On Thu, Mar 02, 2017 at 05:22:48PM +0100, Andreas Abel wrote:
Today a student came to me wondering why a certain function produced a regular result, where he had expected an error. Turned out he had used `concat`, but not on a lists of lists as he had thought, but on a lists of `Either a [b]`.
I keep making the same mistake when using Parsec to process a file; calling `length` on the result (to check how many thingies I have parsed) returns 1, 1 and again 1, no matter how many times I tinker with the code. I feel such a fool after recognising my misstep, every time.
With the Foldable instance for Either, which considers Either a b to be a container of 0-1 elements of b, errors are happily swallowed.
I think this instance is harmful and should be deprecated (and later removed) from base.
There are similarly pointless Foldable instances as well.
See a discussion one year ago, which was heated, but had no consequences.
I suspect it had no consequences because the mother of all battles was "Foldable and Traversable to become importable unqualified"; and once you set the goal (and reap the benefits: Traversable), you have to live with some of the annoying consequences.