#ocaml on 2022-11-14 — irc logs at libera.irclog.whitequark.org

2022-03-30 13:11 companion_cube changed the topic of #ocaml to: Discussion about the OCaml programming language | http://www.ocaml.org | OCaml 4.14.0 released: https://ocaml.org/releases/4.14.0.html | Try OCaml in your browser: https://try.ocamlpro.com | Public channel logs at https://libera.irclog.whitequark.org/ocaml/

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10:05 <neiluj> Hello! How do you get a pointer to the i-th array for an array of arrays? because matrix.(i) gives the i-th row, not a pointer to the rows starting from index i

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10:30 <neiluj> hmm, going to use bigstrings

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11:48 <Fardale> neiluj: there is not really pointer in OCaml. What do you want to do with such pointer?

12:07 <neiluj> just to share the reading of a buffer to some processes, hence they need a pointer to a given segment of the buffer

12:07 <neiluj> bu now I'm switching to bigarrays

12:08 <neiluj> speaking of which, what is the representation of the type 'b supposed to be in https://v2.ocaml.org/api/Bigarray.Genarray.html?

12:09 <neiluj> what is the difference between type 'a and 'b? is it possible to assign to 'a an abstract type? maybe not for 'b?

12:10 <neiluj> it also seems that bigstringaf does some operations faster like blit, also exposes memcmp... but works only for 1D byte arrays, which can't be casted to 'a elements without using Obj.magic

12:12 <sim642> You can see the point of 'b here: https://v2.ocaml.org/api/Bigarray.html#TYPEkind

12:12 <sim642> Basically, multiple low-level types can map to the same Ocaml type

12:15 <neiluj> thanks! so you can only choose between the types that are already provided by the GADT?

12:15 <neiluj> like Char,Int,...

12:16 <neiluj> for instance what if 'a has a representation that require N chars?

12:16 <neiluj> will 'b=Char suffice?

12:18 <sim642> You can't actually arbitrarily choose them

12:18 <neiluj> "Bigarrays can only contain integers and floating-point numbers" so no you're limited to these types

12:20 <neiluj> okay then I'll select byte arrays and add some C bindings for the getter and setter

12:21 <neiluj> is it really worth it to use multidimensional arrays provided by Bigarray instead of handling it yourself?

12:21 <neiluj> seems easy enough, wondering about possible performance gains (wrt. cache usage)

12:22 <sim642> Pretty sure you'd just be duplicating functionality that's already there

12:24 <neiluj> yeah, let's not reinvent the wheel!

12:24 <neiluj> thanks again :)

12:39 <neiluj> no need for C stubs, you can get the bytes that you need with the bigarray .{} syntax, then collect them and get your bytes :)

12:40 <neiluj> may be way less efficient than a memcpy with the right offset though

12:41 <sim642> How would memcpy help though? You'd still need to convert the N bytes to your whatever OCaml type 'a

12:42 <neiluj> no the representation of 'a is just a sequence of N bytes

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12:48 <sim642> I think you can just take 1D slices out of a 2D array then

12:49 <sim642> it might not even copy anything, just remember the offset pointer internally

12:51 <neiluj> exactly

12:52 <neiluj> but then i'd have to get each byte of the slice, and concatenate them

12:52 <sim642> Why? The slice already directly represents those bytes

12:53 <neiluj> oh then I misunderstood. What is a slice? is it an ocaml type?

12:53 <sim642> I just mean Bigarray.Array2.slice_left

12:53 <sim642> That's just an Array1

12:55 <neiluj> oh didn't know that! but is there a way to cast it to bytes without an extra allocation?

12:56 <neiluj> that's what I meant by collecting the bytes of the slice to form a byte sequence Bytes.t

12:57 <sim642> That probably requires an inevitable copy indeed

13:00 <neiluj> but yeah, slice_left is neat! to bad you have to get each element of a slice in this case...

13:00 <sim642> You might be able to use the bigstringaf package though

13:01 <sim642> It seems to provide some low-level C implementations for char bigarray and string/bytes conversions

13:01 <neiluj> good point! indeed there is blit_to_bytes

13:01 <neiluj> happy customer :)

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13:25 <olle> s

13:25 <olle> oops

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