Is there an empty polymorphic variant type? Since the syntax [>] means a polymorphic variant that can have any constructors, it would seem natural for [<] to mean a polymorphic variant that can have no constructors (hence is necessarily empty), but it doesn’t seem to be accepted. The syntax [] for…

https://ocaml.org/manual/emptyvariants.html [image] Empty polymorphic variant type use case Learning Hello. I was wondering with some of my colleagues what the use case could be for an empty polymorphic variant type, apart from making some type that is empty at first but …

I know about empty variants. I’m asking about empty polymorphic variants. Despite its name, the other question seems to be only about empty ordinary variants.

Empty types would be strictly superior replacement for empty polymorphic types, if empty types were considered as a subtype of any type as suggested in Feature request: Make empty types subtype of any other type · Issue #9459 · ocaml/ocaml · GitHub . Note also that empty polymorphic variants are su…

Considering ordinary empty types as a subtype of any type is wrong to me; I’m glad that isn’t the case. But since polymorphic variant types already have subtyping relations, I think it would make sense for them to include an empty version.

I had the same question and opened a feature wish to support [ | ] as the empty polymorphic variant type. If there is positive feedback I may open a Pull Request.

I have just discovered that there do exist polymorphic variant types that are semantically empty even if they are not syntactically empty, such as type empty = [ `Empty of empty ] This type has a constructor, but due to the recursion it is not possible to actually construct an element of it, and w…

It’s actually possible to write: let rec empty : empty = `Empty empty which without the annotation gets type [> `Empty of 'a ] as 'a, so it isn’t really an empty type.

It’s easy enough to adapt that approach to use actually empty types, though, e.g. with a empty variant type nothing = | type empty = [`Empty of nothing ] let abort : type a. empty -> a = function `Empty _ -> . or a polymorphic record field: type nothing = { abort: 'a. 'a } type empty = [`Empty of…

Thank you! You’re right, those modifications work. And for completeness, here is a definition, using that approach, of a family of finite types of all cardinalities that are related by subtyping: type empty = | type zero = [ `Empty of empty ] type 'a suc = [ `Zero | `Suc of 'a | `Empty of empty ]…

That’s because in a turing complete language (with full recursion) any type is inhabited, see for instance How many values does a boolean have? , especially the paragraph about Haskell and bottom which is similar to the solution of @viritrilbia . Bottom is the denotation of non terminating computation…

Empty polymorphic variant type?

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orbitz September 10, 2021, 8:43pm 2

https://ocaml.org/manual/emptyvariants.html

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Empty polymorphic variant type?

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