#ocaml on 2017-07-15 — irc logs at freenode.irclog.whitequark.org

2017-07-13 10:09 companion_cube changed the topic of #ocaml to: Discussions about the OCaml programming language | http://www.ocaml.org | OCaml 4.05.0 release notes: https://caml.inria.fr/pub/distrib/ocaml-4.05/notes/Changes | Try OCaml in your browser: http://try.ocamlpro.com | Public channel logs at http://irclog.whitequark.org/ocaml

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00:02 <SomeDamnBody> I have a graph that I expect to be a tree, but that may not be. I want to detect links between nodes at any point.

00:02 <SomeDamnBody> How might I build an algorithm that does this?

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00:06 <Drup> SomeDamnBody: You want to know if there is a path between two nodes ?

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00:06 <SomeDamnBody> Well, a cross edge I think is the right term.

00:06 <SomeDamnBody> I want to detect the cross edge.

00:06 <Drup> ah, you want to detect if a graph is a DAG or a tree

00:07 <Drup> (or cyclic)

00:07 <SomeDamnBody> I'm not concerned if there is cycles from childs to parent. But it must be a tree.

00:07 <SomeDamnBody> I'm using ocamlgraph. It would be fantastic if I could use some library function to get this for free. lol

00:08 <Drup> yeah, the algorithm is in ocamlgraph

00:08 <SomeDamnBody> So, if A -> B, B -> C, and A -> C, and I start from A, how can I get the edge B -> C returned?

00:08 <SomeDamnBody> what's the algorithm named?

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00:10 <Drup> I don't remember, give me a sec

00:10 <Drup> well, I mean, you could just look for cycles in the undirected version of the graph, that's equivalent

00:13 <Drup> I'm not sure it has a name, really, just doing a BFS will give you what you need.

00:15 <SomeDamnBody> Well, I need to detect them. So, and bfs may allow me to skip cross edges, but I have to be able to remove them.

00:16 <SomeDamnBody> There are other cycles though, as in from parent to leaf. So, if I look for cycles, then I might end up with more than I meant, because if I remove the cycles that are from parent to child, then this could cause a problem, since there are those that should change.

00:19 <Drup> https://en.wikipedia.org/wiki/Depth-first_search#Output_of_a_depth-first_search

00:19 <Drup> http://ocamlgraph.lri.fr/doc/Kruskal.html

00:19 <Drup> http://ocamlgraph.lri.fr/doc/Traverse.Dfs.html

00:20 <Drup> You are not very clear about what you want precisely enough, but I believe this should give you what you want

00:22 <SomeDamnBody> Well a minimum spanning tree may end up giving me something like a cross edge remaining, but some other edge dropped...

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00:23 <SomeDamnBody> Like, for example: I have A -> B, A -> C and that's the acyclic good part.

00:23 <SomeDamnBody> It may be that B -> A

00:23 <SomeDamnBody> That's fine. In fact, I want to preserve that. But it is not ok if B -> C

00:23 <Drup> No, a minimum spanning tree gives a tree. By definition, a tree does not have cross edges

00:23 <SomeDamnBody> Oh ok.

00:24 <Drup> I believe you are confusing trees and DAGs

00:24 <SomeDamnBody> Oh! You're right!

00:24 <SomeDamnBody> That's great.

00:25 <Drup> If you want to extract a DAG out of a graph, you can run a DFS and kill all back edges

00:26 <SomeDamnBody> Kruskal's output may give me a minimum spanning tree, but not tell me which edge was dropped. As in, I still need an edge list at the end, where I can get B -> C, the violator

00:26 <Drup> Well, I mean, that's not difficult, you have two sets of edges, you take the diff ...

00:27 <SomeDamnBody> I was thinking about that, but the back edge would get reported.

00:27 <Drup> (or, as I said, run a DFS)

00:27 <SomeDamnBody> From B -> A

00:28 <SomeDamnBody> I'm trying to build that now with a DFS. I'm just working on it a bit, because the graph is a bit fuzzy in it's structure. I have to be really careful to protect true positives and only remove false positives when I am really sure

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00:44 <Drup> (Since I got curious, the problem of finding a minimum/maximal spanning DAG is called https://en.wikipedia.org/wiki/Feedback_arc_set )

00:45 <Drup> (One thing that is particularly not enjoyable about graph theory is their completely inconsistent naming of problems)

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07:32 <rgrinberg> Drup: what's the situation like with archlinux + opam? does the solver work by default?

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08:11 <adrien> rgrinberg: afaiu, it does now

08:11 <adrien> but it's only been a week (like since friday 8 days ago)

08:22 <rgrinberg> adrien: ok great. finally...

08:23 <adrien> well, try by yourself and report: I'm not an arch user myself, I only have some around me

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09:10 <orbifx[m]> rgrinberg: I've not noticed anything on my arch

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15:00 <spacebat_> I'm trying to get started with batteries, I've used opam to switch to ocaml 4.04.2, opam install batteries seemed to work fine

15:00 <spacebat_> ocamlfind batteries/ocaml gives me a repl with Batteries

15:02 <spacebat_> but open Batteries doesn't work in a simple .ml file - ocamlopt says Error: Unbound module Batteries

15:02 <spacebat_> Same thing with Batteries_uni

15:02 <spacebat_> I've tried putting #require "batteries" at the top of the file but that doesn't work either

15:05 <spacebat_> what am I missing - how do I make an installed library available for use?

