Re: [Caml-list] Polymorphic variant subtyping riddle

[Florian Angeletti <octa@polychoron.fr>]

Hi,

The meaningful difference here is that the type `safe` and `base` are 
recursive.

It is not particularly clear, but the second part of the error message 
is complaining about the difference of type in the argument of `List.

Typically, if your example is extended to:

     type safe = [ `A of safe | `B | `C ]
     type basic = [ `A of basic | `B ]
     let basic :basic = `A `B
     let of_safe :safe -> char = function
        | `A _ -> 'a'
        | `B -> 'b'
        | `C -> 'c'
     let relaxed = (of_safe : safe -> char :> [< safe ] -> char)
    let x = relaxed (basic :> safe)

Then the coercion becomes necessary too.

— octachron.