Re: Functor, Applicative, Monad, Foldable, Traversable instances for (, , ) a b

But length is not a bad name. The length of ((,) a) is exactly one. It's because people keep calling this "the tuple instance", unaware that it is not the tuple instance. It is a conflation in vocabulary, leading to a misunderstanding of concepts and subsequent practical application. Jakub Daniel is exactly right, "It is categorically valid to view (a,-) as a functor. It is no different from 0 being an integer" Is 0 an integer? Is ((,) a) a functor? Is the length of ((,) a) equal to one? Whichever universe you choose to live in, these questions all have the same answer. I would respect the contrary position more if it was also argued that 0 is not an integer. On 31/03/17 07:53, Theodore Lief Gannon wrote:

Two bits from the peanut gallery:

We have an open world of universal instances. The question is not whether it's convenient to have these instances. It's whether there's any likelihood of someone deciding to define them elsewhere if they're not in base. Because then they exist anyway, and without nearly as much discussion and refinement.

Yes, it's an unfortunate situation. Yes, "length" is a bad name. But the realities of the Haskell ecosystem dictate that when a single lawful instance of a base typeclass exists for a base data type, implementing it is pretty much mandatory.

On Thu, Mar 30, 2017 at 2:36 PM, Bryan Richter mailto:b@chreekat.net> wrote:

On Thu, Mar 30, 2017 at 11:18:10PM +0200, Henning Thielemann wrote: > > 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.

Oops, laziness of the bad sort on my own part. :)

> 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.

Well, is there anything to be done for it at this point? Is there even any consensus that this was, in retrospect, a poor choice?

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