January 2017 - Libraries

Proposal: Add readMaybe (and possibly readEither) to Prelude, make Haddocks for read more cautionary
by Simon Jakobi 25 Feb '17

25 Feb '17

read [1] is an easy way to introduce runtime exceptions into programs, but its documentation doesn't sufficiently warn of this danger. read's safe alternatives, Text.Read.readMaybe [2] and Text.Read.readEither [3], are relatively unknown and too hard to find. To guide users (particularly newcomers) away from read and towards readMaybe and readEither, I propose to 1. Add readMaybe to the Prelude 2. Add readEither to the Prelude 3. Change the documentation for read to point out the partiality and to recommend the above alternatives: The haddocks for read currently read: > The read function reads input from a string, which must be completely consumed by the input process. I propose to add a paragraph roughly like this: > Note: read will throw an error if the parse fails. If there's any uncertainty w.r.t. the shape of the input, readMaybe or readEither should be used instead. Name clashes: A cursory code search on Github for readMaybe [4] reveals many existing definitions, mostly with the same type (Read a => String -> Maybe a) and definition. The same search for readEither [5] shows mostly GHC forks. Design issues: I am somewhat doubtful about the benefit of readEither over readMaybe: While readEither does give additional info on the kind of parse failures, that information is encoded in a String error message, from which it must be parsed if it is needed in the program. As the different parts of the proposal can be implemented independently of each other, please vote separately on each part of the proposal. Discussion period: 4 weeks. Cheers, Simon [1] http://hackage.haskell.org/package/base-4.9.0.0/docs/Prelude.html#v:read [2] http://hackage.haskell.org/package/base-4.9.0.0/docs/Text-Read.html#v:readM… [3] http://hackage.haskell.org/package/base-4.9.0.0/docs/Text-Read.html#v:readE… [4] https://github.com/search?l=Haskell&q=readMaybe&type=Code [5] https://github.com/search?l=Haskell&q=readEither&type=Code

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Proposal: Add splitL and splitR to container’s Map APIs
by Joachim Breitner 20 Feb '17

20 Feb '17

Hi, I regularly find myself in the need of splitting a map into two maps. For that we currently have these two functions: split :: Ord k => k -> Map k a -> (Map k a, Map k a) splitLookup :: Ord k => k -> Map k a -> (Map k a, Maybe a, Map k a) Usually, split is useless to me because information (the element right on the border) gets lost. splitLookup preserves the information, but often I want the element at the border to simply be in one of the two output maps. So I would like to see splitL :: Ord k => k -> Map k a -> (Map k a, Map k a) splitR :: Ord k => k -> Map k a -> (Map k a, Map k a) with these properties uncurry union (splitL k m) == m uncurry union (splitR k m) == m all (<= k) (keys (fst (splitL k m)) all (> k) (keys (snd (splitL k m)) all (< k) (keys (fst (splitR k m)) all (>= k) (keys (snd (splitR k m)) These new functions should be added to Data.Map, Data.IntMap (in both variatans each) and, for consistency, Data.Set. Alternative names (in correspondence with Data.Set.lookup{LT,LE,GT,TE}) would be splitLE (instead of splitL) and splitLT (instead of splitR). Corresponding issue: https://github.com/haskell/containers/issues/387 Discussion period: 2 weeks (until Feb 14). Greetings, Joachim -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • https://www.joachim-breitner.de/ XMPP: nomeata(a)joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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Proposal: Make minimumBy and maximumBy go through foldl1
by David Laing 09 Feb '17

09 Feb '17

Hi all, I'm proposing that the implementations of minimumBy and maximumBy be changed from using foldl1 to foldr1. I found this in a GHC trac ticket[0] labelled 'Newcomer' that has more details / discussion on that. The points that stand out to me are: - the Haskell report says that those methods should be implemented in terms of foldl1 (although as instance methods of Foldable there might be some wiggle room there) - it helps solves a space leak (which at first glance feels like it might be a more common problem than the options that foldl1 removes) - from the discussion on the ticket, it seemed to be an agreeable middle ground As a side note: there have been a few proposals in the past to switch these functions to use foldl', which seemed to have stalled. I don't know what the etiquette is around bringing up minor variations on old proposals again, so I apologise if I've breached some kind of protocol here. Although I guess I've already breached one protocol by pushing a patch to Phabricator for review without getting sign-off from the Core Libraries Committee, so at least I'm being even handed with my clumsiness :) Cheers, Dave [0] https://ghc.haskell.org/trac/ghc/ticket/10830

