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Date: Wed, 20 Aug 2008 12:21:02 +0900 (JST)
Message-Id: <20080820.122102.241911903.garrigue@math.nagoya-u.ac.jp>
To: carette@mcmaster.ca
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] Not a variant??
From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
In-Reply-To: <48AB4F06.4070208@mcmaster.ca>
References: <48AB4F06.4070208@mcmaster.ca>
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X-Spam: no; 0.00; o'caml's:01 bug:01 compile:01 caml-list:01 constraint:01 constraint:01 variant:02 variant:02 bounds:02 mcmaster:02 confusing:02 output:02 garrigue:03 garrigue:03 inheritance:03 

From: Jacques Carette <carette@mcmaster.ca>

> The function f1 below works fine -- so type d is seen to be a variant type.
> However, f2 does not compile because type c is *not* a variant type. 
> 
> However, if one looks at O'Caml's output for type c, it visually looks 
> like a variant, while that of d does not.  This is highly confusing.
> 
> This looks like a bug to me -- both c and d should be recognized as 
> variant types.
> 
> Jacques
> 
> type a = [`A ]
> type b = [`B ]
> type 'a c = 'a
>     constraint 'a = [< a | b]
>     ;;

The message is maybe not clear enough: for pattern-matching and
inheritance, you need an "exact" variant type, i.e. a type whose lower
and upper bounds are identical. This is clearly not the case for
c. The standard workaround is to write:

type c0 = [a | b]
type 'a c = 'a constraint 'a = [< c0]

Jacques Garrigue