Re: [Caml-list] illegal permutation of structure fields?

[Andreas Rossberg <rossberg@ps.uni-sb.de>]

Xavier Leroy wrote:
> 
> > file: bla.ml
> > ---------------------------------------------------------------------------
> > module type FOO = sig
> >   val x : int
> >   val y : int
> > end
> > ---------------------------------------------------------------------------
> >
> > file: bla.mli
> > ---------------------------------------------------------------------------
> > module type FOO = sig
> >   val y : int
> >   val x : int
> > end
> > ---------------------------------------------------------------------------
> >
> > This yields the following error:
> >   Illegal permutation of structure fields
> >
> > Why is this illegal? Wouldn't it be very straightforward to normalize
> > the signatures (e.g. sort elements by name) before matching them?
> 
> That's more or less what old versions of OCaml did, but it's
> incorrect.  The reason is that a module signature determine the layout
> of the corresponding structure: if
> 
>   module A : sig val x : int val y : int end
> 
> then A is represented as the tuple (value_of_x, value_of_y), while
> 
>   module B : sig val y : int val x : int end
> 
> is represented as the tuple (value_of_y, value_of_x).

Then the followup question of course is: isn't it trivial to use a
canonical layout instead, where the tuple components are sorted wrt. to
the corresponding fields' label? This way layout is invariant wrt
permutation in signatures. Or is there a particular problem with such a
scheme?

(BTW, the same holds for record and variant types, where Ocaml does not
allow reordering of fields/constructors either.)

Best regards,

	- Andreas Rossberg

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de

"Computer games don't affect kids.
 If Pac Man affected us as kids, we would all be running around in
 darkened rooms, munching pills, and listening to repetitive music."
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