Different types in FoldWhile

[mpdn]

Currently, FoldWhile requires the same type in Continue and Done, but it doesn't seem obvious why it shouldn't be able to use two different types.

It might be possible to add in a backwards compatible way, for example by adding the second type as a type parameter defaulting to the first type parameter.

[bluss]

Marking as breaking-change so that it is not forgotten at a breaking change-point.

[bluss]

Note that std Iterator::try_fold will give you what you want, I think.

[mpdn]

Yeah, that seems like it would do exactly what I want. It does feel a little dirty to use Err for things that aren't really errors, so something like FoldWhile would still be nice if it implemented Try when it eventually stabilizes. However, I'll just use try_fold with Err for now, so thanks!

[scottmcm]

It does feel a little dirty to [...]

I agree -- core uses an enum very much like FoldWhile internally to make things clearer: https://github.com/rust-lang/rust/blob/master/src/libcore/iter/mod.rs#L356-L404

[bluss]

.fold_while() has been deprecated in favour of .try_fold, and that's why this change is not implemented. It could otherwise easily be implemented. #223 #318