February 2015 - Libraries

Proposal: merge either into transformers
by Roman Cheplyaka 03 Jan '17

03 Jan '17

I propose to add the sole module of the 'either' package[1], Control.Monad.Trans.Either, to the transformers package. It provides EitherT, a very basic and fundamental data type. The difference between EitherT and ErrorT is that the latter has an Error constraint, which is used to imlement 'fail'. Note that 'either' depends on the 'semigroupoids' and 'semigroup' packages to provide appropriate instances. The proposal is not to add those instances to 'transformers' to avoid additional dependencies. The instances can then be left in the 'either' package or moved to the 'semigroupoids' and 'semigroup' packages respectively. ('semigroupoids' already depends on 'transformers', while 'semigroups' does not). Compared to the 'either' package, Show, Read, Eq and Ord instances will be dropped to keep the code Haskell2010 (those instances require FlexibleInstances, FlexibleContexts, and UndecidableInstances). The patch is attached. [*] [*] against transformers-0.3.0.0, because the darcs version is not buildable (Control/Monad/Signatures.hs is not in the repository). Roman

22 72

Proposal: add liftA4 and liftA5 to match liftM4 and liftM5
by David Feuer 14 Jul '16

14 Jul '16

Since Applicative is supposed to be important now, I figure we should get these in.

12 33

[containers] Proposal: Change to the Data.Map Monoid
by Nick Partridge 10 Oct '15

10 Oct '15

Hi, Currently the Monoid instance for Data.Map is implemented using union/unions, which are left biased. On key collision, it discards values from the right hand side of `mappend` - https://github.com/ghc/packages-containers/blob/bae098fb0a3994bc2b0ec331300… If you compare this with the Monoid for Maybe, it's like we're defaulting to First as the monoid instance for maps. A more useful instance, however very much a breaking change, would be to make the instance depend on a Monoid (or better yet, a Semigroup) for the values in the map: instance Monoid v => Monoid (Map k v) where mappend = unionWith mappend This lets us build up maps with values in a useful Monoid, and mappend them with gusto. Thoughts? - Nick Partridge Discussion period: 2 weeks.

15 22

Proposal: NF newtype
by Edward Z. Yang 01 May '15

01 May '15

I propose the following (abstract) data type, functions, and instance be added to deepseq (naming amenable to bikeshedding): newtype NF a = NF a -- abstract makeNF :: NFData a => a -> NF a getNF :: NF a -> a instance NFData (NF a) where rnf x = x `seq` () NF is an abstract data type representing data which has been evaluated to normal form; the guarantee specifically is, if NF is in whnf, then it is in nf. Crucially, when we have 'NF a', we ONLY need to seq it in order to assure that it is fully evaluated. This guarantee is sufficient for a variety of cases where normal data is necessary, e.g. when transmitting data over Channels. For example, from the monad-par library 'put_' could be used in place of 'put' with this type signature. put_ :: IVar (NF a) -> (NF a) -> Par () Cheers, Edward

12 25

Generalize filterM to Applicative
by David Feuer 25 Mar '15

25 Mar '15

Currently, filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM p = foldr go (return []) where go x r = do flg <- p x ys <- r return (if flg then x:ys else ys) We could change this to filterM :: (Applicative f) => (a -> f Bool) -> [a] -> f [a] filterM p = foldr go (pure []) where go x r = f <$> p x <*> r where f flg ys = if flg then x:ys else ys

7 12

Proposal: Add (<$>) to Prelude as part of the AMP in GHC 7.10-RC3
by Edward Kmett 10 Mar '15

10 Mar '15

We have a couple of weeks until the third release candidate for GHC 7.10 goes out the door. Along the way with the last couple of release candidates folks have found some problems with the way we implemented the AMP. [1][2] Most notably, we failed to include (<$>) in the Prelude, so the standard idiom of foo <$> bar <*> baz <*> quux doesn't work out of the box! I'd like to include (<$>) in the Prelude in RC3. I'd also like to invite discussion about whether folks believe we should include (<$) out of the box. (<$) has been a member of Functor for a long time, which is only visible if you import it from Data.Functor or bring in Control.Applicative. There is an idiom that you use (<*) and (<$) to point to the parts of the structure that you want to keep the answers from when building longer such Applicative chains. Discussion Period: 2 weeks Thank you, -Edward Kmett [1] http://www.reddit.com/r/haskell/comments/2wzixa/shouldnt_be_in_prelude/ [2] https://plus.google.com/115504368969270249241/posts/URzeDWd7qMp

39 50

Should foldl' be added to the post-"Plan FTP" Prelude
by Bob Ippolito 07 Mar '15

07 Mar '15

(Cross-posted from haskell-cafe as suggested by Carter Schonwald) I would love to have foldl' as part of the Prelude. I use it quite frequently and it seems like a beginner trap to have foldl but not foldl' (which is almost always the better choice). I see that foldMap has been added (in addition to all of Monoid and Traversable), why was foldl' left out? It might be good measure to add foldr' as well, although I use that much less frequently. -bob

