Constraints on definition of `length` should be strengthened

[Henrik Nilsson]

Hi,

On 04/03/2017 10:14 PM, Nathan Bouscal wrote:
> You can't get tuples to behave like they're unbiased. You can try to
> hide the fact that they're biased by getting rid of the only possible
> instances they can support, but that doesn't magically make them
> unbiased. It sounds like you just want to rename tuples to Decorated.
> Maybe that's a good idea, but call it what it is.

While I (so far) disagree, I am trying to fully appreciate this
argument.

The reason is that it seems to me that the above has more to do with
specific syntactic details regarding instance declarations for
partially applied type constructors, than with what (in this case)
tuples fundamentally are in Haskell: essentially Cartesian products.

For the sake of argument, suppose some mechanism were adopted to
mitigate the bias implied by the (inevitable) ordering of arguments
to to type constructors. For tuples, we might imagine some kind
of notation inspired by operator sections as a first step, making the
following instance declaration possible:

     instance Functor (,b) where
         ...

Would tuples then still be biased in the above sense, and if
so why?

Best,

/Henrik





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