- Haskell-Cafe - Haskell.org

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

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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

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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

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[twilio] Project takeover initiation
by Brian C. Jones 28 Jul '19

28 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

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[language-dart] Package takeover initiation
by Sergey Bushnyak 28 Jul '19

28 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

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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/

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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

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Final call for draft papers for IFL 2019 (Implementation and Application of Functional Languages)
by Jurriaan Hage 26 Jul '19

26 Jul '19

Hello, Please, find below the final call for draft papers for IFL 2019. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL ================================================================================ IFL 2019 31st Symposium on Implementation and Application of Functional Languages National University of Singapore September 25th-27th, 2019 http://2019.iflconference.org ================================================================================ ### Scope The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2019 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Topics of interest to IFL include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialization - run-time code generation - partial evaluation - (abstract) interpretation - metaprogramming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques - (industrial) applications ### Keynote Speaker * Olivier Danvy, Yale-NUS College ### Submissions and peer-review Differently from previous editions of IFL, IFL 2019 solicits two kinds of submissions: * Regular papers (12 pages including references) * Draft papers for presentations ('weak' limit between 8 and 15 pages) Regular papers will undergo a rigorous review by the program committee, and will be evaluated according to their correctness, novelty, originality, relevance, significance, and clarity. A set of regular papers will be conditionally accepted for publication. Authors of conditionally accepted papers will be provided with committee reviews along with a set of mandatory revisions. Regular papers not accepted for publication will be considered as draft papers, at the request of the author. Draft papers will be screened to make sure that they are within the scope of IFL, and will be accepted for presentation or rejected accordingly. Prior to the symposium: Authors of conditionally accepted papers and accepted presentations will submit a pre-proceedings version of their work that will appear in the draft proceedings distributed at the symposium. The draft proceedings does not constitute a formal publication. We require that at least one of the authors present the work at IFL 2019. After the symposium: Authors of conditionally accepted papers will submit a revised versions of their paper for the formal post-proceedings. The program committee will assess whether the mandatory revisions have been adequately addressed by the authors and thereby determines the final accept/reject status of the paper. Our interest is to ultimately accept all conditionally accepted papers. If you are an author of a conditionally accepted paper, please make sure that you address all the concerns of the reviewers. Authors of accepted presentations will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal post-proceedings. The program committee will evaluate these submissions according to their correctness, novelty, originality, relevance, significance, and clarity, and will thereby determine whether the paper is accepted or rejected. ### Publication The formal proceedings will appear in the International Conference Proceedings Series of the ACM Digital Library. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication ### Important dates Submission of regular papers: June 15, 2019 Submission of draft papers: August 1, 2019 Regular papers notification: August 1, 2019 Regular draft papers notification: August 7, 2019 Deadline for early registration: August 15, 2019 Submission of pre-proceedings version: September 15, 2019 IFL Symposium: September 25-27, 2019 Submission of papers for post-proceedings: November 30, 2019 Notification of acceptance: January 31, 2020 Camera-ready version: February 29, 2020 ### Submission details All contributions must be written in English. Papers must use the ACM two columns conference format, which can be found at: http://www.acm.org/publications/proceedings-template Authors submit through EasyChair: https://easychair.org/conferences/?conf=ifl2019 ### Peter Landin Prize The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. ### Organization and Program committee Chairs: Jurrien Stutterheim (Standard Chartered Bank Singapore), Wei Ngan Chin (National University of Singapore) Program Committee: - Olaf Chitil, University of Kent - Clemens Grelck, University of Amsterdam - Daisuke Kimura, Toho University - Pieter Koopman, Radboud University - Tamás Kozsik, Eötvös Loránd University - Roman Leschinskiy, Facebook - Ben Lippmeier, The University of New South Wales - Marco T. Morazan, Seton Hall University - Sven-Bodo Scholz, Heriot-Watt University - Tom Schrijvers, Katholieke Universiteit Leuven - Alejandro Serrano, Utrecht University - Tony Sloane, Macquarie University - Simon Thompson, University of Kent - Marcos Viera, Universidad de la República - Wei Ngan Chin, NUS - Jurriën Stutterheim, Standard Chartered Bank ### Venue The 31st IFL is organized by the National University of Singapore. Singapore is located in the heart of South-East Asia, and the city itself is extremely well connected by trains and taxis. See the website for more information on the venue. ### Acknowledgments This call-for-papers is an adaptation and evolution of content from previous instances of IFL. We are grateful to prior organizers for their work, which is reused here. A part of IFL 2019 format and CFP language that describes conditionally accepted papers has been adapted from call-for-papers of OOPSLA conferences.

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Re: [Haskell-cafe] What is the most modern associative list in types?
by Daniel Díaz 24 Jul '19

24 Jul '19

Hi Serguey, I don’t know what extra functionality you require but, for what is worth, my library red-black-record features a Symbol-to-Type map . It’s implemented as a type-level red-black-tree. https://hackage.haskell.org/package/red-black-record-2.0.2.2/docs/Data-RBR.… The map can be manipulated with the Insert, Delete and Value associated type families. Regards, Daniel Díaz > Hello cafe. > > I am trying to express some environment in types to constrain program > behavour. For this I need associative list of types, something along the > lines of [(Symbol, *)] so that I can lookup on "variable" name and get type > associated with it. > > I looked into HList library but it does not provide one most generally > useful way (albeit there are dozen or so of ways in there). > > So where can I get most modern HList-like library? Or what should I read > to get most modern HList myself?

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What is the most modern associative list in types?
by Serguey Zefirov 24 Jul '19

24 Jul '19

Hello cafe. I am trying to express some environment in types to constrain program behavour. For this I need associative list of types, something along the lines of [(Symbol, *)] so that I can lookup on "variable" name and get type associated with it. I looked into HList library but it does not provide one most generally useful way (albeit there are dozen or so of ways in there). So where can I get most modern HList-like library? Or what should I read to get most modern HList myself?

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