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by masklinn
452 days ago
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> the presence of an implicit conversion rule `T -> T?` amounts to the observation that `T <: T?`, where <: is the subtyping relation So you assert that numbers and strings are the same type because PHP and Javascript will implicitly convert back and forth? That any C++ implicit constructor creates a subtyping relationship? > I don't think any language allows this Rust, Swift, Zig, just off the top of my head. C# too, value types is where and why it first introduced explicit nullability (back in C# 2.0). |
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If you can provide (valid!) methods `T -> U` and `U -> T` for two types, why wouldn't `T = U` hold? (Atleast for types where `=` makes sense)
This is the definition I am using:
> S is a subtype of T, written S <: T, if a value of type S can safely be used in any context where a value of type T is expected.
from Pierce's "Software Foundations"
Of course, you may not want to make `String <: Number` and `Number <: String` (and thus `String = Number`), because there is no sensible way to do so; but this is an issue with the way PHP/JS handles subtyping, not with the notion of subtyping itself and certainly does not apply to the example of nullable types.
Unfortunately, I am not familiar enough with C++ to comment on the other question, sorry!