[Haskell-cafe] groupBy huh?
daniel.is.fischer at googlemail.com
Fri Mar 4 01:47:11 CET 2011
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"
> Prelude Data.List> groupBy (<) "abca"
> Prelude Data.List> groupBy (<) [1,2,3,2]
> Prelude Data.List> groupBy (<) [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.
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