Re: [Caml-list] Equality of functional values

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am Montag, den 29.01.2007, 13:04 -0800 schrieb Simon Frost:
> Dear Caml List,
> 
> I'm trying to use a software package written in ocaml (IBAL), however,
> it fails a test due to 'Fatal error: exception Invalid_argument("equal:
> functional value"). It seems that equality isn't defined for functional
> values in OCAML (although I'm not an expert), so I'm wondering what the
> workaround is. This apparently worked fine in ocaml 3.04, but not in
> later versions. I'm using ocaml 3.08.3, and I get this error message
> both on Linux (SUSE 9.1 Profession, x86_64) and Windows XP (x86). Any
> help would be greatly appreciated! I'd rather not have multiple versions
> of ocaml floating around...

As far as I remember there was a slight change for the equality around
3.07 or 3.08. Previous versions of O'Caml always tested for physical
equality first, and only if the two values are not the same they are
compared component by component. So if the two compared functions are
always the same, this equality test never failed. In current O'Caml,
such a physical test is not done. E.g. look at

# let f() = 42;;
val f : unit -> int = <fun>
# f = f;;
Exception: Invalid_argument "equal: functional value".
# f == f;;
- : bool = true

In old O'Caml versions "f = f" worked because the "f == f" test was
implicitly performed first. (This was changed because of an
incompatibility with conventional floating point semantics.)

If it is not possible to remove the functional parts from the compared
values entirely, there is an easy workaround: Wrap the functions into
objects. The equality for objects is well-defined: Objects are only
equal if they are the same.

The code looks now like:

# let obj_f = object method f() = 42 end;;
val obj_f : < f : unit -> int > = <obj>
# obj_f = obj_f;;
- : bool = true

In order to call the wrapped function:

# obj_f # f();;
- : int = 42

Gerd
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