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Storable laws
by David Feuer 16 Dec '17

16 Dec '17

The documentation for pokeByteOff indicates that the following equality holds: pokeElemOff addr idx x = poke (addr `plusPtr` (idx * sizeOf x)) x Notably, this ignores alignment. It thus seems to imply that sizeOf must always be a multiple of alignment; otherwise, non-zero indices could access non-aligned addresses. Was this intentional? If so, I believe sizeOf and alignment should document the law. If not, then I believe the {poke,peek}ElemOff laws need to change to something like pokeElemOff addr idx x = poke (addr `plusPtr` (idx * lcm (sizeOf x) (alignment x))) x

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Discussion: Storable instances for tuples
by David Feuer 16 Dec '17

16 Dec '17

It seems surprising that there are no Storable instances for tuple types. I would expect any tuple of Storable types to be Storable. To provide a sensible FFI experience, the order of the fields should be preserved. Since Storable generally aligns values, I believe elements should be aligned within tuples as well, in the default fashion used for C structs. We could offer a Packed newtype to override this behavior. Why store tuples like this, and not the way Data.Vector.Unboxed does? A moment's reflection will show that it simply can't. A Ptr is a single pointer and must remain one; there's no room in it to deal with multiple arrays or extra offset info.

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Re: Proposal: add `Proxy`fied versions of `bitSizeMaybe` and `finiteBitSize`
by Ryan Scott 14 Dec '17

14 Dec '17

I don't have a particular opinion on this proposal, but I did want to chime in to note that there are several other functions in base besides biteSizeMaybe, finiteBitSize, sizeOf, and alignment that take a dummy argument. The last time I checked, all of these functions also fit the bill [1]: * Data.Bits.isSigned :: Bits a => a -> Bool * Data.Data.dataTypeOf :: Data a => a -> DataType * GHC.Generics.datatypeName :: Datatype d => t d f a -> String * GHC.Generics.moduleName :: Datatype d => t d f a -> String * GHC.Generics.isNewtype :: Datatype d => t d f a -> Bool * GHC.Generics.packageName :: Datatype d => t d f a -> String * GHC.Generics.conName :: Constructor c => t c f a -> String * GHC.Generics.conFixity :: Constructor c => t c f a -> Fixity * GHC.Generics.conIsRecord :: Constructor c => t c f a -> Bool * GHC.Generics.selName :: Selector s => t s f a -> String * GHC.Generics.selSourceUnpackedness :: Selector s => t s f a -> SourceUnpackedness * GHC.Generics.selSourceStrictness :: Selector s => t s f a -> SourceStrictness * GHC.Generics.selDecidedStrictness :: Selector s => t s f a -> DecidedStrictness * Prelude.floatRadix :: RealFloat => a -> Integer * Prelude.floatDigits :: RealFloat => a -> (Int, Int) * Text.Printf :: PrintfArg a => a -> ModifierParser Ryan S. ----- [1] This list was taken from http://hackage.haskell.org/package/proxied-0.3/docs/Data-Proxied.html

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Proposal: `Enum` class overhaul
by M Farkas-Dyck 13 Dec '17

13 Dec '17

I propose to add some new methods of `Enum`: class Enum a where ... predMay, succMay :: a -> Maybe a toEnum' :: Integer -> Maybe a fromEnum' a -> Integer Rationale for `fromEnum'` and `toEnum'`: The docs for `Enum` now say the minimal complete definition is `toEnum` and `fromEnum`, but this is not enough to have sane default instances of the other methods, for example: data ABC = A | B | C deriving Show instance Enum ABC where toEnum 0 = A toEnum 1 = B toEnum 2 = C fromEnum A = 0 fromEnum B = 1 fromEnum C = 2 main = print [A ..] -- [A,B,C,*** Exception: Non-exhaustive patterns in function toEnum In this case one could merely derive `Enum`, but not in some other cases, e.g. numeric types or GADTs. It is not possible to do better defining `toEnum` and `fromEnum` alone. If we default-define `toEnum'` and `fromEnum'` and their evil (i.e. partial) syblings in terms of each other, the user need merely define the total methods. Using `Integer` rather than `Int` allows these methods to not fail for types larger than an `Int`, which are not uncommon on 32-bit systems. Rationale for `predMay` and `succMay`: I include these partly for completeness, but `predMay` can not now be defined in general, and `succMay` only cumbersomely in terms of `enumFrom`. Note: All rationales imply "unless one uses `unsafePerformIO`". I'd rather not, myself.

