Re: [Caml-list] Behaviour of an OCaml program: any possible GC improvement?

[David House <dhouse@janestreet.com>]

On Thu, Oct 25, 2012 at 7:48 AM, David MENTRE <dmentre@linux-france.org> wrote:
>> Some improvement is also
>> possible with simple tricks, e.g. if you have something like
>>
>> Number 0
>>
>> where Number is from an algebraic type, and you use this term frequently,
>> consider to add a constant for it,
>>
>> let n0 = Number 0
>>
>> so this isn't reallocated all the time
>
> Thanks for the trick.

As Martin pointed out, this isn't necessary for Number 0. Constant
expressions are statically allocated. If you have two functions
returning Number 0, you'll get two of them, but they'll still be
statically allocated (so calling that function does not allocate for
Number).

This is actually even true of string literals, which are mutable:

# let f () = "foo";;
val f : unit -> string = <fun>
# let x = f ();;
val x : string = "foo"
# let y = f ();;
val y : string = "foo"
# x.[0] <- 'g';;
- : unit = ()
# y;;
- : string = "goo"

But other, similar tricks are worthwhile. For example, in async, it is
very common to write "return ()" for the deferred that is immediately
determined with value (). But doing so does a function call and
allocates a deferred. So we have [Deferred.unit] for this common case.