Re: [Caml-list] Comparing variant types

[Dmitry Bely <dmitry.bely@gmail.com>]

On Fri, Apr 29, 2011 at 1:33 PM,  <luc.maranget@inria.fr> wrote:
>> > (* performs a simple comparison *)
>> > let f a = a <> Right
>> >
>> > (* calls out to C to do compare_val *)
>> > let g (a:dir) b = a <> b
>> >
> ...
>>
>> let g (a:dir) b = a != b
>>
>> - Dmitry Bely
>>
>
> As a general rule, don't do that! :)  (using <> or !=
> for writing  'g').
>
>
> For <>, you'll get hurt by data types with non-unique representation
> (such as Set), as already pointed out.
> It is ok to use structural equality when it is not the case, but
> your programm is not as robust as you may want it to be.
>
> For !=, it is much worse, as soon as you add a non-constant
> constructor to your data type, your code is wrong
> (cf. [1] != [1])
>
> If you aim at robust  code. A recommended (tiresome) alternative
> is to write your own equality function once for all, in
> the following style.
>
> type dir = Left | Right | Up | Down | No_op
>
> let dir_equal d1 d2 = match d1,d2 with
> | (Left, Left)
> | (Right,Right)
> | (Up, Up)
> | (Down,Down)
> | (No_op,No_op)
>  -> true
> | (Left,(Right|Up|Down|No_op))
> | (Right,(Left|Up|Down|No_op))
> | (Up,(Left|Right|Down|No_op))
> | (Down,(Left|Right|Up|No_op))
> | (No_op,(Down|Up|Right|Left))
> -> false

Yes, but Ethan's primary concern was efficiency. You dir_equal would
hardly fit (I'm afraid it's even more slow than <> because it's
translated into switch/case block). But generally speaking, of course
robustness is always the first priority.

- Dmitry Bely