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Maintenance of streaming and streaming-bytestring
by Andrew Martin 11 Dec '17

11 Dec '17

David Feuer brought this up two weeks ago [0]. The maintainer of streaming and streaming-bytestring has been unreachable, and provided his continued absence, I would like to become a maintainer of these two libraries. I have put up PRs fixing issues [1] and I've offered on the issue tracker to become a comaintainer [2]. I use this library a lot at work, so I have an interest in keeping it in good shape. [0] https://mail.haskell.org/pipermail/libraries/2017-August/028141.html [1] https://github.com/michaelt/streaming-bytestring/pull/11 [2] https://github.com/michaelt/streaming-bytestring/issues/19 -- -Andrew Thaddeus Martin

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Add foldMapM to Data.Foldable
by Andrew Martin 07 Dec '17

07 Dec '17

Several coworkers and myself have independently reinvented this function several times: foldMapM :: (Foldable g, Monoid b, Monad m) => (a -> m b) -> g a -> m b foldMapM f xs = foldlM (\b a -> mappend b <$> (f a)) mempty xs I would like to propose that this be added to Data.Foldable. We have the triplet foldr,foldl,foldMap in the Foldable typeclass itself, and Data.Foldable provides foldrM and foldlM. It would be nice to provide foldMapM for symmetry and because it seems to be useful in a variety of applications. -- -Andrew Thaddeus Martin

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Proposal? foldAlt :: (Foldable t, Alternative f) => t a -> f a
by Fumiaki Kinoshita 29 Nov '17

29 Nov '17

We found this quite useful, but we are not 100% about the name and documentation. -- | Try—in the 'Alternative' sense—to return all the values in a 'Foldable' -- container. -- -- @ -- foldAlt ≡ 'listToMaybe' :: [a] -> 'Maybe' a -- foldAlt ≡ 'maybeToList' :: 'Maybe' a -> [a] -- foldAlt ≡ 'MaybeT' . 'return' :: ('Monad' m) => 'Maybe' a -> 'MaybeT' m a -- foldAlt ≡ 'Pipes.ListT' . 'Pipes.each' :: ('Monad' m) => [a] -> 'Pipes.ListT' m a -- foldAlt ≡ id :: 'Maybe' a -> 'Maybe' a -- @ foldAlt :: (Foldable t, Alternative f) => t a -> f a foldAlt = getAlt . foldMap (Alt . pure) {-# INLINE foldAlt #-} I propose adding this to either `Data.Foldable` or `Control.Applicative`. Any thoughts?

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Monoid Instance for ReaderT
by Andrew Martin 28 Nov '17

