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[Caml-list] Documentation error - #myvariant
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 Date: 2004-04-29 (11:18) From: Keith Wansbrough Subject: [Caml-list] Documentation error - #myvariant
Hi.. the OCaml documentation, section 4.2 on polymorphic variants,

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?