.