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

We can have an instance that generates a type error: instance TypeError (Text "Functor for tuples is prohibited") => Functor ((,) a) and similarly for Foldable, Traversable, etc. But at this point it's too late to make this change, so +1 to the proposal. On Fri, Mar 31, 2017 at 12:53 AM, 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 wrote:

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

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On Thu, 30 Mar 2017, Bryan Richter wrote:

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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. :)

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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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