On 04/09/2017 12:51 AM, Nathan Bouscal wrote:
- You're trying to establish an analogy between these tuple instances and non-law-abiding instances, but that analogy really doesn't work. These are the only law-abiding instances that the types can possibly have.
Yes. But that is, actually, more of a syntactic accident than any deeply mathematical property: mathematically, there are n ways to make an n-tuple a functor, And if we, for "consistency", continue to add functor instances for tuples for n > 2, the situation, in the view of many reasonable people at least, becomes increasingly absurd, or at least it becomes increasingly clear that that the utility we get is a smaller and smaller part of what we really need. Simply because tuples actually are "morally unbiased", as Vladislav Zavialov phrased it; i.e., there are perfectly legitimate uses where the fields are of equal importance: there is no a priori "pay load" field, and no a priori "annotation" fields. And in fact, that is *exactly* what tuples originally were in Haskell, and how many very reasonably people still prefers to think about them. And of course, if, in a hypothetical future version of Haskell where we could make all possible functor instances for tuples, the question becomes: which one do we pick? The answer might well be "none" (in the prelude, at least). (A newtype wrapper approach, e.g. a la numeric Monid instances, would be both unpalatable and ultimately pointless.) So, in summary, I'd find the argument for the present tuple instances much more compelling if it *mathematically* were the case that the only law-abiding instances are those ones. But that is not the case. Best, /Henrik This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.