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15:07 <octachron> spacebat_, for compiling a file, a first step would be: "ocamlfind ocamlopt -package batteries a.ml"

15:09 <octachron> a second step would be to use a build system, "ocamlbuild" works for small project (completed by oasis for bigger ones); similarly, "jbuilder" is becoming quite popular

15:10 <octachron> (and there are other options available)

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15:11 <spacebat_> ok, I now get Error: No implementations provided for the following modules: Batteries referenced from wat.cmx

15:14 <def`> t

15:14 <spacebat_> I see that a builder would make sense, but I'm trying to understand the most basic steps right now

15:15 <spacebat_> one I can actually compile something by making the library available to the compiler, the next challenge is to make merlin see it also

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15:16 <spacebat_> because neither can see anything that I install

15:16 <octachron> spacebat_, for merlin, you need to add "PKG library1_name,library2_name"

15:17 <spacebat_> wat.ml is in $HOME and the content of ~/.merlin is one line PKG batteries

15:18 <spacebat_> I've restarted merlin but it can't resolve Batteries

15:19 <spacebat_> so it feels like I have something deeply or fundamentally wrong because none of the advice I've found seems to work

15:19 <octachron> spacebat_, for the compilation step, you need "-linkpkg"

15:19 <def`> spacebat_: what is your editor, os, and what tells you it can't resolve Batteries?

15:19 <spacebat_> I'm trying this with opam installed via homebrew on OSX and installed from the opam website for debian

15:20 <def`> (focusing on merlin part :P)

15:22 <spacebat_> oh, now merlin is working, not sure what I changed, but that's a relief

15:23 <spacebat_> it was highlighting anything do to with Batteries in red

15:23 <def`> ah good :)

15:24 <spacebat_> I just tried adding -linkpkg to the command line and it was successful but I don't know what it did

15:25 <spacebat_> what would be the output of a successful invokation of ocamlfind ocamlopt -package batteries -linkpkg wat.ml

15:25 <spacebat_> I'm looking for an executable that I can run

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15:29 <octachron> probably "a.out" aka the default executable name

15:30 <octachron> you may want to precise the output name with "-o executable_name"

15:30 <spacebat_> ah true, didn't expect that c-like behaviour

15:31 <spacebat_> thanks so much, I've got it working from the command line and have merlin support working in emacs

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15:31 <octachron> spacebat_, note also if your aim is to understand OCaml compilation process, it would be simpler to try first to compile a set of files with no external dependencies

15:32 <octachron> like this you would only have to understand ocamlopt behavior not ocamlopt + ocamlfind (ocamlfind is mostly useful here to locate the right files for external dependencies)

15:33 <spacebat_> one more thing - a few months back I tried accessing environment variables in an ocaml repl and distinctly remember the function returning an option type

15:33 <spacebat_> but batteries and core seem to throw an exception if the variable is not found in the environment

15:35 <spacebat_> I guess I'm asking which is more idiomatic and which you'd prefer

15:36 <spacebat_> exceptions strike me as going against the grain of the type support in ocaml but of course there's a lot I don't know

15:36 <octachron> spacebat_, this sounds strange for core, generally only "exn" suffixed functions throw an exception

15:37 <octachron> otherwise, I would say that for accessing environment, an option sounds better, because the existence of the variable is uncertain

15:37 <spacebat_> ah, my mistake again - Core.Sys.getenv is string -> string option

15:38 <spacebat_> and you're right, there's an _exn alternative

15:38 <octachron> i.e exception for exceptional behaviors (or careful control flow sometimes), and option in uncertain times

15:39 <spacebat_> thanks again, this is clearer now

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15:50 <lostman> is it possible to use jbuilder to build a library that binds to some c++ code? need to build the c++ code (including a c-style wrapper using extern c) and then bindings in ocaml

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16:31 <rgrinberg> lostman: it should be possible. but i'm not sure how well it's documented. If you want an example, check out jst's re2 bindings i suppose

16:36 <lostman> rgrinberg: thanks. can I make it work with ctypes? I've been trying to figure it out today but didn't get too far

16:38 <rgrinberg> The answer is similar. It should be possible but not pretty or well documented

16:39 <rgrinberg> Study async_ssl to see how it's done

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22:48 <LACampbell> hello. For the second time now, I've been able to `opam search` a package, but when I opam install a package it tells me "your request can't be satisfied: - No package matches $name". is there anything I should check?

22:52 <LACampbell> I've done opam update and opam remote. I just switched to 4.05 recently

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22:53 <dxtr> How would I write my match case to get a variable for both the items in a tuple and the tuple itself?

22:53 <dxtr> Said tuple is in a list too

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22:56 <LACampbell> dxtr: why not just nest matches?

22:56 <octachron> dxtr, "match l with (x,y as tuple) :: q -> …"

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22:57 <octachron> LACampbell, you could check "opam info $name"

22:58 <LACampbell> octachron: and install the deps manually? worth a shot

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23:00 <LACampbell> oh look at that, I can't install ppx_deriving - but it gives me a reason

23:01 <octachron> LACampbell, ah yes, ppx_deriving is not yet available with 4.05

23:04 <LACampbell> well that solves that mystery.

23:04 <LACampbell> thanks octachron

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