28 Nov '17

There's a thread from earlier this year where someone asked about a Monoid instance for ReaderT: https://mail.haskell.org/pipermail/haskell-cafe/2017-March/126588.html Several other people showed interest in the instance, but in the end, the request was dismissed since there are other valid monoid instances for ReaderT. Conseqently, the PR was closed ( https://hub.darcs.net/ross/transformers/issue/38) This final comment ( https://mail.haskell.org/pipermail/haskell-cafe/2017-March/126605.html) concludes with: > In the absence of a principled reason to prefer one over the others and a general consensus, I think it’s better not to choose. This is the point I'd like to argue. There were three possible monoid instances for ReaderT discussed. They are: instance (Applicative m, Monoid a) => Monoid (ReaderT r m a) instance (Alternative m) => Monoid (ReaderT r m a) instance (Monoid (m a)) => Monoid (ReaderT r m a) I believe there is good reason to prefer the first instance over the other two. The third instance requires FlexibleContexts. This takes it outside the realm of Haskell98 and Haskell2010, which the transformers library stays within. In fact, transformers pioneered the Eq1, Ord1, etc. classes that ended up in base specifically to avoid FlexibleContexts, and deviating from this now seems against the spirit of the library, since transformers intends that it is able to be built with any Haskell compiler, not just GHC. The Alternative-based instance is reasonable, but we already have another instance that for ReaderT that is implemented this way: the Alternative instance! Here it is: instance Alternative m => Alternative (ReaderT r m) Here I'm going to loosely adapt an argument that Gabriel Gonzalez makes when defending his Monoid instance for ListT ( https://www.reddit.com/r/haskell/comments/4r5bcj/listtransformer_a_beginner… ): > Also, with the Alternative class the only thing you can do is concatenate collections (there's no way to write the cartesian product because the whole class is higher-kinded) so it's meaning is always unambiguous. However, with the Monoid class there are two possible behaviors (append or cartesian product). Given that the Alternative class has to be append, I prefer to give the Monoid class the other behavior. From the end user's perspective, if you use Alternative you always know exactly what you are getting so there is no disadvantage to using Alternative. > The other reason that it's nice to give the Monoid class the other behavior is that it works really nicely when chaining monoid instances as described in this post. The Alternative instance for ReaderT has to be Alternative-based. There is no way around this. Given that we have two possibilities for the Monoid instance, I would prefer to see the one that gives us something different from an instance we already have. Why is it beneficial to have the different behavior instead of the existing behavior? In part, it's because we now have a greater number of behaviors from the typeclasses that most people are familiar with. Additionally, we actually end up in a better situation when the Monoid instance isn't what we need. Let's consider two scenarios to measure the inconvenience that a user faces when the Monoid instance isn't the one that they want: 1) The Monoid instance for ReaderT, WriterT, StateT, etc. is Monoid-based but we wish are in a scenario where we want the Alternative-based one. 2) The Monoid instance for ReaderT, WriterT, StateT, etc. is Alternative-based but we wish are in a scenario where we want the Monoid-based one. In scenario (1), the workaround is trivial. In Data.Monoid, we have Alt, which recovers a Monoid instance from an Alternative instance: newtype Alt f a = Alt {getAlt :: f a} instance Alternative f => Monoid (Alt f a) where mempty = Alt empty mappend = coerce ((<|>) :: f a -> f a -> f a) The same Alt data type is reusable with ReaderT, WriterT, and StateT to recover the Alternative-based instance we wanted: myActions :: [ReaderT IO ()] runUntilSuccess :: ReaderT IO () runUntilSuccess = coerce (fold (coerce myActions :: [Alt (ReaderT IO) ()])) But what about scenario (2)? Let's say that we now instead want the Monoid-based instance that requires all of the actions to succeed. myActions :: [ReaderT IO ()] runAll :: ReaderT IO () runAll = ... What do we put in place of the ellipsis? We can just write it out by hand, but it would be nicer if this Monoid instance was reusable in some way. Let's say that we introduce a newtype wrapper for the Monoid-based Monoid instance: newtype MonoidReaderT m a = MonoidReaderT ... instance (Applicative m, Monoid a) => Monoid (MonoidReaderT m a) But now if we want something like this for StateT or WriterT, they're going to need their own newtype wrappers as well. That's unfortunate. In scenario (1), we needed a single newtype wrapper (that already exists in base!) to recover everything that we lost. In scenario (2), we need a newtype wrapper per type. So, by providing an instance that gives us more behaviors from haskell's blessed set of typeclasses, we end up in a better situation when we need to recover what we don't have. Finally, and this is the weakest part of my argument since it's pure speculation, I think that the Monoid-based instance is more useful in practice. For some loosely analogous historical precedent, I would point to the Monoid instance for IO. In July 2014, Edward Kmett pointed out that one issue with admitting a Monoid instance for IO was that two possible instances existed ( https://mail.haskell.org/pipermail/glasgow-haskell-users/2014-July/025124.h…) One of them monoidally concatenated the results, and the one was identical to IO's Alternative instance. Sounds familiar. Ultimately though, in November 2014, when Gabriel Gonzalez proposed the instance that monoidally concatenated results, nobody objected ( https://mail.haskell.org/pipermail/libraries/2014-November/024310.html) Every person who responded on the mailing list wanted the Monoid-based instance, not the one that was a copy of Alternative. (And it's always recoverable via Alt in the event that someone does want it). So, and again I want to stress that I consider this a weaker part of the argument since it's more rooted in human preferences rather than logic, I think the Monoid-based instance is just generally more useful in practice. In my own experience, I often have wanted the Monoid-based instances, but I have never wanted the Alternative-based instances With all that said, I would like to ask that the Monoid-based Monoid instances for ReaderT, WriterT, StateT, AccumT, etc. be considered. I would also appreciate any feedback or further discussion of this issue. -- -Andrew Thaddeus Martin

