[thoughtpolice]

Two very clear problems are that it introduces more complexity up front for beginners (because now to understand `length` you need to understand the concept of type classes) and the change had the possibility to break some extant code, due to it changing some type inference properties.

In practice, I think #2 was a fairly uncommon case, and the fixes were trivial and backwards compatible, so the cost was deemed pretty acceptable. #1 is a point of debate, because 'beginner' needs context (perhaps experienced programmers that are Haskell beginners would get over it quickly, but non-programmers would be substantially more confused, etc). Unfortunately I don't think we have a lot of truly empirical evidence on #1.

[HelloNurse]

My empirical evidence is against #1: up-front productive learning beats permanent uncertainty.

- I can fold a list, but what about other types? Which of the similarly or identically named functions dispersed among a bewildering number of library modules is the right one? Are they any different?

- I can fold a Foldable? What is it? Let's find a tutorial about Foldable. Ooh, nice! That takes care of folding anything that can be folded! If I'm not folding a list I only need to find where the relevant Foldable instance is (very easy) or to write one myself (reasonably easy).