I find that few practical folds have that property. I mean, you're not going to actually implement "sum" - there's a library function for that. Your reduces are

I find that few practical folds have that property. I mean, you're not going to actually implement "sum" - there's a library function for that. Your reduces are going to be domain specific, and then order often matters, and explicitly accumulating the result on the left (or right) hand side often has a performance impact (eg when building lists).