[Chris_Newton]

I don’t see any disagreement that some type checker could catch this unintended behaviour. Many popular languages have checkers that would. The question here appears to be whether TypeScript’s type checker could do it without other consequences that are considered unacceptable.

[Geminidog]

Read carefully. The distinction here is that the type checker must allow for intended behavior within JavaScript while checking for an error.

The type checking I am talking about is not a sum type. It is not that the function can take a two different possible types. It's the fact that the parameter function can mutate into two different types depending on the usage. It has (<arity 1 or 2>) not (<arity 1> or <arity 2>) if you catch my meaning.... Or in other words the concrete type is not evaluated when you pass the function as a parameter but only when it is called with a certain amount of parameters... which is not something type checkers I know about look for.

[Chris_Newton]

The fundamental problem with the example under discussion seems to be that while the behaviour might not be intended by the programmer, it is working as specified as far as the language is concerned and changing that specification to make the unwanted behaviour fail a type check could have additional and unwanted side effects.

Perhaps I’m not correctly understanding your idea around arity as part of the function types, but so far it’s not obvious to me how what I think you’re describing helps to resolve that contradiction. Are you suggesting a way the type system could be changed without causing those additional, unwanted side effects?

Do you by any chance have a more rigorous definition or even a formal semantics for your proposed arity types that you could share, so the rest of us can understand exactly what you’re proposing here?

[Geminidog]

> The fundamental problem with the example under discussion seems to be that while the behaviour might not be intended by the programmer, it is working as specified as far as the language is concerned and changing that specification to make the unwanted behaviour fail a type check could have additional and unwanted side effects.

You don't need to change the behavior of the program. You can change the type checker to catch the unwanted error.

>Perhaps I’m not correctly understanding your idea around arity as part of the function types, but so far it’s not obvious to me how what I think you’re describing helps to resolve that contradiction. Are you suggesting a way the type system could be changed without causing those additional, unwanted side effects?

It's not formalized anywhere to my knowledge and I'm not willing to go through the rigor to do this in the comments. But it can easily be explained.

Simply put, what is the type signature of a function that can accept either two variables or one variable? I've never seen this specified in any formal language.

To fix this specific issue you want the type signature here to specify only certain functions with a fixed arity.

When some external library is updated with a function that previously had arity 1 to <arity 1 or 2> that could be thought of as type change that should trigger a type error.

Right now type checker recognizes F(a) and F(a, b=c) (where c is a default parameter that can be optionally overridden) as functions with matching types.

  F(a) == F(a, b=c)
  F(a,b) == F(a, b=c) <-----(F(a,b) in this case is a function where b is NOT optional) 
  F(a) != F(a, b) 

From the example above you can see the type checker lacks transitivity (a == c and b == c does not imply a == b), because the type of a function with an optional parameter is not really well defined or thought out.

This is exactly the problem the author is describing. The type checker assumes that when the library changed F(a) to F(a, b=c) that the types are still equivalent, but this breaks transitivity so it's a bad choice and will lead to strange errors because programmers assume transitivity is a given.

You don't see this problem in other type checkers because JavaScript is weird in the sense that you can call a function of arity 1 with 5 parameters.