8 10

fgl-5.5.0.1
by C Maeder 06 Mar '15

06 Mar '15

Hi Ivan, could you make a new release of fgl that works with ghc-7.10? I could compile it locally after the following changes: Data/Graph/Inductive/Query/Monad.hs +import Control.Applicative +import Control.Monad (ap, liftM) +instance Monad m => Functor (GT m g) where + fmap = liftM + +instance Monad m => Applicative (GT m g) where + pure = return + (<*>) = ap Data/Graph/Inductive/Monad/IOArray.hs -{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} +{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-} I'm not sure why FlexibleContexts is now needed and I'm not sure how the package version number should change. Cheers Christian

6 8

Proposal to deprecate and then drop fromJust
by Erik de Castro Lopo 02 Mar '15

02 Mar '15

Hi all, We currently have in the Data.Maybe module from base: fromJust :: Maybe a -> a which newbies often find using hoogle or other search tools and is a huge trap. *Every* usage of the above can be replaced by using fromMaybe :: a -> Maybe a -> a which forces the user to provide a default value for the case where the Maybe a is a Nothing. The idea would be to deprecate fromMaybe for 7.12 and remove it completely later. Comments? Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

30 67

Re: [Haskell-cafe] A Proposed Law for Foldable?
by Gershom B 28 Feb '15

28 Feb '15

I think I can now float an amended law that is a further step in the right direction. First I will give the law, and then I will attempt to both motivate why the amended treatment is needed, and why the weakening of the initial proposal is a legitimate approach. Finally, I will discuss some remaining issues that it would be nice to examine, including some sketches of related but alternate approaches. So, the new law: === for a lawful Foldable f, and given a fresh newtype GenericSet = GenericSet Integer deriving (Eq, Ord), and mkGenericSet = GenericSet, where GenericSet is otherwise fully abstract then forall (g :: forall a. f a -> Maybe a), (x :: f GenericSet). maybe True (`Foldable.elem` x) (g x) === True == You may recall that the prior formulation asked that this condition hold for all a., (x :: f a), and so required an external notion of equality. Here, we ask that it only hold at a _specified type_ -- i.e. "GenericSet". So, in what sense was the older formulation "too strong"? Here is a minimal case. data TBox a where TAny :: a -> TBox a TString :: String -> TBox String TBox is just a box with some optional type information, and it should be able to be equipped with the obvious Foldable instance. However, we can now write a function such as discrim :: forall a. TBox a -> Maybe a discrim (TAny x) = Just x discrim (TString _) = "zoinks" This is to say that if we _know_ that TBox contains a string, we can now take _any_ string out of it. Clearly, the existence of such functions means that the obvious Foldable instance cannot satisfy the stronger law. By contrast, the straightforward Foldable instance for TBox still satisfies the weaker law, as we only test the action on "GenericSet" -- and since we have defined GenericSet as a fresh newtype, we know that it cannot have a tag to be matched on within a GADT we have already defined. Hence, our law, though it gives a universal property, does not give this property itself "universally" _at every type a_ but rather at a "generic type". This is by analogy with the technique of using "generic points" in algebraic geometry [1] to give properties that are not "everywhere true" but instead are true "almost everywhere", with a precise meaning given to "almost" -- i.e. that the space for which they do not hold _vanishes_. This is to say, a Foldable satisfying the proposed law will not have the property hold "at all types," but rather at "almost all types" for an analogously precise meaning of "almost". I believe that a statement of why GenericSet is indeed "generic" in some topological sense is possible, although I have not fully fleshed this out. To my knowledge, the introduction of this sort of approach in an FP context is new, but I would welcome any references showing prior art. Aside from dealing with GADTs in some fashion, the new approach has a few other useful features. First, by formulating things directly in terms of "Foldable.elem" we omit the need for some sort of notion of equality in the metalogic -- instead we can use the "Eq" typeclass directly. Furthermore, this happens to sidestep the "toList" problem in the initial approach -- it directly allows search of potentially infinite structures. There are still a few things that could potentially be better. 1) The "fully internal" approach means that we are now subject to the "bias" of "||" -- that is to say that _|_ || True -> _|_, but True || _|_ -> True. So we do not fully capture the searchability of infinite structures. This may indicate that a retreat to a metalogical description of "elem" with unbiased "or" is in order. 2) While we can generically characterize a Foldable instance on some f "almost everywhere" this is at the cost of giving it _no_ characterization at the points where our generic characterization fails. It would be nice to establish some sort of relation between the generic characterization and the action at specified points. However, I am not quite sure how to present this. An alternate approach could be to specify a Foldable law for any `f` that first takes `f` (which may be a GADT) to a related type `f1` (which must be an ordinary ADT) that squashes or omits dictionaries and equality constraints, and likewise takes the Foldable instance to a related instance on f1, and then provides a condition on f1. So rather that retreating from universal to generic properties, we instead take hold of the machinery of logical relations directly to establish a law. I would be