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Date: Wed, 19 Feb 2003 09:13:33 +0100
From: Alessandro Baretta <alex@baretta.com>
Organization: Baretta srl -- www.baretta.com
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To: Ocaml <caml-list@inria.fr>
Subject: [Caml-list] Baffling typing problem
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I have a typing problem concerning polymorphic variants I 
can't figure out myself. The code is long and boring, so, if 
I can avoid it, I will not post it. I'll try to condense it 
to a more easily comprehensible form.

let make_data = function
   | "whoa" -> `WHOA(`Foo, [ `Frob; `Twid; `Knob ])
   | "whiz" -> `WHIZ(`Bar, [ `Twid; `Twid; `Frob; `Frob])
   | "bang" -> `BANG(`Foo, [ ], [ ] )

and data = make_data "whoa"

If I pass this to the toplevel I get no error message:
           Characters 16-169:
Warning: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
""
   ................function
     | "whoa" -> `WHOA(`Foo, [ `Frob; `Twid; `Knob ])
     | "whiz" -> `WHIZ(`Bar, [ `Twid; `Twid; `Frob; `Frob])
     | "bang" -> `BANG(`Foo, [ ], [ ] )
val make_data :
   string ->
   [> `BANG of [> `Foo] * 'a list * 'b list
    | `WHIZ of [> `Bar] * [> `Frob | `Twid] list
    | `WHOA of [> `Foo] * [> `Frob | `Knob | `Twid] list] =
   <fun>
val data :
   _[> `BANG of _[> `Foo] * '_a list * '_b list
     | `WHIZ of _[> `Bar] * _[> `Frob | `Twid] list
     | `WHOA of _[> `Foo] * _[> `Frob | `Knob | `Twid] list] =
   `WHOA (`Foo, [`Frob; `Twid; `Knob])

I attempt to compile it with ocamlc I get this error message:
File "prova.ml", line 1, characters 16-169:
Warning: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
""
File "prova.ml", line 6, characters 11-27:
The type of this expression,
_[> `BANG of _[> `Foo] * '_a list * '_b list
   | `WHIZ of _[> `Bar] * _[> `Frob | `Twid] list
   | `WHOA of _[> `Foo] * _[> `Frob | `Knob | `Twid] list],
contains type variables that cannot be generalized

Is there any way around this limitation?

Alex

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