iko (Apr 20 2023 at 09:02): iko (Apr 20 2023 at 09:02):How would you feel about an implementation like this?
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Folkert de Vries (Apr 20 2023 at 09:04):takeWhile usually only returns the part of the list that it takes, right? this is more like a split
Folkert de Vries (Apr 20 2023 at 09:04):(which is still useful!)
iko (Apr 20 2023 at 09:05):I think this is the behaviour that is expected in the issue: https://github.com/roc-lang/roc/issues/5138
I agree though, it doesn't really match the name
iko (Apr 20 2023 at 09:07):maybe breakOn?
iko (Apr 20 2023 at 09:09):Not sure about the Predicate type itself though. I don't think it can be made anonymous.
Folkert de Vries (Apr 20 2023 at 09:10):I think that is too fancy, I don't think the predicate should be recursive
Folkert de Vries (Apr 20 2023 at 09:11):it would be more efficient to fold to find the index, and then slice, for instance
iko (Apr 20 2023 at 09:18):Folkert de Vries said:
it would be more efficient to fold to find the index, and then slice, for instance
I think the type I proposed still allows this sort of implementation
The recursive predicate allows you to make a decision using an arbitrart number of elements. I think this is the general case for what was described in the issue. Using a simple boolean predicate would not allow for making a decision based on two consecutive elements.
Folkert de Vries (Apr 20 2023 at 09:20):this is true, but would that be convenient to use in practice?
Folkert de Vries (Apr 20 2023 at 09:20):this makes me think of a fold, which is a traversal that keeps some state
iko (Apr 20 2023 at 09:22):In practice the type I proposed can be used as just a simple boolean predicate (Take and Break) without using NeedMore.
Folkert de Vries (Apr 20 2023 at 09:24):yes I understand, but is the inclusion of NeedMore really worth it. it makes the signature more complicated
Folkert de Vries (Apr 20 2023 at 09:25):and its usecases can be covered in other ways
Folkert de Vries (Apr 20 2023 at 09:25):perhaps to resolve this: can you create an example that uses NeedMorethat might be included in the documentation?
iko (Apr 20 2023 at 09:25):I think we are actually debating if the function proposed in the Github issue is needed in builtins at all.
Folkert de Vries (Apr 20 2023 at 09:25):then let's see if we like that code
iko (Apr 20 2023 at 09:26):sure
Folkert de Vries (Apr 20 2023 at 09:26):yeah I think we should include the simpler (standard) version of takeWhile : List a, (a -> Bool) -> List a
Folkert de Vries (Apr 20 2023 at 09:27):and anything fancier than what that can do, can be done with List.walk and variants, plus a List.splitAt : List a, Nat -> List a
Richard Feldman (Apr 20 2023 at 12:26):makes sense to me!
Luke Boswell (Apr 20 2023 at 12:27):Good discussion, thanks for clarification Folkert. I copied what I had into the issue, glad to see some design in here :smile:
iko (Apr 20 2023 at 14:51):To be clear: we decided to not implement takeWhile as described in the Github issue, and instead decided to add a more traditional boolean-based takeWhile?
Richard Feldman (Apr 20 2023 at 16:01):yep!
Last updated: Jul 26 2025 at 12:14 UTC