Re: Improving the instances of Data.Functor.{Product,Sum}

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 mailto:rae@richarde.dev> wrote:

...

On Mar 14, 2020, at 4:14 AM, Eric Mertens mailto:emertens@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

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