[Caml-list] Typechecking madness

[Caml-list] Typechecking madness

[Alex Baretta]

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?

Alex

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Re: [Caml-list] Typechecking madness

[Jacques Garrigue]

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.
Can you send me a sufficiently small program exhibiting this
behaviour?

        Jacques Garrigue

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Re: [Caml-list] Typechecking madness

[Mikael Brockman]

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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Re: [Caml-list] Typechecking madness

[Alex Baretta]

Mikael Brockman wrote:
> Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp> writes:
> 
> 
>>From: Alex Baretta <alex@baretta.com>

>>>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

>>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.
> 

Allow me to point out that I was not using the toplevel (REPL, as you 
call it); rather I was compiling a PXP document transformation with the 
toplevel. There actually was a type error in my code, and I did manage 
to figure it out. The error message did not really help, though.

Alex

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