Re: [Haskell-cafe] groupBy huh?
Jacek Generowicz schrieb: Hi Cafe, It seems that I don't understand what groupBy does. I expect it to group together elements as long as adjacent ones satisfy the predicate, so I would expect ALL four of the following to give one group of 3 and a group of 1. Prelude :m + Data.List Prelude Data.List groupBy () abcb [abcb] Prelude Data.List groupBy () abca [abc,a] Prelude Data.List groupBy () [1,2,3,2] [[1,2,3,2]] Prelude Data.List groupBy () [1,2,3,1] [[1,2,3],[1]] What am I missing? http://hackage.haskell.org/packages/archive/utility-ht/0.0.5.1/doc/html/Data-List-HT.html#v:groupBy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] groupBy huh?
Hi Cafe, It seems that I don't understand what groupBy does. I expect it to group together elements as long as adjacent ones satisfy the predicate, so I would expect ALL four of the following to give one group of 3 and a group of 1. Prelude :m + Data.List Prelude Data.List groupBy () abcb [abcb] Prelude Data.List groupBy () abca [abc,a] Prelude Data.List groupBy () [1,2,3,2] [[1,2,3,2]] Prelude Data.List groupBy () [1,2,3,1] [[1,2,3],[1]] What am I missing? Thanks. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] groupBy huh?
Excerpts from Jacek Generowicz's message of Fri Mar 04 00:18:07 + 2011: Prelude Data.List groupBy () [1,2,3,2] [[1,2,3,2]] This is wired. However if you think about the algorithm always using the first element of a list and comparing it against the next elements you get 1 2 ok, same group 1 3 dito 1 2 dito Thus you get [[1,2,3,2]] Marc Weber ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] groupBy huh?
On Friday 04 March 2011 01:18:07, Jacek Generowicz wrote: Hi Cafe, It seems that I don't understand what groupBy does. I expect it to group together elements as long as adjacent ones satisfy the predicate, so I would expect ALL four of the following to give one group of 3 and a group of 1. Prelude :m + Data.List Prelude Data.List groupBy () abcb [abcb] Prelude Data.List groupBy () abca [abc,a] Prelude Data.List groupBy () [1,2,3,2] [[1,2,3,2]] Prelude Data.List groupBy () [1,2,3,1] [[1,2,3],[1]] What am I missing? That groupBy expects an equivalence relation (iirc, that was documented some time, seems to be gone, there's only a hint left at the docs for group equality test). It tests subsequent elements against the first of the group, not adjacent elements. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] groupBy huh?
On 2011 Mar 4, at 01:39, Marc Weber wrote: Excerpts from Jacek Generowicz's message of Fri Mar 04 00:18:07 + 2011: Prelude Data.List groupBy () [1,2,3,2] [[1,2,3,2]] This is wired. However if you think about the algorithm always using the first element of a list and comparing it against the next elements you get 1 2 ok, same group 1 3 dito 1 2 dito Thus you get [[1,2,3,2]] OK, that works, but it seems like a strange choice ... On 2011 Mar 4, at 01:47, Daniel Fischer wrote: On Friday 04 March 2011 01:18:07, Jacek Generowicz wrote: What am I missing? That groupBy expects an equivalence relation ... Bingo! Now it makes sense. (iirc, that was documented some time, seems to be gone, there's only a hint left at the docs for group equality test). Hmph. In my opinion, explicitly including the words equivalence relation would immensely improve the documentation. Thank you for you clarifications, gentlemen. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] groupBy huh?
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 3/3/11 20:09 , Jacek Generowicz wrote: 1 2 ok, same group 1 3 dito 1 2 dito Thus you get [[1,2,3,2]] OK, that works, but it seems like a strange choice ... Stability is often valued in functions like this: the order of elements is not altered. - -- brandon s. allbery [linux,solaris,freebsd,perl]allber...@gmail.com system administrator [openafs,heimdal,too many hats]kf8nh -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.11 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk1wPPIACgkQIn7hlCsL25WF3ACfZrwM2OxutJZgadhaSCcpjoEv Bg4AnA+V/H3tfCovwwnw8qrlaw5I92C4 =WJev -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] groupBy huh?
Hi, Am Freitag, den 04.03.2011, 01:18 +0100 schrieb Jacek Generowicz: It seems that I don't understand what groupBy does. I expect it to group together elements as long as adjacent ones satisfy the predicate, so I would expect ALL four of the following to give one group of 3 and a group of 1. Prelude :m + Data.List Prelude Data.List groupBy () abcb [abcb] Prelude Data.List groupBy () abca [abc,a] Prelude Data.List groupBy () [1,2,3,2] [[1,2,3,2]] Prelude Data.List groupBy () [1,2,3,1] [[1,2,3],[1]] What am I missing? this comes up repeatedly. Also see http://hackage.haskell.org/trac/ghc/ticket/1408 Greetings, Joachim -- Joachim nomeata Breitner mail: m...@joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nome...@joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nome...@debian.org signature.asc Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] groupBy huh?
On 3/3/11 7:18 PM, Jacek Generowicz wrote: Hi Cafe, It seems that I don't understand what groupBy does. I expect it to group together elements as long as adjacent ones satisfy the predicate, so I would expect ALL four of the following to give one group of 3 and a group of 1. Prelude :m + Data.List Prelude Data.List groupBy () abcb [abcb] Prelude Data.List groupBy () abca [abc,a] Prelude Data.List groupBy () [1,2,3,2] [[1,2,3,2]] Prelude Data.List groupBy () [1,2,3,1] [[1,2,3],[1]] What am I missing? The behavior is that it's comparing subsequent elements to the first element of the current chunk. I'm not sure how often that'd be a desirable behavior compared to the one you and I would expect. Of course, the API only specifies the behavior of groupBy on equality-like predicates IIRC. So technically either behavior is permissible... This should be FAQed on the documentation a bit better. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] groupBy huh?
On 3/3/11 8:14 PM, Brandon S Allbery KF8NH wrote: -BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 3/3/11 20:09 , Jacek Generowicz wrote: 1 2 ok, same group 1 3 dito 1 2 dito Thus you get [[1,2,3,2]] OK, that works, but it seems like a strange choice ... Stability is often valued in functions like this: the order of elements is not altered. Making it stable and also comparing adjacent elements is entirely doable. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] groupBy huh?
On 2011 Mar 4, at 02:14, Brandon S Allbery KF8NH wrote: -BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 3/3/11 20:09 , Jacek Generowicz wrote: 1 2 ok, same group 1 3 dito 1 2 dito Thus you get [[1,2,3,2]] OK, that works, but it seems like a strange choice ... Stability is often valued in functions like this: the order of elements is not altered. I'm failing to see how the behaviour I expected would change the order of elements: the elements would still come out in exactly the same order, it is just the boundaries between the groups would be in different places. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
