| Suppose you have a type hierarchy where a Tabby is a type of Cat that is a type of Animal. Now, say you have a function that takes a Cat as an argument and returns a Cat. What types can you use instead of the Cats and still remain type safe? Well, you could pass in a Tabby or return a Tabby and everything would be fine. This is a subtype rule that the compiler can check. What about something a little trickier? Say you have a map function that maps from a list of Cats to another list of Cats. The map function takes two arguments: a list of Cats and an arbitrary function that takes a Cat and returns a Cat. For each item in the input list, the function is called on it and the returned Cat is added to the output list. Now, can we replace the Cat to Cat input function with a function that has different types and still have the types make sense? Let's try a function that takes a Tabby as an argument and returns a Cat (usually denoted Tabby->Cat). This won't work as we could have something that isn't a Tabby in the input list. What about a function Animal->Cat? This would work OK as every Cat in the input list is also an Animal. Similarly, what if we try a Cat->Tabby function? This would work too as a Tabby is always a Cat. What about Cat->Animal? No, as it might produce a Dog and we only want Cats. Notice the difference between the arguments and return types? For arguments we can replace a Cat with an Animal but, for returns, we can only go the other way. We say that we are contravariant in the argument type but covariant in the return type. There are other forms. We say we are invariant if we can't change the type from the original stated type. We say we are bivariant if we can replace Cat with either Animal or Tabby. So, what did Typescript do wrong? As I understand it, Typescript was (is?) bivariant in both argument and return types. This means that the type system would, say, accept a Tabby->Cat function for a Cat->Cat function argument and then fail at runtime. Is this a heinous crime? A matter of opinion I guess. It's definitely wrong from a type system perspective but plenty of type systems let this sort of thing pass and are successful. BTW this subtyping complexity isn't just for function arguments. It's also an issue with collection types e.g. what can yuo replace a list of Cats with? But this comment is way too long already. |