Improving the instances of Data.Functor.{Product,Sum}
John Ericson
john.ericson at obsidian.systems
Sat Apr 18 18:47:33 UTC 2020
Big +1 to fixing these issues.
https://github.com/phadej/some/pull/21 is a PR I'd like to see merged,
but which cannot build because of base shunning flexible instances. We
at Obsidian frequently use `Sum` and `Product` (or when it usually fails
their `Data.Generic` equivalents, `:+:` and `:*:`) with GADTs that are
*not* functors, and anything but the `Flexible*` instances prevents
that. (Yup, `QuantifiedConstraints` instead of *1 is also no good.)
Do note that none of specific problems with `Flexible*` raised in this
seem to apply in this case (no multi-param type class, no missing `~`,
etc.), only the general wariness. As mentioned, the `Data.Generics` ones
get it the way I want, and I do try to use them accordingly, but if we
can be "right" there why can't we be right here? (And wouldn't it be
nice of one was a newtype or alias of the other...)
If we can all do that, secondarily I would actually like to *keep* the
*1 classes, but give them `QuantifiedConstraints` super-classes:
> (forall a. Eq (f a)) => Eq1 f
It doesn't look like this has been brought up yet, but the idea is that
one implements a *1 class because of the extra power that the lift*
methods provide----the functionality on the type parameter need not be
canonical---not as a crude hack around the lack of `QuantifiedConstraints`.
I would also advocate writing a `Lift1` for this reason: "shallowly
quoting" a functor of syntax (i.e. `liftLift id`) is quite useful and
very much in the spirit of the lisp quasi-quoting that our quoting and
splicing is based one.
Finally, while the change is breaking, no CPP should be needed and there
is less room for user-error as the new quantified constraint super
classes forces anything which used the old instances to also support the
new instances---you can unthinkingly fix the the type errors and end up
with the right instances for old and new base.
John
P.S. Maybe one might argue that the the new ret-conned use for the *1
classes is much more niche so they belong outside of base. I always like
anything to do with breaking up base, so that's fine with me too. Just
don't say they are altogether useless because of `QuantifiedConstraints`.
On 3/15/20 4:43 PM, Carter Schonwald wrote:
> as an additional point of data,
> https://github.com/ekmett/bytes/pull/49#issuecomment-580924670 and the
> indirectly linked ticket
> https://gitlab.haskell.org/ghc/ghc/issues/17767#note_251123
> is an example of how not trivial a pretty simple use of quantified
> constraints / related machinery can be!
>
>
> a good "guiding light" for core libraries in haskell perhaps should be
> "what choices jointly give the best type inference, composability,
> generality, performance, and usability" and when we hit a trade off
> for these parameters, make sure we understand those and have very well
> understood tradeoffs.
>
> @daniel for these sorts of instance renovation proposals, its
> probably best to show case "heres the type and instance and uses" in
> each of the K different flavors and what gets better/easier/harder
> simpler. I honestly dont do it as often as I should myself!
>
> In this case, putting together a tiny repo with different module doing
> the various flavors and how they work might be best for grounding
> this. bonus points if its cabalized so that folks on different lagging
> rates of ghc versions can poke around :)
>
> (one idea i've been thinking about is how to best make various phases
> of possibly nontrivial proposals easy to evaluate and compare for
> impact and benefit, but thats a discussion for another time)
>
> as ever, be well all on this bizarre spring we're all experiencing
>
> -Carter
>
>
> On Sat, Mar 14, 2020 at 11:44 AM Oleg Grenrus <oleg.grenrus at iki.fi
> <mailto:oleg.grenrus at iki.fi>> wrote:
>
> QuantifiedConstraints are not buggy, but they are not _complete_
> (I do mean that as "buggy" = not sound, properly expressive =
> complete).
>
> There are at least three issues:
>
> 1. Instance definition need UndecidableInstances (in my opinion
> this is big deal)
> 2. Instances are not elegant (easy to write, but not elegant).