interested in being pointed to related work along these lines as well. 3) An additional drawback of the "Generic Point" approach as given is that we chose to derive only two particular typeclasses -- Eq and Ord. An alternate approach would be to quantify over all a, but then give the property in terms of say "newtype Generify a = Generify a deriving (...)" which derives all classes on "a" that do not feature "a" in a positive position. Doing this would also mean a retreat from a fully internal notion of equality, of course... Anyway, this is clearly still work in progress, but I would appreciate any feedback on the direction this is going, or references that may seem useful. Cheers, Gershom [1] https://en.wikipedia.org/wiki/Generic_point On Thu, Feb 12, 2015 at 6:33 PM, Edward Kmett <ekmett(a)gmail.com> wrote: > With Foldable we *do* have a very nice law around foldMap. > > A monoid homomorphism g is a combinator such that > > g mempty = mempty > g (mappend a b) = mappend (g a) (mappend (g b) > > For any monoid homomorphism g, > > foldMap (g . f) = g . foldMap f > > We can use that to construct proofs of an analogue to "the banana split > theorem" for foldr, but rephrased in terms of foldMap: > > foldMap f &&& foldMap g = foldMap (f &&& g) > > Getting there uses the fact that fst and snd are both monoid homomorphisms. > > There are also laws relating the behavior of all of the other combinators > in Foldable to foldMap. > > Ultimately the reasons for the other members of the class are a sop to > efficiency concerns: asymptotic factors in terms of time or stack usage > matter. > > -Edward > > > On Thu, Feb 12, 2015 at 5:22 PM, Kim-Ee Yeoh <ky3(a)atamo.com> wrote: > >> (removing Libraries, since not everyone on HC is on that list) >> >> I do not know all of the context of Foldable, but one I do know that's >> not been mentioned is the implicit rule-of-thumb that every type class >> should have a law. >> >> So the undercurrent is that if Foldable doesn't have a law, should it >> even be a type class? This has led to efforts to uncover laws for Foldable. >> >> Worth discussing in a separate thread is the criterion itself of "if it >> doesn't have a law, it's not a type class". Useful sometimes, but that's >> not the sine qua non of type classes. >> >> >> >> -- Kim-Ee >> >> On Fri, Feb 13, 2015 at 2:47 AM, Gershom B <gershomb(a)gmail.com> wrote: >> >>> For a long time, many people, including me, have said that "Foldable has >>> no laws" (or Foldable only has free laws) -- this is true, as it stands, >>> with the exception that Foldable has a non-free law in interaction with >>> Traversable (namely that it act as a proper specialization of Traversable >>> methods). However, I believe that there is a good law we can give for >>> Foldable. >>> >>> I earlier explored this in a paper presented at IFL 2014 but >>> (rightfully) rejected from the IFL post-proceedings. ( >>> http://gbaz.github.io/slides/buildable2014.pdf) That paper got part of >>> the way there, but I believe now have a better approach on the question of >>> a Foldable law -- as sketched below. >>> >>> I think I now (unlike in the paper) can state a succinct law for >>> Foldable that has desired properties: 1) It is not "free" -- it can be >>> violated, and thus stating it adds semantic content. 2) We typically expect >>> it to be true. 3) There are no places where I can see an argument for >>> violating it. >>> >>> If it pans out, I intend to pursue this and write it up more formally, >>> but given the current FTP discussion I thought it was worth documenting >>> this earlier rather than later. For simplicity, I will state this property >>> in terms of `toList` although that does not properly capture the infinite >>> cases. Apologies for what may be nonstandard notation. >>> >>> Here is the law I think we should discuss requiring: >>> >>> * * * >>> Given Foldable f, then >>> forall (g :: forall a. f a -> Maybe a), (x :: f a). case g x of Just a >>> --> a `elem` toList x >>> * * * >>> >>> Since we do not require `a` to be of type `Eq`, note that the `elem` >>> function given here is not internal to Haskell, but in the metalogic. >>> >>> Furthermore, note that the use of parametricity here lets us make an >>> "end run" around the usual problem of giving laws to Foldable -- rather >>> than providing an interaction with another class, we provide a claim about >>> _all_ functions of a particular type. >>> >>> Also note that the functions `g` we intend to quantify over are >>> functions that _can be written_ -- so we can respect the property of data >>> structures to abstract over information. Consider >>> >>> data Funny a = Funny {hidden :: a, public :: [a]} >>> >>> instance Foldable Funny where >>> foldMap f x = foldMap f (public x) >>> >>> Now, if it is truly impossible to ever "see" hidden (i.e. it is not >>> exported, or only exported through a semantics-breaking "Internal" module), >>> then the Foldable instance is legitimate. Otherwise, the Foldable instance >>> is illegitimate by the law given above. >>> >>> I would suggest the law given is "morally" the right thing for Foldable >>> -- a Foldable instance for `f` should suggest that it gives us "all the as >>> in any `f a`", and so it is, in some particular restricted sense, initial >>> among functions that extract as. >>> >>> I do not suggest we add this law right away. However, I would like to >>> suggest considering it, and I believe it (or a cleaned-up variant) would >>> help us to see Foldable as a more legitimately lawful class that not only >>> provides conveniences but can be used to aid reasoning. >>> >>> Relating this to adjointness, as I do in the IFL preprint, remains >>> future work. >>> >>> Cheers, >>> Gershom >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >>> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

4 10