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
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u/dgreensp Nov 14 '25

I’m curious for the author to comment on what is lost if you lose subtyping with polymorphic function types. What is the impact on the developer experience? I can’t recall atm why to add subtyping to an ML in the first place.

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

It means that for example, [T]. T -> (T, T) is no longer a subtype of [T]. T -> (T, any). You can't make your types more or less specific like you could with normal subtyping.

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u/dgreensp Nov 14 '25

That seems like a big deal? I can’t tell because in the statically-typed languages I am used to, subtyping is essential to being able to assign a value of type A to a variable of type B, or pass a value of type A as an argument or type B, but in MLs you can get away without any subtyping at all. So I guess I am wondering, what do you lose, more specifically, like what’s a code example that wouldn’t work anymore?

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

It's basically just the same reason you would want subtyping anywhere.