Derived Functor instance for void types

[Ryan Scott]

I think I mostly agree with everything you've said, but I'm not sure I
understood all the details, so let me try to recap:

> In the case it doesn't occur at all or occurs only positively, er... we don't care. We're not case analyzing it =)

Indeed there's only one stock class you can derive where the type
occurs only positively, and that's Read, which already has nice
behavior for empty datatypes:

    data Empty
    deriving instance Read Empty
    ====>
    instance Read Empty where
      readPrec = parens pfail

So indeed, this an easy case.

> In the case of it occurring in both positive and negative position we have the option to 'pass it through' to exchange the bottom. In the case of changing the behavior of Functor and the like we're not actually changing the definedness of the result, we're merely choosing between "equivalent" bottoms. This is a rather exceptional case, but the ability to preserve placed bottoms in a structure in case they have meaning to the user seems quite valuable.

To be clear, this corresponds to David's proposal to use -XEmptyCase, right?

> In the case of negative position only, the current behavior is more defined than the stricter behavior Ryan proposes. I personally very much favor keeping Eq, Ord, etc. as defined as possible for Void, V1 and the like with the existing behavior. 'a' occurs in negative position only for these cases.

What do you mean by "the current behavior" here? For instance, if you had:

    data Empty
    deriving instance Eq Empty

Are you defining "the current behavior" to mean this? That is, what
GHC currently spits out today:

    instance Eq Empty where
      (==) = error "Void =="

Or by "current behavior", do you mean the (manually written) instance
that Void has:

    instance Eq Void where
      _ == _ = True

These two instances are quite different, after all.

> Similarly the existing practice of not doing wasted work and producing more defined results, also seems valuable. Aiming for "consistency" here seems to be pursuing a goal that doesn't actually help anyone and just makes stuff less defined.

Fully agree.

Ryan S.

On Mon, Jan 16, 2017 at 12:55 PM, Edward Kmett <ekmett at gmail.com> wrote:
> There are really four cases to consider for deriving purposes.
>
> Some type T occurs positively, negative, not at all, or in both.
>
> * In the case it doesn't occur at all or occurs only positively, er... we
> don't care. We're not case analyzing it =)
>
> * In the case of negative position only, the current behavior is more
> defined than the stricter behavior Ryan proposes. I personally very much
> favor keeping Eq, Ord, etc. as defined as possible for Void, V1 and the like
> with the existing behavior. 'a' occurs in negative position only for these
> cases.
>
> * In the case of it occurring in both positive and negative position we have
> the option to 'pass it through' to exchange the bottom. In the case of
> changing the behavior of Functor and the like we're not actually changing
> the definedness of the result, we're merely choosing between "equivalent"
> bottoms. This is a rather exceptional case, but the ability to preserve
> placed bottoms in a structure in case they have meaning to the user seems
> quite valuable.
>
> Similarly the existing practice of not doing wasted work and producing more
> defined results, also seems valuable. Aiming for "consistency" here seems to
> be pursuing a goal that doesn't actually help anyone and just makes stuff
> less defined.
>
> -Edward
>
> On Mon, Jan 16, 2017 at 9:25 AM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
>>
>> To be clear, if you have:
>>
>>   data V a deriving Functor
>>
>> David is proposing in #13117 [1] that the derived instance use
>> -XEmptyCase instead of the current error-based implementation:
>>
>>   instance Functor V where
>>    fmap _ x = case x of {}
>>
>> This seems fine and well, but given that #13117 is basically a
>> continuation of another discussion in #7401 [2], I feel like if we're
>> going to tackle the issue of derived instances for empty data
>> declaration, we should solve it for all stock classes.
>>
>> In particular, there are two thorny issues to consider here. What if
>> we have this instead?
>>
>>   data V2 deriving Eq
>>
>> What instance should this produce? Reid Barton proposed in #10577 [3]
>> that it should be:
>>
>>   instance Eq V2 where
>>     a == _ = case a of {}
>>
>> But we have a choice here, since (==) has multiple arguments! The
>> definition could also conceivably be:
>>
>>   instance Eq V2 where
>>     _ == b = case b of {}
>>
>> Is there a uniform policy we can decide for functions with multiple
>> arguments like this? In fmap, it's unambiguous since there's only one
>> argument of type f a.
>>
>> Another issue to consider is that if we adopted this convention for
>> derived Eq instances for empty datatypes, we'd actually be going
>> against the convention set for Data.Void. As noted in [4], this is the
>> current Eq instance for Void:
>>
>>   instance Eq Void where
>>     _ == _ = True
>>
>> I'm not proposing that we change this definition, since there are many
>> good reasons to have it this way (see the thread in [5] for Edward
>> Kmett's convincing argument in favor of the current Eq Void instance).
>> Rather, I'm asking if we would be OK with having this
>> discrepancy--that is, deriving Eq for your own Void2 type would
>> produce a different instance. Personally, I'd be fine with it, but I
>> think we should ask for the community's input as well.
>>
>> So sorry to hijack this thread, David, but we really should answer
>> these two questions as well:
>>
>> 1. What do we do in ambiguous cases like derived (==) implementations
>> for empty datatypes?
>> 2. Are we fine with derived instances for empty datatypes sometimes
>> being different than the corresponding instances for Data.Void?
>>
>> Ryan S.
>> -----
>> [1] https://ghc.haskell.org/trac/ghc/ticket/13117
>> [2] https://ghc.haskell.org/trac/ghc/ticket/7401#comment:46
>> [3] https://ghc.haskell.org/trac/ghc/ticket/10577
>> [4] https://mail.haskell.org/pipermail/libraries/2015-July/025959.html
>> [5] https://mail.haskell.org/pipermail/libraries/2015-July/025965.html
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