On Thu, 30 Mar 2017, Bryan Richter wrote:
My intuition, based on observed usage in Haskell and elsewhere, is that tuples are used as anonymous product types. Thus, I am interested in the rationale for making them Functors because I feel it has denied me a common, valuable tool. I no longer feel that I should use pairs as product types. By making "(,) a" a Functor, it is no longer an anonymous product type —— it is an anonymous Reader. Are anonymous Readers really all that much more valuable than anonymous product types? I cannot conceive it. I must be missing something.
Pairs as functors are Writers. The function type got a Functor instance that corresponds to Readers. Functions as Readers (and Either as exception monad) must have happened in the GHC-6 version range and Pairs as Writers is quite recent and happened together with moving Monoid to Prelude (because Writers need Monoid for the left pair element). I think the only reason is that some programmers got "laziness" wrong and try to use primitive types for everything instead of using (and importing) dedicated types.