[Caml-list] Naming polymorphic variant types

["Nick Alexander" <nalexander@amavi.com>]

Hi all,

Here's a file...

module type TEST = sig
  type outer = [`A | `B]
  type inner = [`B]

  val x : outer -> outer
  val y : inner
end

module Test : TEST = struct
  type outer = [`A | `B]
  and inner = [`B]

  let x o = match o with
  | `A -> `A
  | `B -> `A

  let y = `B
end

let p = Test.y
let q = Test.x p

That when evaluated, produces...

# #use "C:/Program Files/Objective Caml/test.ml";;
module type TEST =
  sig
    type outer = [ `A | `B]
    and inner = [ `B]
    val x : outer -> outer
    val y : inner
  end
module Test : TEST
val p : Test.inner = `B
File "C:/Program Files/Objective Caml/sql5.ml", line 21, characters 15-16:
This expression has type Test.inner = [ `B] but is here used with type
  Test.outer = [ `A | `B]
The first variant type does not allow tag(s) `A

I'm confused.  Can I get an explanation for why the parameter to a
function with a sum variant type needs all summands to be possible
arguments?  I'm sure I'm misunderstanding the subtyping relationship, but
I can't hack it.  This all grew out of a desire to document code by naming
polymorphic variant types, a questionable capability at best.
Any light appreciated,
Nick Alexander


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