this thread would greatly benefit from example code. sounds like a XY problem.
KristerV
wanton7
cmo
Couldn’t you reduce over it, accumulating each value unless it matches, when you’d apply the function to it, and keep on accumulating the remaining values?
Or with recursion you could exit when you find it and append the tail to what you’ve accumulated.
Remembering to reverse it at the end.
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wanton7
Thanks, this code will be used for lists of in range of 1000-10000 elements during processing. That’s why I would like finding of the element to be fast and to cause minimal new allocations. Also I would like that the loop returns if element is found to exit fast as possible, so I don’t reduce is the way to go here. Matching of element doesn’t happen that often so fast path needs to be the path that returns nil. It’s not that of big problem currently that I have do extra call Enum.at but asked this question to find out if there is function like that already available that returns both index and the element that I’ve missed. I can of course create my own function to do it.
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wanton7
it seems that Enum.find_index is using Enumerable.reduce. I’ll just create my own function that is copy of Enum.find_index that returns both 
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LostKobrakai
Enum.reduce_while(list, {:not_found, 0}, fn el, {:not_found, index} ->
if el.id == id, do: {:halt, {:found, index, el}}, else: {:cont, {:not_found, index + 1}}
end)
4
nacengineer
For those wanting a use case. A contrived use case
You have 1 CSV row and you know it’s in sync with the headers which you also have but you don’t have a guarantee on the header order across report generations (I know this is a bit contrived but it’s the reality of the stupid system we’re exporting these CSVs from which we don’t have much control over).
So you need to figure out which index the header is at then fetch the corresponding index within the CSV row that you have.
At least this was my use case.
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LostKobrakai
Sounds like you’d do better keeping rows in maps keyed by header and then using the order of headers in the final report to turn your row maps into a list of values.
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