> 3. QuantifiedConstraints resists to be abstracted over
>
> See
> https://gist.github.com/phadej/266d68cf5cc1229c3548b7965f4335f8 for
> the standalone code file.
>
> For the reasons below I wouldn't recommend using
> QuantifiedConstraints.
> I very like them, but I'm not convinced the feature is ready for
> "prime time".
> To put into perspective, I think code classes of `singletons` are
> more ready to
> be included in `base` than changing instances of Data.Functor.Sum
> and .Product.
> I do use singletons in my code more than QuantifiedConstraints. :)
>
> I'm very worried how this change will affect libraries like `free` and
> `recursion-schemes` and what builds on top of them.
> This is not only change to `base`, it strongly guides how downstream
> libraries should be written (or changed) as well.
>
> On the other hand, I don't feel strongly about
>
> instance (Eq (f a), Eq (g a)) => Eq (Product f g a)
>
> Yet, in the light of `free` it is "a step backwards".
> See https://hackage.haskell.org/package/free-5/changelog
>
> ---
>
> Simple example with `newtype Fix f = Fix (f (Fix f))`
>
> class Eq1 f where
> liftEq :: (a -> b -> Bool) -> f a -> f b -> Bool
>
> class Eq1 f => Ord1 f where
> liftCompare :: (a -> b -> Ordering) -> f a -> f b -> Ordering
>
> instance Eq1 f => Eq (Fix f) where
> (==) = eq where eq (Fix x) (Fix y) = liftEq eq x y
>
> instance Ord1 f => Ord (Fix f) where
> compare = cmp where cmp (Fix x) (Fix y) = liftCompare cmp x y
>
> works. It's boilerplate, but it's already written.
>
> However, if we want to use QuantifiedConstraints,
> then we
>
> 1. need UndecidableInstances
> 2. and then the code turns out to be less elegant:
>
> instance (forall x. Eq x => Eq (f x)) => Eq (Fix f) where
> (==) = eq where eq (Fix x) (Fix y) = x == y
>
> instance (forall x. Ord x => Ord (f x)) => Ord (Fix f) where
> compare = cmp where cmp (Fix x) (Fix y) = compare x y
>
> fails to compile with
>
> GHCi, version 8.8.3: https://www.haskell.org/ghc/ :? for help
> [1 of 1] Compiling Main ( Ord1.hs, interpreted )
>
> Ord1.hs:28:10: error:
> • Could not deduce (Ord x)
> arising from the superclasses of an instance declaration
>
> we need to write Ord instance differently
>
> instance (forall x. Ord x => Ord (f x), forall x. Eq x => Eq (f
> x)) => Ord (Fix f) where
> compare = cmp where cmp (Fix x) (Fix y) = compare x y
>
>
> This _cannot_ be made to work:
>
> forall x. Ord x => Ord (f x)
>
> doesn't entail
>
> forall x. Eq x => Eq (f x)
>
> If you try to write defaultLiftEq using liftCompare
>
> defaultLiftEq :: Ord1 f => (a -> b -> Bool) -> f a -> f b -> Bool
> defaultLiftEq eq x y = EQ == liftCompare _problem_ x y
>
> then you will se a problem.
>
> ---
>
> Third issue is that We cannot abstract over QuantifiedConstraints.
>
> Take an example `Dict`. We can use `Dict` for various things.
>
> data Dict :: (k -> Constraint) -> k -> * where
> Dict :: c a => Dict c a
>
> It nicely uses PolyKinds extension so we can write:
>
> eqInt :: Dict Eq Int
> eqInt = Dict
>
> eq1List :: Dict Eq1 []
> eq1List = Dict
>
> And we can use Dict to *manually* thread information
>
> entail :: Dict Ord1 f -> Dict Eq1 f
> entail Dict = Dict
>
> The selling pitch of QuantifiedConstraints, that we could get this
> for free.
