Re: Derived Functor instance for void types

16 Jan 2017

      It’s possible that empty case produces exactly the same eventual result. Simply looking at the core I was seeing a new bottom being potentially introduced.

Given:

data C a = D Void Void

fmap1, fmap2 :: (a -> b) -> C a -> C b
fmap1 _ = coerce
fmap2 _ = \case {}

I get:

fmap4 = \ @ a_a2FE @ b_a2FF _ tpl_B2 -> tpl_B2
fmap1 = fmap4 `cast` ...
fmap5 = \ @ b_a2FX -> patError "Demo.hs:12:11|case"#
fmap2 = \ @ a_a2FW @ b_a2FX _ _ -> fmap5

And with the goal being the avoidance of new bottoms, fmap1’s generated core looked nicer.
...
On Jan 16, 2017, at 3:38 PM, David Feuer  wrote:
Semantically, this is precisely equivalent to the empty case. Is there some reason to prefer it?
On Jan 16, 2017 6:35 PM, "Eric Mertens" mailto:emertens@gmail.com> wrote:
A possible derived fmap implementation for any empty data type could be `fmap _ = coerce`. This would ensure that any bottom was passed through. Of course this would run into problems with an explicit "type role" declaration on the empty data type.
-- 
Eric
On Mon, Jan 16, 2017 at 1:08 PM Ryan Scott mailto:ryan.gl.scott@gmail.com> wrote:
For reference, Data.Void.Void currently has no Enum or Bounded
instance. It does have an Ix instance [1]:
instance Ix Void where
    range _     = []
    index _     = absurd
    inRange _   = absurd
    rangeSize _ = 0
Ryan S.
-----
[1] http://hackage.haskell.org/package/base-4.9.1.0/docs/src/Data.Void.html#line... http://hackage.haskell.org/package/base-4.9.1.0/docs/src/Data.Void.html#line...
On Mon, Jan 16, 2017 at 4:03 PM, David Feuer mailto:david.feuer@gmail.com> wrote:
...
I have no intuition about what Enum should do about void datatypes;
it's too broken. Bounded has one sensible default (minBound and
maxBound should produce an error message naming the type and explain
that it has no values). Ix gets pretty weird here. I would think
range = const []
index _ x = case x of
inRange _ _ = False   OR  inRange _ x = case x of {} (is it sensible
to ask if _|_ is in an empty range?)
rangeSize = const 0
On Mon, Jan 16, 2017 at 3:37 PM, Ryan Scott mailto:ryan.gl.scott@gmail.com> wrote:
...
Thanks for the clarification. I definitely agree that the current
error-based behavior for classes where T occurs in a negative position
is suboptimal. If I understand your position correctly, then you
advocate determining what the derived code for methods in which T
occurs negatively on a case-by-case basis? How about I just list all
of the stock derivable classes, give example instances, and see if you
agree with them?
data Empty a deriving (Eq, Ord, Read, Show, Functor, Foldable,
Traversable, Lift, Generic, Data)
  ===>
  instance Eq (Empty a) where
    _ == _ = True
  instance Ord (Empty a) where
    compare _ _ = EQ
  instance Read (Empty a) where
    readPrec = parens pfail
  instance Show (Empty a) where
    showsPrec _ = absurd
  instance Functor Empty where
    fmap _ = absurd
  instance Foldable Empty where
    foldr _ z _ = z
    foldMap _ _ = mempty
  instance Traversable Empty where
    traverse _ = absurd
  instance Lift (Empty a) where
    lift = absurd
  instance Generic (Empty a) where
    from = absurd
    to   = absurd
  instance Data (Empty a) where
    gfoldl _ = absurd
    gunfold _ _ c = constrIndex c of {}
    toConstr = absurd
    dataTypeOf = mkDataType "Empty" []
absurd :: Empty a -> b
  absurd x = case x of {}
Note I didn't list Bounded, Enum, or Ix because GHC currently forbids
you from deriving instances of those for empty datatypes, even with
-XStandaloneDeriving on. Do you think we should revisit this?
Ryan S.
On Mon, Jan 16, 2017 at 3:09 PM, Edward Kmett mailto:ekmett@gmail.com> wrote:
...
By current behavior, I was referring to the behavior of Void, not deriving.
I think the current deriving behavior on this front is suboptimal by the
reasoning given in my previous post.
-Edward
On Mon, Jan 16, 2017 at 1:51 PM, Ryan Scott mailto:ryan.gl.scott@gmail.com> wrote:
...
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 mailto:ekmett@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 mailto:ryan.gl.scott@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 https://ghc.haskell.org/trac/ghc/ticket/13117
> [2] https://ghc.haskell.org/trac/ghc/ticket/7401#comment:46 https://ghc.haskell.org/trac/ghc/ticket/7401#comment:46
> [3] https://ghc.haskell.org/trac/ghc/ticket/10577 https://ghc.haskell.org/trac/ghc/ticket/10577
> [4] https://mail.haskell.org/pipermail/libraries/2015-July/025959.html https://mail.haskell.org/pipermail/libraries/2015-July/025959.html
> [5] https://mail.haskell.org/pipermail/libraries/2015-July/025965.html https://mail.haskell.org/pipermail/libraries/2015-July/025965.html
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