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Storing lifted and unlifted pointer types in the same Array#
by Reiner Pope 20 Nov '17

20 Nov '17

Hi libraries@, In GHC.Prim, it is documented that Array# and ArrayArray# have the same runtime representation. I'd like to exploit this to put unlifted and lifted pointers into the same array (for example, unlifted pointers at even indices and lifted pointers at odd indices). The reason I want to do this is to save some array header words, and also to improve the cache locality of my data, so that a read of a (lifted, unlifted) pointer pair only needs to touch a single cache line rather than two. I believe it is safe to do this (put unlifted and lifted pointers in the same array), as long as I always read at the same liftedness and type that I ran the write at for that index. Can you confirm? Specifically, I'm expecting to use a code pattern like the following, to read an unlifted value from an Array#, by coercing the Array# to ArrayArray#, then doing the unlifted read there: coerceToArrayArray :: Array# a -> ArrayArray# coerceToArrayArray = unsafeCoerce# coerceToUnlifted :: forall (b :: TYPE 'UnliftedRep). ArrayArray# -> b coerceToUnlifted = unsafeCoerce# indexUnliftedFromArray :: forall a (b :: TYPE 'UnliftedRep). Array# a -> Int# -> b indexUnliftedFromArray arr i = coerceToUnlifted (indexArrayArrayArray# (coerceToArrayArray arr) i) Similarly, I'm expecting to do writes by coercing the array to a MutableArrayArray# and doing the writes to that. Reiner

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Proposal: add ordNub somewhere in containers
by Gershom B 13 Nov '17

13 Nov '17

There have been many discussions over the years about adding an efficient order preserving nub somewhere to our core libraries. It always comes down to the same issue: an efficient nub wants to be backed by an efficient `Set`, but the API of the `nub` itself doesn't make reference to any other data structures besides lists. So it feels a bit conceptually strange to put an efficient nub anywhere besides `Data.List` even though it can't go there without inverting our dependency tree in a weird way or inlining an efficient set implementation into the middle of it. Nonetheless, the convenience of having a good `nub` lying around in a core library is undeniable, and after writing the "usual" one in my code for the zillionth time, I decided to raise an issue about it: https://github.com/haskell/containers/issues/439 I was promptly directed here to make a proper proposal. So, here: 1) I propose two new functions, `ordNub` and `intNub` with the standard implementation (from https://github.com/nh2/haskell-ordnub) import qualified Data.Set as Set ordNub :: (Ord a) => [a] -> [a] ordNub l = go Set.empty l where go _ [] = [] go s (x:xs) = if x `Set.member` s then go s xs else x : go (Set.insert x s) xs and the same implementation, but specialized to `Int` and using `IntSet`s. The rationale for the names is that the former has a long history of use in folklore, and the latter is the obvious specialization of it. 2) I propose these functions be added to a new module in the `containers` library: `Data.Containers.ListUtils`. This can also potentially in the future add efficient list intersection, etc. as documented on the above reference link. The rationale for the new module is that it can provide a meaningful home for such functions which operate on lists, but require other data structures to be implemented efficiently... Discussion period: 2 weeks. --Gershom

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