Re: Constraints on definition of `length` should be strengthened

Am 06.04.2017 um 21:18 schrieb amindfv@gmail.com:

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On 6 April 2017 at 23:56, wrote: I don't totally understand this viewpoint. It sounds like what you're saying is it's unfortunate that tuples (and everything else) are biased in Haskell, but because they are we're obligated to make all the legal instances we can for them.

E.g. if I define a datatype "data Foo x y z", I'm powerless and sort of obligated to define "instance Functor (Foo x y)" if there's a legal one, regardless of if that's what I want Foo to mean.

Is Foo going to be widely used or only an internal data type to your own code?

For the sake of comparison, let's say it's going to be widely used. It's also a structure which isn't (conceptually) biased.

If we're starting from a place of feeling that it's a shame Haskell is unable to have unbiased structures, then probably an "if we knew then what we know now" version of Haskell would have them. So then why knowingly create instances we think a "better Haskell" wouldn't have?

Is the argument that if it's public-facing, someone's going to define the instance and so we should do it canonically? If so, this feels to me a little like "you can't fire me, I quit!" - doing what we don't want before someone else has a chance to.

I think the situation is analogous to honoring class laws. There is no way in Haskell to enforce them. But there is an unwritten code of conduct (sic!) for library writers, to define only (public) instances that adhere to the stated laws. Why can't we not state that a certain data type is intended to by symmetric in its arguments? And expect that others respect that intent and refrain from defining asymmetric Functor, Foldable, etc instances for it? Cheers Ben