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Date: Fri, 29 Apr 2011 11:33:55 +0200
To: Dmitry Bely <dmitry.bely@gmail.com>
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User-Agent: Mutt/1.5.18 (2008-05-17)
Subject: Re: [Caml-list] Comparing variant types

> > (* 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



let g a b = not (dir_equal a b)

Notice that 'let f a = a != Right' and 'let f a = a <> Right' are
less dangerous, but well...


--Luc