Re: [Haskell-cafe] groupBy without Ord?

On 22/03/14 18:29, martin wrote:

Am 03/22/2014 06:40 PM, schrieb Kim-Ee Yeoh:

...

On Sat, Mar 22, 2014 at 11:51 PM, martin mailto:martin.drautzburg@web.de> wrote:

How can I groupBy a List whose elements are only instances of Eq but not of Ord?

If you take a look at the code for groupBy:

groupBy :: (a -> a -> Bool) -> [a] -> [[a]] groupBy _ [] = [] groupBy eq (x:xs) = (x:ys) : groupBy eq zs

Cool!

I have a database background and the SQL "group by" does of course not assume any ordering. So I often wonder, where you would use Haskell's groupBy WITHOUT sorting first, but I assume there are situations, where this is useful.

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One example that comes to mind is filtering consecutive-same elements: | Prelude Data.List> map head $ group "hello world!!!" | "helo world!" -- Mateusz K.