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Selection Monad
by Jakub Daniel 02 Feb '17

02 Feb '17

Hello, I apologize in case this is not the right place to bring this question/proposal up. Some time ago I stumbled upon the Selection Monad [1] (also referred to as Search from the search package on hackage). Its relation to the Continuation Monad and the usefulness demonstrated in [1] made me wonder whether it would be nice to include Selection Monad in the core libraries along the Continuation Monad (in mtl and transformers) with all the business of selections attaining continuations. I can imagine the pattern to be too little recognised to justify such an addition, yet the theoretical connection to the Continuation Monad seems to be an interesting one and worth being addressed. Best, Jakub Daniel [1] Jules Hedges. *Monad transformers for backtracking search*. In *Proceedings of MSFP 2014. *https://arxiv.org/abs/1406.2058

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Random Number Update
by Dominic Steinitz 31 Jan '17

31 Jan '17

I wanted to give an update on the status of random numbers in Haskell. 1. It is well known that the random number generator package https://hackage.haskell.org/package/random gives unexpected results. 2. Most people do *not* use it. I believe https://hackage.haskell.org/package/mwc-random is a popular choice but developers are free to use e.g. Mersenne Twister, PCG (Permuted Congruential Generator), TF (ThreeFish) and many others. 3. Approximately 2 years, I made a proposal to replace the algorithm within random (https://hackage.haskell.org/package/random) with that used by tf-random (https://hackage.haskell.org/package/tf-random) which is used by QuickCheck. In summary, the response to this was that someone should do more research with the result that nothing happened. 4. In the meantime, random (https://hackage.haskell.org/package/random) is *no longer* a core library. It's just a library with the same status as e.g. mwc-random. However, it has one difference: it uses the name for its module: "System.Random". Other RNGs use "System.Random.MWC", "System.Random.PCG", "System.Random.Mersenne" etc. As a maintainer of random (https://hackage.haskell.org/package/random) my proposal now is to deprecate all of it. I am not clear what the policy is on namespace usage. Could every RNG use the module name "System.Random"? Or is this somehow reserved? If the latter then I propose that *nothing* uses this name and that all RNGs should add a suffix indicating which algorithm they use. I note that the Haskell Platform contains tf-random so users of this will still be able to generate (better) random numbers. If someone comes along in the future, as I hope they do, and implements e.g. Guy Steele's splitmix algorithm then this can occupy the name "System.Random.Splitmix" and have the package name "random-splitmix". The advantages of doing this are: 1. Neophyte (and experienced) Haskellers do not accidentally use an RNG which gives unexpected results. 2. No-one will any longer be able to write blogs or papers about this embarrassing aspect of Haskell. I believe the co-maintainer of random (https://hackage.haskell.org/package/random) Carter Schonwald, has a different view on this matter but it is best he speaks for himself rather than me imperfectly trying to reflect his thinking. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Win32 non-backwards compatible API change
by Tamar Christina 27 Jan '17

27 Jan '17

Hi All, I want to revive and push forward a change first proposed in 2014 for an API change in the Win32 package. The discussion at that time took place here https://github.com/haskell/win32/issues/24 In short, In the Windows API it is common that when a function accepts a pointer to something, that NULL also be valid and defaults to some action. Such an example is the FindWindow call. https://msdn.microsoft.com/en-us/library/windows/desktop/ms633499(v=vs.85).… HWND WINAPI FindWindow( _In_opt_ LPCTSTR lpClassName, _In_opt_ LPCTSTR lpWindowName ); In this case, both the ClassName and the WindowName are allowed to be NULL. The Haskell binding to this function is findWindow :: String -> String -> IO (Maybe HWND). So we've lost the ability to use the default parameters. The proposal is to change such functions to accept Maybe: findWindow :: String -> String -> IO (Maybe HWND) becomes findWindow :: Maybe String -> Maybe String -> IO (Maybe HWND) Which also would more accurately match the original API. This is of course a backwards incompatible change. Any objections to this change? Thanks, Tamar

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Re: Random Number Update
by Dominic Steinitz 26 Jan '17

