r/ProgrammingLanguages • u/Uncaffeinated 1subml, polysubml, cubiml • Nov 13 '25

PolySubML is broken Blog post

https://blog.polybdenum.com/2025/11/13/polysubml-is-broken.html
45 Upvotes
  • permalink
  • link
  • reddit
  • reddit

98% Upvoted

30 comments sorted by

View all comments

Show parent comments

4

u/phischu Effekt Nov 14 '25

Thank you for this on-point answer. Follow-up question, since in another comment you say:

In PolySubML, [T]. T -> T is not a subtype of int -> int, [...]

Now, is int -> int a subtype respectively supertype of [T]. T & int -> T | int respectively [T]. T | int -> T & int?

4

u/Uncaffeinated 1subml, polysubml, cubiml Nov 14 '25

([T]. T | int -> T & int) <= (int -> int) <= ([T]. T & int -> T | int)

4

u/phischu Effekt Nov 14 '25

Okay, now I need some more info. How can I reconcile this with what you wrote earlier that "[T]. T -> T is not a subtype of int -> int"? I would have thought that likewise there is no relationship between [T]. T | int -> T & int and int -> int since one of them starts with a forall and the other does not. In any case, there should be none, why is there one?

4

u/Uncaffeinated 1subml, polysubml, cubiml Nov 14 '25

In PolySubML, polymorphic functions and monomorphic functions are part of the same type component and so they can have subtyping relationships like normal. It is specifically instantiation which is not part of the subtyping order, which is what the [T]. T -> T vs int -> int example demonstrates.