- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] ANN: To Kata Haskellen Evangelion
by Olaf Klinke 01 Aug '19

01 Aug '19

Isn't the Eq vs Ord problem similar to all sub-classes, e.g. Applicative vs. Monad, Semigroup vs. Monoid, Foldable vs. Traversable? What is the recommended way of coding? The Traversable module for example has default implementations of fmap and foldMap, as Functor and Foldable are superclasses. This suggests it is okay to define the subclass method first and derive the superclass methods from them. But arguments have been put up against this practice in particular cases, e.g. for AMP [1]. Olaf [1] https://mail.haskell.org/pipermail/haskell-cafe/2019-July/131259.html

7 13

RfA: wcwidth
by Jason Dusek 01 Aug '19

01 Aug '19

Hi All, This is a Request for Adoption for WCWidth, a low traffic package that provides the functionality of wcwidth(3) <http://man7.org/linux/man-pages/man3/wcwidth.3.html>: “The wcwidth() function returns the number of columns needed to represent the wide character c.” https://github.com/solidsnack/wcwidth/ Although there have been just 3 issues and 2 PRs in 10 years, that’s still indicative of a need for maintenance and life has drawn me away from these avocations. A contributor suggested I reach out with an RfA. Hopefully, the community can be better served with a new maintainer. Kind Regards, Jason

1 0

Call for Participation: Summer BOB 2019 (August 21, Berlin)
by Stefan Wehr 31 Jul '19

31 Jul '19

Here's a quick reminder on the upcoming BOB conference, happening at the third day of ICFP at the same venue. For those going to ICFP anyway: your ticket also gives you access to all BOB talks. For those not going to ICFP: come to BOB conference and also get access to the ICFP talks of day 3! ======================================================================== Summer BOB 2019 Conference “What happens if we simply use what’s best?” August 21, 2019, Berlin co-located with ICFP 2019 http://bobkonf.de/2019-summer/ Program: http://bobkonf.de/2019-summer/program.html Registration: http://bobkonf.de/2019-summer/registration.html ======================================================================== Are you interested in technologies beyond the mainstream, that are a pleasure to use, and effective at getting the job done? BOB is the forum for developers, architects and builders to explore and discover the best tools available today for building software. Our goal is for all participants to leave the conference with new ideas to improve development back at the ranch. Summer BOB is a one-time-only event, in the spirit of the spectacular Winter BOB. The International Conference on Functional Programming is coming to town, and Summer BOB will be right in the middle of it, on the last day of ICFP proper, prior to all the workshops. Summer BOB will feature two tracks: one from practitioners, and one from researchers, and foster communication and cross-pollination between these communities. BOB features two tracks of seven talk each: One research track with invited talks, and one track by practitioners, designed to cross-pollinate and inspire.http://bobkonf.de/2019-summer/program.html Topics include distributed programming, testing, linear algebra, functional design patterns, type systems, formal methods, and interactive development. We are committed to diversity: We aim at exploring a wide range of tools in a welcoming and friendly crowd of diverse people. To that end, a number of support options for participants from groups under-represented in tech are available.http://bobkonf.de/2019-summer/registration.html

1 0

Re: [Haskell-cafe] type variable is ambiguous in a non-injective type family
by Anthony Clayden 30 Jul '19

30 Jul '19

Anthony> I am interested in how you got into such a pickle in the first place: Anthony> Why set `AllowAmbiguousTypes`? Did you understand what that means? ----------------------^^^^^^^^^^^^^^^^^^^^ >I read > > https://stackoverflow.com/questions/49684655/how-dangerous-is-allowambiguou… > > > which seems to indicate that using it is "a perfectly reasonable thing" Hmm, I don't think that's how you got into the pickle. That SO answer (and there are plenty on the subject) says the "combination" of `AllowAmbiguousTypes` + `TypeApplications` is reasonable. But your first post had `AllowAmbiguousTypes` without `TypeApplications`. That is not "a perfectly reasonable thing"; no SO answer says it is. If you had already looked at that SO answer, how did you not use `TypeApplications`? Let me suggest how you got into the pickle, because there is a repeated experience here which is poor for learners: You wrote the method signature for `toBool`. That's ambiguous. GHC gave you an error message suggesting `AllowAmbiguousTypes` might get the method signature to compile -- which it did. `AllowAmbiguousTypes` works at the declaration site, not the usage site. Which is why: * It doesn't make sense to switch it on everywhere -- as one of the SO comments suggests. * It does need you to take action at the usage site, but GHC's message doesn't tell you that. * So your first post showed more `ambiguous` rejections at the usage site. * What's more there was a red herring about non-injective type families. * And to fix it per Li-yao's suggestion needs a whole bunch more extensions, * including the deeply flawed `ScopedTypeVariables`, * when the simpler `PatternSignatures` would be more appropriate -- except that's now deprecated. * `AllowAMbiguousTypes` is module-wide: that means you get no validation of any of your method sigs; although probably you mean only a few of the to be ambiguous. * Then any/every usage site for any method might turn out to be ambiguous. IMO GHC has got itself into a total mess in this bit of the language, and the error messages lead learners rapidly into deep water. I think `AllowAmbiguousTypes` might as well be spelled `AllhopeAbandonTypes`. Those other extensions might be the best approach for your full requirements, but might not. So as to other possible designs: * I didn't want to get into the FunDeps vs TypeFamilies war. * If method `toBool` is only going to be called on the result of `toSubAmbi`, I would go > toBool :: a -> Bool -- not ambiguous > > instance Ambi blah where > type SubAmbi blah = ... > toSubAmbi x = ... > toBool x = case toSubAmbi x of { ... -> True; _ -> False } Then in your `whatsWrong` function, call only `toBool`. So the instances are a little more verbose but the usage sites are more succinct and don't need all those extensions. AntC