> Above Ord (Fix) example however makes me suspicious. Let's try:
>
> eqQList :: Dict (forall x. Eq x => Eq (f x)) []
> eqQList = undefined
>
> But it doesn't work!
>
> Ord1.hs:53:28: error:
> • Expected kind ‘(* -> *) -> Constraint’,
> but ‘Eq (f x)’ has kind ‘*’
> • In the first argument of ‘Dict’, namely
> ‘(forall x. Eq x => Eq (f x))’
> In the type signature:
> eqQList :: Dict (forall x. Eq x => Eq (f x)) []
> |
> 53 | eqQList :: Dict (forall x. Eq x => Eq (f x)) []
> |
>
> Ord1.hs:53:36: error:
> • Expected a type, but ‘Eq (f x)’ has kind ‘Constraint’
> • In the first argument of ‘Dict’, namely
> ‘(forall x. Eq x => Eq (f x))’
> In the type signature:
> eqQList :: Dict (forall x. Eq x => Eq (f x)) []
> |
> 53 | eqQList :: Dict (forall x. Eq x => Eq (f x)) []
> | ^^^^^^^^
>
> Then we remember that we have seen that,
> we **cannot define** type synonyms for quantified constraints
>
> type Eq1' f = forall x. Eq x => Eq (f x)
>
> errors with
>
> Ord1.hs:56:33: error:
> • Expected a type, but ‘Eq (f x)’ has kind ‘Constraint’
>
> Luckily GHC-8.10.1 (which is not released at the moment of writing)
> will give us ability to say
>
> type Eq1' :: (* -> *) -> Constraint
> type Eq1' f = forall x. Eq x => Eq (f x)
>
> This is promising! Let's try to fix eqQList
>
> eqQList2 :: Dict Eq1' []
> eqQList2 = undefined
>
> But that doesn't work. Type-aliases have to be fully applied!
>
> How in Haskell we fix issues when type-aliases (of classes) need
> to be partially
> evaluated? We defined
>
> class ... => Example a b c
> instance ... => Example a b c
>
> Third try
>
> eqQList3 :: Dict Eq1'' f
> eqQList3 = Dict
>
> The type signature is accepted, but the implementation is not
>
> Ord1.hs:67:12: error:
> • Could not deduce (Eq (f x)) arising from a use of ‘Dict’
>
> At this point I'm clueless.
>
> ---
>
> Best regards,
> Oleg
>
> P.S. If we would like to take QuantifiedConstraints somewhere into
> use, then IMO we should start with MonadTrans class. IIRC it's
> well motivated in the paper. But UndecidableInstances is very
> unfortunate.
>
> On 14.3.2020 16.10, chessai . wrote:
>> I can second Richard's estimation of QuantifiedConstraints, I
>> have used them a lot in my own code since they were in HEAD. I
>> consider it a sufficiently stable feature to include in base or
>> any library.
>>
>> On Sat, Mar 14, 2020, 4:16 AM Richard Eisenberg <rae at richarde.dev
>> <mailto:rae at richarde.dev>> wrote:
>>
>>
>>
>>> On Mar 14, 2020, at 4:14 AM, Eric Mertens
>>> <emertens at gmail.com <mailto:emertens at gmail.com>> wrote:
>>>
>>> The last thing I'd heard about quantified constraints was
>>> that they were buggy and I've been avoiding relying on them.
>>> (I should probably review that assumptions at some point.)
>>
>> Without expressing an opinion about chessai's proposal (which
>> I have not really thought about): quantified constraints are
>> in good shape and ready for prime time. They have limitations
>> (e.g. you can't mention a type family to the right of the
>> =>), but when they are valid, they work well. I'll never
>> swear that a feature is bug-free, but I think it's reasonable
>> to consider using quantified constraints in `base`.
>>
>> Richard
>>
>>
>> _______________________________________________
>> Libraries mailing list
>> Libraries at haskell.org <mailto:Libraries at haskell.org>
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org <mailto:Libraries at haskell.org>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
More information about the Libraries
mailing list