On Mon, 3 Apr 2017, David Feuer wrote:

That leaves a few trouble spots:

1. There are types that some people think shouldn't have Functor/Foldable/Traversable instances at all, or that some people would like to have Functor and maybe even Traversable instances for without wanting Foldable instances. The latter is impossible because of a superclass constraint. One essential issue here seems to be one of perspective: is Foo x y a container of ys, decorated with xs, or is it a container of xs and ys? Different people tend to think about this differently, and thus form different intuitions.

I don't know if anyone has a problem with interpreting a custom data type Foo x y as a container of ys decorated with xs - if it is defined for that purpose. Discussion arose solely about the cases Foo = (,), Foo = (,,) x and so on. E.g. I actually proposed to define a custom data type like Decorated x y instead of (x,y) in case you want to have a Foldable instance.

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