2 1

Re: [Haskell-cafe] type variable is ambiguous in a non-injective type family
by Henry Laxen 29 Jul '19

29 Jul '19

First, thank you Li-yao Xia for your clear and helpful response. I was able to continue thanks to you. Thanks for the question Anthony, I look forward to learning something. I hope I'll be explain this clearly. I am writing some games, which can share some code, such as logging in, and broadcasting messages to players. I started by writing just one game, with no consideration for how and what to share. This let to a gamestate which contained things like: newtype GameId = GameId Int data Game1 { _game1TimeStarted :: UTCTime, _game1PlayerId :: GameId, ... _game1RandomGen :: StdGen, _game1Commands :: [Game1Command] } the actual state is a more complicated than this, because I split it into a Static part and a Mutable part that is in a TVar (Map GameId Game1MutableState) Next I start writing Game2, which I discover will do the same kind of broadcasting of messages (Commands) as Game1. So I would like to use the same code in Game2 as in Game1 that handles the broadcasting. I came up with: (this is part of the actual class I am using) class Game a where type Command a :: * type Mutable a :: * type Static a :: * toGameId :: a -> GameId toMutable :: a -> IO (Maybe (Mutable a)) everyone :: Mutable a -> [Sink] instance Game Game1State where type Command Game1State = Game1Command type Mutable Game1State = Game1Mutable type Static Game1State = Game1Static toGameId ss = ss ^. game1Static . game1GameId toMutable ss = do let tVar = ss ^. game1Mutable gId = toGameId ss gm <- liftIO (readTVarIO tVar) return $ M.lookup gId (unGameMap gm) everyone = mapMaybe _game1Sink . M.elems . _game1SPMap similarly, I make Game2State an instance of Game. Now in the code that broadcasts commands I have: sendBroadCasts :: (Show (Command a) , Game a) => [BroadCast a] -> a -> IO () ... sendOneCommand :: (Show (Command a) , Game a) => a -> BroadCastReceiver -> Command a -> IO () sendOneCommand ss BroadCastToEveryone command = do mbMut <- toMutable ss case mbMut of Nothing -> return () Just mut -> do let sinks = everyone mut mapM_ (`sendCommand` command) sinks but GHC scolds me with: src/Games/BroadCasts.hs:53:12-23: error: … • Couldn't match type ‘Mutable a’ with ‘Mutable a0’ Expected type: IO (Maybe (Mutable a0)) Actual type: IO (Maybe (Mutable a)) NB: ‘Mutable’ is a non-injective type family The type variable ‘a0’ is ambiguous • In a stmt of a 'do' block: mbMut <- toMutable ss In the expression: ... After incorporating Li-yao Xia's suggestion, the code becomes: sendOneCommand :: forall a. (Show (Command a) , Game a) => ----------------- ^^^^^^^^^ this is added a -> BroadCastReceiver -> Command a -> IO () sendOneCommand _ (BroadCastToSome sinks) command = mapM_ (`sendCommand` command) sinks sendOneCommand ss BroadCastToEveryone command = do mbMut <- toMutable ss case mbMut of Nothing -> return () Just mut -> do let sinks = everyone @a mut -------------------------- ^^ this is added mapM_ (`sendCommand` command) sinks and GHC is happy. I hope I have explained this sufficiently clearly. I look forward to your insights. Best wishes, Henry Laxen >>>>> "Anthony" == Anthony Clayden <anthony_clayden(a)clear.net.nz> writes: Anthony> Hi Henry, both replies you've received are excellent, I won't expand on them. Anthony> I am interested in how you got into such a pickle in the first place: Anthony> Why set `AllowAmbiguousTypes`? Did you understand what that means? ----------------------^^^^^^^^^^^^^^^^^^^^ I read https://stackoverflow.com/questions/49684655/how-dangerous-is-allowambiguou… which seems to indicate that using it is "a perfectly reasonable thing" Anthony> Did you think it odd that despite having that set, you get error Anthony> `type variable ... is ambiguous`? Anthony> There are ways to achieve what you want without Anthony> `AllowAmbiguousTypes` nor `TypeApplications`, but that would need Anthony> a rather different design. So what lead you into this design? Anthony> AntC -- Nadine and Henry Laxen The rest is silence Villa Alta #6 Calle Gaviota #10 Never try to teach a pig to sing Chapala It wastes your time +52 (376) 765-3181 And it annoys the pig

