Is `type as` documented anywhere?

1

I’m trying to do some bizarre stuff with poly variants. I suspect what I want to do is not possible (and should not be possible), and I’ll follow up with some questions once I figure out what those questions are. :smiley:

In the meantime, I have a simpler question. I noticed in the lower bound docs that a type is being assigned as a:

type basicBlueTone<'a> = [> #Blue | #DeepBlue | #LightBlue ] as 'a

Intuitively, I can see what’s going on here, but I want to know how far I can push it. Can I define a type with a as generic parameter as well as other fields, for example?

I found the (lack of) docs on covariance ticket, but I can’t tell if it’s a related concept.

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2

Hi, perhaps this will help: OCaml - Labels and variants

If you want something with fields, I assume you mean an object, in which case yes:

type person<'a> = {
  ..
  "id": int,
  "name": string,
} as 'a

This means that the type person has at least the id and name fields, and possibly more. Here we’re defining the type explicitly, which we sometimes need to do; more commonly, it is inferred from usage, like:

let greet = person => {
  print_int(person["id"]) // int argument
  print_endline("")
  print_endline(person["name"]) // string argument
}
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3

I’m trying to get my head around this stuff at the moment to support expandable Result.t error types and although I’m still trying to get my head around the covariant/contravariant annotations, I found the information on OCaml objects useful (as the implementation and hence type system implications) are similar:

https://dev.realworldocaml.org/objects.html

4

This may be relevant for you: https://github.com/yawaramin/prometo/blob/ef7229d3cbfe9980e32feef8e882785b49a96434/src/Yawaramin__Prometo.rei#L15

It is in Reason syntax but obviously there is a direct correspondence to ReScript.

1 Like