What :> does in OCaml?

Learning
1

I’ve come across

let cast x = (x :> (unit, write_error) result Lwt.t)

here

and this operator :> is not in pervasives and I couldn’t find on google/don’t know how to find, because it ignores the :> in the search.

2

This is a explicit type coercion from a subtype to a supertype. See for instance https://ocaml.org/releases/4.10/htmlman/objectexamples.html#s:using-coercions for more explanations .

2 Likes
3

Another example of :> being used for upcasting: https://ocaml.org/releases/4.10/htmlman/privatetypes.html#ss:private-types-abbrev

And it also works for upcasting values of polymorphic variants:

type t = [`t]
type u = [t | `u]

let t: t = `t
let u: u = (t :> u)
2 Likes