26 Jan '17

I’ll try and address the points. 1. I proposed a low cost solution as advocated by Sven and Henrik about 2 years ago, that of using tf-random. This was rejected because apparently better RNGs might exist. Now as we all know there is no such thing as the best RNG. My reasoning for tf-random was that if it were good enough for QuickCheck then it would certainly be good enough for naive users. A case of the best driving out the good? Not that best exists in this case. 2. QuickCheck were bitten very hard! 3. > Prelude> import System.Random > Prelude System.Random> let dieRoll = fst . randomR (1,6) . mkStdGen :: Int -> Int > Prelude System.Random> map dieRoll [1..20] > [6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6] 4. In addition to the problems adumbrated above, random also has a range > Prelude System.Random> newStdGen >>= return . genRange > (1,2147483562) which makes it fail even testu01’s SmallCrush if used naively. random *may* work in certain cases but I would strongly advise against using it when there are so many other packages without the above problems. I would also add that for e.g. MCMC it is often very hard to see if something is working or not working because of the Monte Carlo noise; something apparently working does not necessarily validate a particular RNG. I can’t say I agree with the reasoning that it is better to carry on using something that could be giving incorrect results silently rather than breaking things so that people can take action. Anyhow as I have already said I feel I have done my bit for the Haskell Community. I agree with Carter that this is out of scope for the Core Libraries Committee. I wish you all luck and will step down from being maintainer. > On 25 Jan 2017, at 11:26, libraries-request(a)haskell.org <mailto:libraries-request@haskell.org> wrote: > > Send Libraries mailing list submissions to > libraries(a)haskell.org <mailto:libraries@haskell.org> > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries> > or, via email, send a message with subject or body 'help' to > libraries-request(a)haskell.org <mailto:libraries-request@haskell.org> > > You can reach the person managing the list at > libraries-owner(a)haskell.org <mailto:libraries-owner@haskell.org> > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Libraries digest..." > > > Today's Topics: > > 1. Re: Selection Monad (Jakub Daniel) > 2. Add Zero and One to Data.Functor (mirroring V1, U1) > (Baldur Blöndal) > 3. Re: Random Number Update (Sven Panne) > 4. Re: Random Number Update (Henning Thielemann) > 5. Re: Random Number Update (Andreas Abel) > 6. Re: Random Number Update (Henrik Nilsson) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 24 Jan 2017 17:23:58 +0000 > From: Jakub Daniel <jakub.daniel(a)gmail.com <mailto:jakub.daniel@gmail.com>> > To: Eric Mertens <emertens(a)gmail.com <mailto:emertens@gmail.com>> > Cc: libraries(a)haskell.org <mailto:libraries@haskell.org> > Subject: Re: Selection Monad > Message-ID: > <CACaCNTSj5T0GnGkWgk_VO8vaL4dZYao16QUTCvYMXvsAyUw1oQ(a)mail.gmail.com <mailto:CACaCNTSj5T0GnGkWgk_VO8vaL4dZYao16QUTCvYMXvsAyUw1oQ@mail.gmail.com>> > Content-Type: text/plain; charset="utf-8" > > Hi Eric, > > Just to clarify, my question was not about endorsing another library but > rather extending the ones currently maintained by the committee with > definition of a notion closely related to what is already contained in the > libraries. It just so happens that Edward Kmett implemented the notion in a > standalone library. However to me it does not make much sense to include > ContT but not SelT (currently under the name Search) in the core libraries, > despite their connection. I understand if this does not change the answer > though. > > Best Regards, > Jakub Daniel > > > > On Tue, 24 Jan 2017 at 17:39 Eric Mertens <emertens(a)gmail.com <mailto:emertens@gmail.com>> wrote: > >> Hi Jakub, >> >> The core libraries committee helps maintain libraries that are already >> critical to the Haskell ecosystem. It doesn’t select new libraries to be >> recommended to users. It’s possible that the search package is a hidden gem >> that more people need to know about. To accomplish that people will need to >> write software using it, market it with blog posts, and bring it up in >> discussions. >> >> Best Regards, >> Eric Mertens >> >> On Jan 24, 2017, at 5:57 AM, Jakub Daniel <jakub.daniel(a)gmail.com <mailto:jakub.daniel@gmail.com>> wrote: >> >> Hello, >> >> I apologize in case this is not the right place to bring this >> question/proposal up. Some time ago I stumbled upon the Selection Monad [1] >> (also referred to as Search from the search package on hackage). Its >> relation to the Continuation Monad and the usefulness demonstrated in [1] >> made me wonder whether it would be nice to include Selection Monad in the >> core libraries along the Continuation Monad (in mtl and transformers) with >> all the business of selections attaining continuations. I can imagine the >> pattern to be too little recognised to justify such an addition, yet the >> theoretical connection to the Continuation Monad seems to be an interesting >> one and worth being addressed. >> >> Best, >> >> Jakub Daniel >> >> [1] Jules Hedges. *Monad transformers for backtracking search*. In *Proceedings >> of MSFP 2014. *https://arxiv.org/abs/1406.2058 <https://arxiv.org/abs/1406.2058> >> >> _______________________________________________ >> Libraries mailing list >> Libraries(a)haskell.org <mailto:Libraries@haskell.org> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries <http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries> >> >> >> >

