One could define fold as a generic operation that replaces the empty list with a constant (for example an identity element) and replaces the "cons" operator with a function that takes an element and a partial result. This is consistent with other ways to define generalized folds over recursive data structures. For example for trees you can replace the leaf node with a constant and replace the internal node with a binary function. The key idea here is that fold directly reflects the natural structure of the underlying data structure. ie, the way you would draw a picture of the underlying data structure is exactly the picture of the execution order. If we are consistent with this, then fold will be defined for lists to be the same as foldRight for lists. For more detailed exposition to motivate this definition, see the paper "Merging Monads and Folds for Functional Programming", by Meijer and Jeuring. <http://dspace.library.uu.nl/bitstream/handle/1874/20910/jeuring_95_mergingmonads.pdf>
One could also define a fold (as scala does) to be not-necessarily-linear -- the execution order is unspecified and left up to the implementer. So in scala, a fold can split up a list into many pieces (sublists) and process the sublists in parallel, and then fold them up to obtain the final answer. In this case, it is not necessaria foldLeft or a foldRight. It could actually be a fold-from-the-middle or even fold-from-many-places-in-the-middle. In order to make sure the answer is well-defined, the folding operator is required to be associative. IMO it would be a good idea to avoid confusion with these alternative semantics by not reusing the name "fold" at all, or else adopting one of the conventional meanings (if we can agree what the most common meaning is). On Thursday, October 20, 2016 at 11:32:12 AM UTC-4, Max Goldstein wrote: > > Normally I'm opposed to syntax or name changes. But I think this or some > variation is a good idea. (Maybe foldl becomes fold, since it's usually the > one you want.) > > That said, it's all subject to Evan's approval. > -- You received this message because you are subscribed to the Google Groups "Elm Discuss" group. To unsubscribe from this group and stop receiving emails from it, send an email to elm-discuss+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
