Constraints on definition of `length` should be strengthened

[Henning Thielemann]

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.