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[Caml-list] Unbound type variables
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Date: -- (:)
From: Olivier Andrieu <andrieu@i...>
Subject: Re: [Caml-list] Unbound type variables
 Patrick M Doane [Thursday 13 September 2001] :
 >
 > While writing code today, I came across an annoying problem with using
 > polymorphic variants and objects. The basic code fragment is this:
 > 
 > class c = object
 >   method m x =
 >     match x with
 >       | `A -> ()
 >       | `B -> ()
 >       | `C -> ()
 > end
 > 
 > which produces an error:
 > 
 > The method m has type
 >   [< `A | `B | `C] -> unit
 > where .. is unbound

 > ... there isn't anyway I'm aware of to be able to
 > bind that row variable to the type of the class.  

yes you can (if I understand you correctly) :

 class ['a] c = object
   method m : 'a -> unit = function
       | `A -> ()
       | `B -> ()
       | `C -> ()
 end

and you get

 class ['a] c : 
   object
    constraint 'a = [< `A | `B | `C] 
    method m : 'a -> unit 
   end

-- 
   Olivier
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