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To: caml-list@inria.fr
Subject: [Caml-list] Documentation error - #myvariant
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Date: Thu, 29 Apr 2004 12:18:54 +0100
From: Keith Wansbrough <Keith.Wansbrough@cl.cam.ac.uk>
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Hi.. the OCaml documentation, section 4.2 on polymorphic variants,
Advanced use, explains that

  To make this even more confortable, you may use type definitions as
  abbreviations for or-patterns. That is, if you have defined type
  myvariant = [`Tag1 int | `Tag2 bool], then the pattern #myvariant is
  equivalent to writing (`Tag1(_ : int) | `Tag2(_ : bool)).

But this is not correct!  Consider

  type de = [`D | `E of de];;
  
  type def = [`D | `E of def | `F of def];;
  
  let rec deproc2 rfun =
    function
      | `D -> print_string "D"; `D
      | `E(x) -> print_string "E"; `E(rfun x)
  
  let rec deproc3 x = deproc2 deproc3 x
  
  let rec defproc2 rfun =
    function
  (*    | (`D | `E(_)) as x -> deproc2 rfun x *)
      | #de as x -> deproc2 rfun x
      | `F(x) -> print_string "f"; `F(rfun x)
  
  let rec defproc3 : def -> def =
    fun x -> defproc2 defproc3 x
  
This gives the following error (in OCaml 3.07+2):

  File "polyvar2.ml", line 19, characters 29-30:
  This expression has type def = [ `D | `E of def | `F of def ]
  but is here used with type [< `D | `E of de | `F of de ]
  Type def = [ `D | `E of def | `F of def ] is not compatible with type
    de = [ `D | `E of de ] 

(Note that the expression mentioned is the final x on the last line.)
Replacing the #de line with the commented line above it, however,
yields

  val defproc3 : def -> def = <fun>

as expected.  It looks to me like #de means (`D | `E(_:de)), rather
than (`D | `E(_)) as I expected; except that I'm not even sure what
`E(_:de) means in this case - does it do type-directed matching at
runtime?

Could the documentation please be made more accurate at this point?

Thanks.

--KW 8-)

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