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Date: Tue, 01 Apr 2008 11:42:34 +0900 (JST)
Message-Id: <20080401.114234.146324100.garrigue@math.nagoya-u.ac.jp>
To: rathereasy@gmail.com
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] type error
From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
In-Reply-To: <f74178430803311927u3c1cbd2at8d30e6a85fb42d10@mail.gmail.com>
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X-Spam: no; 0.00; abbreviation:01 unifying:01 exception:01 caml-list:01 inherit:01 coerce:02 coerce:02 defined:02 defined:02 covariant:02 garrigue:03 garrigue:03 jacques:03 jacques:03 nagoya-u:04 

From: "Jacques Le Normand" <rathereasy@gmail.com>
> thanks for all the help so far; it's been very educational
> there's a type error I can't get my head around:
> 
> 
> class a =
> object
> end
> 
> and b =
> object
>   inherit a
>   method d (e : b) = (e :> a)
> end
> 
> 
> gives the error:
> 
> The abbreviation b expands to type < d : b -> a > but is used with type <  >
> 
> why is this?

You cannot coerce to a type that is not yet (fully) defined.
In practice, what happens is that (e :> a) is just interpreted as
(e : a), so that you are unifying a and b, with the above error.

As an exception to the above rule, you can coerce self toward the
currently defined class (i.e. coerce self to a inside a), but only if
self is covariant in itself. Here you coerce b to a inside b, so this
doesn't apply.

Jacques Garrigue