Add Iterable.takeUntil() or similar

[CeylonMigrationBot]

[@jvasileff] The method:

{Element*} takeUntil(Boolean stopping(Element element))

would be similar to takeWhile, with the following differences:

• The passed in function would return true to signal the end of the stream, rather than false
• The element that caused the "stop" would be included in the resultant stream (takeWhile discards the "final" element).

This is useful if your goal is to take all elements up to and including the "element you are looking for". found could also be used as the name for the passed in function.

[Migrated from ceylon/ceylon.language#739]

[CeylonMigrationBot]

[@quintesse] Although I understand why you want this I personally feel it's wrong to use takeUntil for this. Logically the only difference between a takeWhile and a takeUntil should be the fact that the stop condition is inverted. I think people might find it hard to remember which of the two is the one that includes and which is the one that excludes the element at the point the stop condition is met.

I'd rather have a boolean argument inclusive or a different name for the method itself, ie takeWhileInclusive (personally I prefer being verbose but clear).

[CeylonMigrationBot]

[@jvasileff] Yeah, I agree that takeUntil does not imply inclusion of the stopper.

There's a similar discussion here, with the following names suggested:

• takeUntil
• takeUntilInclusive
• takeWhileInclusive
• skipAfter
• terminateAfter

[CeylonMigrationBot]

[@tombentley] Can this wait till 1.3?

[CeylonMigrationBot]

[@gavinking] Yes. (OTOH if someone can think of a decent name for it, it could be implemented in 5 mins.) In the absence of such a name I will reassign it.

[CeylonMigrationBot]

[@quintesse] The other option would be to introduce a delay() method that takes a first value and a function that together work to return each value one invocation "late", eg seq.takeWhile(delay(true, (c)=>c!=',')). Just brilliant ;)

[CeylonMigrationBot]

[@gavinking] @quintesse interesting idea...

[CeylonMigrationBot]

[@jvasileff] IMO delay is too complicated to obviate the need for the unnamed function, but very cool @quintesse!

I almost adopted it for my project, but terminateAfter(p) can return the nonempty {Element+}, and takeWhile(delay(true, p)) can't.

[CeylonMigrationBot]

[@luolong] I second that. As for names, what about these:

• Iterable.takeWhile, Iterable.takeUntil
• Iterable.skipWhile, Iterable.skipUntil

The convention would be that methodes with *While are inclusive and methods with *Until are exclusive.

(I have no idea what would be the use cases for skipAfter and terminateAfter -- to me it seems like an alias to takeWhile/takeUntil)

[CeylonMigrationBot]

[@quintesse]
@luolong sure, we could do that but to me that just means that people will continuously look up the docs to see what was the difference again between the two.

We had this (very long) discussion with span() and measure() and even though I've done work on the implementation of those two I still confuse them (and even worse when using x..y vs x:y because you won't even get documentation hints).

Now if this was a common convention in other languages and everybody was accustomed to the idea perhaps.... maybe, just maybe, we could make a comparison with while loops vs do-until loops and make it easier to remember that way. Hmmm.

[CeylonMigrationBot]

[@FroMage] Sorry, wrong thread. I deleted my comment.