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Make combine faster #29

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Make combine faster #29

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4f5956b
Faster combine
jfmengels Oct 20, 2021
a7ac9ee
Inline the call to List.reverse
jfmengels Oct 21, 2021
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20 changes: 18 additions & 2 deletions src/Result/Extra.elm
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Original file line number Diff line number Diff line change
Expand Up @@ -143,8 +143,24 @@ mapBoth errFunc okFunc result =
Also known as `sequence` on lists.
-}
combine : List (Result x a) -> Result x (List a)
combine =
List.foldr (Result.map2 (::)) (Ok [])
combine list =
combineHelp list []
|> Result.map List.reverse
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If you move the List.reverse into the [] -> branch of combineHelp then you don't need to Result.map over it or pipe. Will probably only add 0.000001% better performance 😅

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@jfmengels jfmengels Oct 21, 2021 •
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Oh, you'd be surprised... It's not the same kind of improvement, but for small lists, it's a noticeable improvement (compared to the version I already proposed).

https://ellie-app.com/fDcGZNzBFzGa1

Screenshot from 2021-10-21 20-30-32
Screenshot from 2021-10-21 20-31-15
Screenshot from 2021-10-21 20-32-08

You're absolutely right, and I don't know why I didn't do this in the first place (well, probably because adding List.reverse was something I realized I needed to do a lot later 🤦).

I updated the PR 👍

I didn't benchmark on the Err case but it should also improve performance 👍



combineHelp : List (Result x a) -> List a -> Result x (List a)
combineHelp list acc =
case list of
head :: tail ->
case head of
Ok a ->
combineHelp tail (a :: acc)

Err x ->
Err x

[] ->
Ok acc


{-| Map a function producing results on a list
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