Re: [MirageOS-devel] Blog post on Irmin/CueKeeper

[Thomas Leonard <talex5@xxxxxxxxx>]

On 23 June 2015 at 16:44, Daniel BÃnzli <daniel.buenzli@xxxxxxxxxxxx> wrote:
> Le mardi, 23 juin 2015 Ã 16:08, Thomas Leonard a Ãcrit :

>> I should probably replace most of the binds with this (being careful
>> that they don't return an existing signal, though).
>
> Yes that's the reason why I prefer weak references no need to ask yourself 
> such questions.
>
>> Can we define "A handler that isn't needed" as "A handler that would
>> be garbage collected if the GC ran now"?
>>
>> (or, if we had a glabal sink, "A handler that isn't a dependency of
>> the global sink")
>
> I'm not totally sure about what I'm going to write as I haven't got these 
> things in my head. But I'm wondering if we couldn't maybe replace the weak 
> refs by a reference counting mechanism (FRP is a dag of nodes, no cycle 
> problems) and perform strong stops on the node once the count reaches `0`. 
> The only thing that would be needed is a retain/release on the output nodes. 
> retain you already need to do it in some way (otherwise your node gets gc'd). 
> You'd still need to do releases but this would be hidden behind the 
> sinks-like setters.

Sounds reasonable. To check I understand, taking this example from the
React docs:

"It is important to keep references on effectful events and signals.
Otherwise they may be reclaimed by the garbage collector. The
following program prints only a 1.

let x, set_x = S.create 1
let () = ignore (S.map print_int x)
let () = Gc.full_major (); List.iter set_x [2; 2; 3]"

Under the ref-counting scheme, this program would always print
nothing, because the map signal's refcount is always zero, and it
therefore never connects to the x signal. It will be GC'd, but that
won't affect its behaviour, since it wasn't doing anything anyway.

With your Stdout module (or some other way to increment the ref
counter) we'd have:

let x, set_x = S.create 1
let () = Stdout.set (S.map string_of_int x)
let () = Gc.full_major (); List.iter set_x [2; 2; 3]"

This would always print all the values of x.

I guess the more complex case is S.bind, where the bind returning a
signal would increase its count (and decrease the count of the
previous signal), requiring signals to register and unregister
themselves during evaluation. It sounds like it could work, though.


-- 
Dr Thomas Leonard        http://roscidus.com/blog/
GPG: DA98 25AE CAD0 8975 7CDA  BD8E 0713 3F96 CA74 D8BA

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