Re: [Caml-list] Guards vs. conditionals

[Sven Luther <luther@dpt-info.u-strasbg.fr>]

On Thu, Dec 26, 2002 at 12:06:52AM -0700, Matt Gushee wrote:
> Hi, all--
> 
> I'm just curious about something. I've been noticing that in many cases
> (if not always) a pattern match with guard expressions in OCaml is
> equivalent to a conditional statement. E.g.:
> 
>   let foo =
>     function
>     | i when i >= 500 -> true
>     | i -> false
>     
> does the same thing as
> 
>   let foo i =
>     if i >= 500 then true
>     else false
> 
> And I wrote a pair of small programs, each of which invokes one of these
> functions on a random integer 100,000 times. The executables, whether
> byte-compiled or native, are almost exactly the same size and, according
> to a round of informal tests with GNU time, run at the same speed. This
> suggests that the compiled code is for all practical purposes the same.
> 

Yes, i guess the exact same code it generated for both.

> So, my question is, is there any objective reason to prefer the
> pattern-match version over the conditional, or vice versa? Or is it just
> a matter of coding style?

It is just a matter of coding style. I think that the if version is
maybe easier to do prooves on or something such, and that guard version
is easier to read and maybe better when there are more than one
condition, but the compiler does not know when the guards cover all the
cases, and may output a warning when non is needed :

consider :

let foo = function
| i when i > 500 -> 1
| i when i = 500 -> 2
| i when i < 500 -> 3

Which will output a warning.

Friendly,

Sven Luther
>         
> -- 
> Matt Gushee                 When a nation follows the Way,
> Englewood, Colorado, USA    Horses bear manure through
> mgushee@havenrock.com           its fields;
> http://www.havenrock.com/   When a nation ignores the Way,
>                             Horses bear soldiers through
>                                 its streets.
>                                 
>                             --Lao Tzu (Peter Merel, trans.)
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