Re: [Caml-list] Magic with fun (type t) ... ?

[Alain Frisch <alain@frisch.fr>]

On 1/26/2011 11:33 PM, David Allsopp wrote:
>> # f ();;
>> - : exn = E 0
>> # f None;;
>> - : exn = E 0

>
> # f () = f None;;
> - : bool = true


There are two distinct problems here, neither of which has been 
introduced in 3.12.

First, the toplevel uses types to display exception arguments only when 
it knows exactly their type. Otherwise, there is a fallback heuristic to 
look at runtime tags.

# module X : sig val x : exn end = struct exception E of int option let 
x = E None end;;
module X : sig val x : exn end
# X.x;;
- : exn = X.E 0


Second, exception values values are compared structurally; this means 
that the constructors are compared textually and when they match, the 
arguments are compared even if they don't have the same type (yes, this 
can probably yield to segfaults).

# exception E of unit;;
exception E of unit
# let e1 = E ();;
val e1 : exn = E ()
# exception E of int option;;
exception E of int option
# let e2 = E None;;
val e2 : exn = E None
# e1 = e1;;
- : bool = true


Of course, pattern matching on exceptions uses the true equality between 
constructors (with a generative semantics). This is only a problem with 
the generic comparison functions.


Cf:
http://caml.inria.fr/mantis/view.php?id=1248
http://caml.inria.fr/mantis/view.php?id=4531
http://caml.inria.fr/mantis/view.php?id=4765
http://caml.inria.fr/mantis/view.php?id=5087



Alain