The square vs rectangle example is a great one for showing how to not use types at all, IMO.
> As a type, this relationship is reversed: you can use a rectangle everywhere you can use a square (by having a rectangle with the same width and height), but you cannot use a square everywhere you can use a rectangle (for example, you can’t give it a different width and height).
Which is a good thing (yes, I'm a proponent of strong typing, like in Rust). If you want that square to be used in places where only a rectangle can be, you should either be explicitly required to cast it to a rectangle type or have a language that can define and handle contravariance for a consumer method, where it would be OK to accept a square as a rectangle.
EDIT: As this is getting upvoted and I said it slightly wrong: "where it would be OK to accept a rectangle where a square (or it's supertype, the rectangle) is acceptable." Goes ways to prove that strong typing is the better solution, though, I guess. :-)
> As a type, this relationship is reversed: you can use a rectangle everywhere you can use a square (by having a rectangle with the same width and height), but you cannot use a square everywhere you can use a rectangle (for example, you can’t give it a different width and height).
Which is a good thing (yes, I'm a proponent of strong typing, like in Rust). If you want that square to be used in places where only a rectangle can be, you should either be explicitly required to cast it to a rectangle type or have a language that can define and handle contravariance for a consumer method, where it would be OK to accept a square as a rectangle.
EDIT: As this is getting upvoted and I said it slightly wrong: "where it would be OK to accept a rectangle where a square (or it's supertype, the rectangle) is acceptable." Goes ways to prove that strong typing is the better solution, though, I guess. :-)