duh! I tried scanr (\x acc -> x : takeWhile (/= x) acc) [] [1,2,3,4,5,3] which gives [[1,2,3,4,5],[2,3,4,5],[3,4,5],[4,5,3],[5,3],[3],[]] which kind of makes sense. M

On 24 Sep 2017, at 20:08, mike h wrote:

I’m looking at how to take from a list until a duplicate is found. e.g.

takeUntilDup [1,2,3,4,3] = [1,2,3,4]

I found this implementation

takeUntilDup = foldr (\x r -> x : takeWhile (/= x) r) []

It works but I can’t see how!!? The accumulated result is built up in r so with input [1,2,3,4,3] then, at the point when r = [1, 2, 3, 4], the fold is about to use the number 3. i.e. it does takeWhile (/=3) [1,2,3,4] which gives [1,2]

Please, how does this work?

Thanks Mike

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