Re: [Caml-list] Typechecking madness

[Mikael Brockman <phubuh@phubuh.org>]

Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp> writes:

> From: Alex Baretta <alex@baretta.com>
> > I'm sure the typechecker knows what he's doing, but the following error 
> > message does look strange.
> > 
> > This expression has type
> >    'a. (('a Pxp_document.node #Pxp_document.extension as 'a) -> unit) -> 
> > unit
> > but is here used with type
> >    'b. (('b Pxp_document.node #Pxp_document.extension as 'b) -> unit) -> 
> > unit
> > 
> > Is this due to the two '#' signs on Pxp_document.extension, yielding two 
> > different ellipsis type variables?
> 
> Indeed, this is a strange error message.
> As far as I can see, it really says "This expression has type t but is
> here used with type t". 'a and 'b are bound variables, so the ellipsis
> is not the problem.
> Even if the type checking is correct in detecting a type error, this
> is not a valid error message.

Actually, the first expert FAQ [1] addresses a (valid) such error
message.  Briefly, this kind of error occurs when you have defined two
types with the same name, which is easy to do when you're using the
REPL.

> Can you send me a sufficiently small program exhibiting this
> behaviour?
> 
>         Jacques Garrigue
> 
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Footnotes: 
[1] http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#message_incomprehensible

(Sorry, Jacques, for sending you this message once in private.  I keep
expecting R in Gnus to reply to the list. :-))

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