[Haskell-cafe] groupBy huh?
jacek.generowicz at cern.ch
Fri Mar 4 02:09:13 CET 2011
On 2011 Mar 4, at 01:39, Marc Weber wrote:
> Excerpts from Jacek Generowicz's message of Fri Mar 04 00:18:07
> +0000 2011:
>> Prelude Data.List> groupBy (<) [1,2,3,2]
> This is wired. However if you think about the algorithm always using
> first element of a list and comparing it against the next elements you
> 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").
Hrrrrmph. In my opinion, explicitly including the words "equivalence
relation" would immensely improve the documentation.
Thank you for you clarifications, gentlemen.
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