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Re: Add Zero and One to Data.Functor (mirroring V1, U1)
by Ryan Scott 26 Jan '17

26 Jan '17

You linked to the Trac issue [1], but for the purposes of motivating this proposal, I think it's important that we reiterate why you want this. First of all, I think we should be clear that you're proposing to add these in the modules: * Data.Functor.One * Data.Functor.Zero Right? Not literally "Data.Functor", as the title might mislead you into believing. Also, for a reason why you might want these - there are currently analogs in the GHC.Generics [2] module: * data U1 a = U1 -- isomorphic to One * data V1 a -- isomorphic to Zero But this is an unfortunate situation, since I'd rather not recommend that folks import a generics library to use what should be general-purpose datatypes. Now, as for the proposal itself: * I'm weakly +1 in favor of Zero. I think it'd be nice to have a proper * -> * counterpart to Void in base (that isn't V1, obviously). * I'm weakly -1 in favor of One, simply because we already have Data.Proxy, which came first. Ryan S. ----- [1] https://ghc.haskell.org/trac/ghc/ticket/13177 [2] http://hackage.haskell.org/package/base-4.9.1.0/docs/GHC-Generics.html

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Add Zero and One to Data.Functor (mirroring V1, U1)
by Baldur Blöndal 26 Jan '17

26 Jan '17

Deadline of 2 weeks. Original discussion: ⁽¹⁾ data Zero a data One a = One Same definitions as in ‘trivia’ package,⁽²⁾ if a consensus is reached we can update the documentation for Free:⁽³⁾ “Free Zero” is isomorphic to Identity. “Free One” is isomorphic to Maybe. ⁽¹⁾ https://ghc.haskell.org/trac/ghc/ticket/13177 ⁽²⁾ https://hackage.haskell.org/package/trivia ⁽³⁾ https://hackage.haskell.org/package/free-4.12.4/docs/Control-Monad-Free.html

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Proposal: Introduce GHC.TypeNats module with natVal :: forall n proxy. KnownNat n => proxy n -> Natural
by Oleg Grenrus 24 Jan '17

24 Jan '17

Currently we have in the GHC.TypeLits module natVal :: forall n proxy. KnowNat n => proxy n -> Integer However, the result integer is never negative. Numeric.Natural module is in base since base-4.8.0.0. --- I propose that we introduce new module GHC.TypeNats with natVal and natVal' natVal :: forall n proxy. KnownNat n => proxy n -> Natural and natVal' :: forall n. KnownNat n => Proxy# n -> Natural and other KnownNat related terms and type families. --- This change would propagate to give more precise type to someNatVal: someNatVal :: Natural -> SomeNat which is currently someNatVal :: Integer -> Maybe SomeNat --- The alternative would be to change the types in GHC.TypeLits, but - it would be breaking change - hard to shim (where new module is implementable for older base, in e.g. base-compat) - we lose opportunity to separate Nat and Symbol data kinds --- Additionally we'd deprecate the GHC.TypeLits natVal, natVal' and someNatVal --- As related "nice to know", `Nat` and `Natural` "agree" on negation: > (1 - 2) :: Natural *** Exception: arithmetic underflow > natVal (Proxy :: Proxy (1 - 2)) <interactive>:9:1: error: • No instance for (KnownNat (1 - 2)) arising from a use of ‘natVal’ • In the expression: natVal (Proxy :: Proxy (1 - 2)) In an equation for ‘it’: it = natVal (Proxy :: Proxy (1 - 2)) --- The change is quite straightforward to do, and I'm willing to implement it even for GHC 8.2 Oleg Grenrus

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