2 1

Re: [Haskell-cafe] type variable is ambiguous in a non-injective type family
by Anthony Clayden 29 Jul '19

29 Jul '19

> I really don't understand what is going on here. ... -------------------------------------------------------------------------------- > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE AllowAmbiguousTypes #-} > {- > > src/Games/Ambiguous.hs:15:16: error: … > • Couldn't match expected type ‘SubAmbi a0’ > with actual type ‘SubAmbi a’ > NB: ‘SubAmbi’ is a non-injective type family > The type variable ‘a0’ is ambiguous > > -} Hi Henry, both replies you've received are excellent, I won't expand on them. I am interested in how you got into such a pickle in the first place: Why set `AllowAmbiguousTypes`? Did you understand what that means? Did you think it odd that despite having that set, you get error `type variable ... is ambiguous`? There are ways to achieve what you want without `AllowAmbiguousTypes` nor `TypeApplications`, but that would need a rather different design. So what lead you into this design? AntC

1 0

[twilio] Project takeover initiation
by Brian C. Jones 29 Jul '19

29 Jul '19

Hello, I have attempted to contact the author via PR and direct email with no response in 3 weeks. This is my final attempt before engaging the Hackage admins. https://github.com/markandrus/twilio-haskell/pull/67 Brian Jones

3 2

[language-dart] Package takeover initiation
by Sergey Bushnyak 29 Jul '19

29 Jul '19

Hi all, I tried to contact the author via github issues and direct email with no response in couple weeks. This is my statement on desire to takeover package maintainership forhttps://hackage.haskell.org/package/language-dart <https://hackage.haskell.org/package/language-dart> -- Sergey Bushnyak

1 0

Munich Haskell Meeting, 2019-07-31 @ 19:30
by Leah Neukirchen 28 Jul '19

28 Jul '19

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Wednesday, July 31 at Augustiner-Gaststätte Rumpler at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jul-2019/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

1 0

type variable is ambiguous in a non-injective type family
by Henry Laxen 28 Jul '19

28 Jul '19

Dear Cafe, I really don't understand what is going on here. I've searched, but I don't really understand the links that I found. The closest I've come is: https://sulzmann.blogspot.com/2013/01/non-injective-type-functions-and.html but I don't understand it enough to see my way to a solution. Here is a minimal example of what I'm trying to do: -------------------------------------------------------------------------------- {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE AllowAmbiguousTypes #-} module Ambiguous where class Ambi a where type SubAmbi a :: * toSubAmbi :: a -> SubAmbi a toBool :: SubAmbi a -> Bool whatsWrong :: Ambi a => a -> IO () whatsWrong a = do let s = toSubAmbi a b = toBool s if b then print "True" else print "False" {- src/Games/Ambiguous.hs:15:16: error: … • Couldn't match expected type ‘SubAmbi a0’ with actual type ‘SubAmbi a’ NB: ‘SubAmbi’ is a non-injective type family The type variable ‘a0’ is ambiguous • In the first argument of ‘toBool’, namely ‘s’ In the expression: toBool s In an equation for ‘b’: b = toBool s • Relevant bindings include s :: SubAmbi a (bound at /home/henry/nadineloveshenry/bin/nlh/src/Games/Ambiguous.hs:14:5) a :: a (bound at /home/henry/nadineloveshenry/bin/nlh/src/Games/Ambiguous.hs:12:12) whatsWrong :: a -> IO () (bound at /home/henry/nadineloveshenry/bin/nlh/src/Games/Ambiguous.hs:12:1) | Compilation failed. -} -------------------------------------------------------------------------------- Could any of you Haskell sages please explain how to do this? Best wishes, Henry Laxen -- Nadine and Henry Laxen The rest is silence Villa Alta #6 Calle Gaviota #10 Never try to teach a pig to sing Chapala It wastes your time +52 (376) 765-3181 And it annoys the pig

3 2