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From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
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Date: Fri, 7 Dec 2012 11:45:07 +0900
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To: Francois Berenger <berenger@riken.jp>
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Subject: Re: [Caml-list] What is the fastest? Pattern matching or if then
 else

On 2012/12/07, at 11:33, Francois Berenger <berenger@riken.jp> wrote:

> Hello,
> 
> I always wondered what is the fastest (at least for integers):
> 
> match n with
>  0 -> (* do something *)
> | _ -> (* do something else *)
> 
> or
> 
> if n = 0 then
>  (* do something *)
> else
>  (* do something else *)
> 
> Sometimes I have some code that would be more beautiful
> if I was using only pattern matching rather than
> pattern matching interspersed with if then else directives.
> 
> Is one significantly faster than the other?
> 
> Is it the same cost at runtime?

$ ocaml -dlambda
        OCaml version 4.00.1

# fun n -> match n with 0 -> "this" | _ -> "that";;
(function n/1016 (if (!= n/1016 0) "that" "this"))
- : int -> string = <fun>
# fun n -> if n then "this" else "that";;
(function n/1017 (if n/1017 "this" "that"))
- : bool -> string = <fun>

In most cases, the two are going to be exactly equivalent.
Here there is a slight difference because of the handling of the constant 0.
But I would expect that to be optimized out in the native code compiler.
So the short answer is: do not let your sense of beauty be disturbed by that.

	Jacques