#ocaml on 2016-05-15 — irc logs at freenode.irclog.whitequark.org

2016-04-27 14:02 flux changed the topic of #ocaml to: Discussions about the OCaml programming language | http://www.ocaml.org | OCaml MOOC http://1149.fr/ocaml-mooc | OCaml 4.03.0 announced http://ocaml.org/releases/4.03.html | Try OCaml in your browser: http://try.ocamlpro.com | Public channel logs at http://irclog.whitequark.org/ocaml

00:00 <Algebr> okay why oh why doesn't stdlib's Array come with filter

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00:02 <pierpa> because it would be a very wasteful function, I guess

00:02 <mrvn> Algebr: because you don't know how long each array is.

00:04 <Algebr> mrvn: how and where is this relevant

00:04 <mrvn> Algebr: you have to allocate the result arrays with the right size. you can't efficiently append to it as you filter.

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00:06 <mrvn> "filter : ('a -> bool) -> 'a array -> 'a list * 'a list" might make sense but not "-> 'a array * 'a array"

00:06 <Algebr> ahhh

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00:16 <pierpa> here's an efficient Array.filter: https://bpaste.net/show/795fc7864444

00:16 <pierpa> only, is a bit non tail recursive :)

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00:26 <pierpa> and this works for any array: https://bpaste.net/show/95f071a977ec

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00:36 <mrvn> ahh right, I was thinking partition, not filter.

00:36 <mrvn> same reason though

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00:37 <pierpa> partition can be done in the same way

00:38 <mrvn> yeah, just needs 2 counts

00:38 <mrvn> or count and len-count

00:38 <pierpa> yes, can be tuned in several way

00:38 <mrvn> you should make 2 passes over the array to make it tail recursive

00:39 <pierpa> and call the function 2 times for each element. No way! :D

00:40 <mrvn> pierpa: no, first pass call f a.(i), store the result in an array and count the trues. Second pass copy the trues in one array (and the false in a second for partition).

00:41 <pierpa> storing the result in an array kind of defeats the whole machinery, no?

00:41 <mrvn> pierpa: only way to make it tail recursive without calling f twice I think

00:42 <pierpa> well, yes. In the end the most practical way is to build a list

00:42 <pierpa> and maybe convert to an array if one really really really wants an array in the end

00:46 <pierpa> (This is just for the sake of a fun exercise. If one has fun with this kind of things :)

00:48 <pierpa> then if there could be a way to not use Array.make count a.(0), which I don't like, that would be super

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01:07 <Algebr> okay lwt...lwt(timeout): uncaught exception after timeout:

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05:30 <Bahman> Greetings.

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05:35 <Algebr> hi

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10:59 <companion_cube> Algebr: of course there is CCArray.filter ;)

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16:54 <cyc> Hi, is there recently (like with 4.03) a change in the behavior of linking with C code?

16:54 <cyc> I have a program that refuse to compile, and it failed to find the module implemented by my C code

16:55 <Algebr> like linking?

16:55 <cyc> Algebr, exactly

16:58 <Algebr> Is there a small example?

16:59 <cyc> wait a minute, I will provide

16:59 <adrien> which architecture? what are the compialtion calls and linking calls?

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17:04 <cyc> adrien, linux 64bits (arch)

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17:05 <flux> cyc, do you have an .mli file that defines the externals without a matching .ml file?

17:05 <cyc> flux, yes

17:05 <flux> I believe this case is then different with 4.03

17:05 <cyc> http://pastebin.com/5G6dwzwB

17:05 <cyc> Here my small example. I can't compile it.

17:06 <cyc> It fails to find module Test

17:07 <cyc> It builds with old 4.02.3

17:07 <flux> I'm not sure how it is fixed, though. perhaps you need to replicate the definitions in the .ml

17:07 <flux> (I mean I don't know if there's a shorter solution)

17:07 <cyc> flux, you mean declare them as external in a new .ml and not just in .mli?

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17:08 <flux> well, if you don't have anything else in the .mli why not just rename it to .ml

17:08 <flux> but otherwise an .ml must provide whatever the .mli provides

17:08 <adrien> or symlink

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17:08 <adrien> (although that's bad for sources)

17:08 <flux> .ml and .mli format are not the same in general..

17:08 <adrien> yeah, that only works in limited cases :)

17:08 <flux> alternatively you can just provide only the signature (val foo : int -> int) in the .mli and then the external part in the .ml file for implementation

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17:09 <Algebr> I read there is a small runtime penalty for that but makes the abstraction cleaner.

17:09 <cyc> flux, i try to create a .ml with the external definitions, and just the val declaration .mli

17:09 <cyc> sorry, it works

17:09 <cyc> thx you

17:09 <cyc> But why does it change?

17:10 <flux> I imagine it's a by-product of some other change, not something that was explicitly planned..

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17:10 <flux> and I do wonder if the old functionality will ever be brought back

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20:56 <Omerta> hi, does anyone know how to use deriving show with polymorphic types?

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21:58 <Typs> Hello guys

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22:01 <Typs> I have a little question, I want to create a function : unit -> ('a,'b)list

22:02 <Typs> but I don't know how to make the ('a,'b)list

22:02 <Typs> can someone help me ?

22:03 <Drup> This is a homework ? :)

22:03 <Typs> yep haha

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22:03 <mrvn> There is only one ('a, 'b) list you can make out of thin air: The empty list.

22:05 <Typs> ok but it will just be a 'a list no ?

22:05 <mrvn> which can be unified with ('b, 'c) list

22:06 <Typs> Ok I think I see the thing thanks :)

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22:10 <mrvn> you can annotate the type if you must or write an mli file

22:10 <Typs> yeah I've got type('a,'b)dict = ('a *'b)list

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22:11 <Typs> and I'm trying to write let new_dict () =

22:12 <mrvn> There is no trying, only doing.

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