- Haskell-Cafe - Haskell.org

Forcing the kind in data
by Michael Roth 26 Jul '16

26 Jul '16

Hi, if I have: data Foobar a b = Foobar it has kind: * -> * -> * How can I force the kind to: (* -> *) -> * -> * ? Thank you!

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Different behaviour with -XAllowAmbiguousTypes in 7.10.3b and 8.0.1
by Max Amanshauser 26 Jul '16

26 Jul '16

Hi, while porting a library to Haskell, which deals with persisting finite state automata to various stores, depending on the user's choice and the instances provided for types s(tate) e(vent) a(ction), I ran into different behaviour in ghc 7.10.3b and 8.0.1 related to ambiguity checks. This is a minimised and somewhat contrived example: > -- testme.hs > > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE AllowAmbiguousTypes #-} > > module TestMe where > > data MyStore = MyStore > > class FSMStore st s e a where > fsmRead :: st -> String -> s > > instance (Num s) => FSMStore MyStore s e a where > fsmRead st i = 23 > > get :: (FSMStore st s e a) => st -> String -> s > get st i = fsmRead st i With GHC 8.0.1 I get: > *TestMe> :l test.hs > [1 of 1] Compiling TestMe ( test.hs, interpreted ) > > test.hs:19:12: error: > • Could not deduce (FSMStore st s e0 a0) > arising from a use of ‘fsmRead’ > from the context: FSMStore st s e a > bound by the type signature for: > get :: FSMStore st s e a => st -> String -> s > at test.hs:18:1-47 > The type variables ‘e0’, ‘a0’ are ambiguous > Relevant bindings include > st :: st (bound at test.hs:19:5) > get :: st -> String -> s (bound at test.hs:19:1) > These potential instance exist: > instance Num s => FSMStore MyStore s e a > -- Defined at test.hs:12:10 > • In the expression: fsmRead st i > In an equation for ‘get’: get st i = fsmRead st i > Failed, modules loaded: none. However, when I remove the type signature for get: > Prelude> :l test.hs > [1 of 1] Compiling TestMe ( test.hs, interpreted ) > Ok, modules loaded: TestMe. > *TestMe> :t get > get :: FSMStore st s e a => st -> String -> s > *TestMe> get MyStore "asdf" > 23 GHC inferred the exact same type I provided, but this time it compiles successfully. When going back to GHC 7.10.3b, without type signature: > GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help > :Prelude> :l test.hs > [1 of 1] Compiling TestMe ( test.hs, interpreted ) > > test.hs:19:1: > Could not deduce (FSMStore st s e0 a0) > from the context (FSMStore st s e a) > bound by the inferred type for ‘get’: > FSMStore st s e a => st -> String -> s > at test.hs:19:1-23 > The type variables ‘e0’, ‘a0’ are ambiguous > When checking that ‘get’ has the inferred type > get :: forall st s e a. FSMStore st s e a => st -> String -> s > Probable cause: the inferred type is ambiguous > Failed, modules loaded: none. So my questions are: *) What's with the different behaviour depending on whether the type sig is inferred or provided? *) Is GHC 7.10.3b or 8.0.1 closer to the correct behaviour w.r.t. -XAllowAmbiguousTypes? -- Regards, Max Amanshauser.

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Use GHC API in standalone executable?
by Phyx 26 Jul '16

26 Jul '16

I guess this depends on what you want to do. I've seen various level of GHC API usage in standalone applications, but it just depends on how much of the compiler pipeline you want to use. Their are ways to override the paths or provide stubs for programs ghc expects to be there. I've packaged API calls into shared libraries myself before, in that case I was mostly accessing the compiler frontend.

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Is there any way in GHC plugin to refer exposed but defined in a hidden module?
by Hiromi ISHII 25 Jul '16

25 Jul '16

Hi Cafe, I'm currently writing a simple GHC Typechecker plugin to augment typelevel naturals with a presburger arithmetic solver [^1]. I want to use type-class constraints to give premisses to the solver, for example, `Empty a` constraint means "a is false (empty type)". So I have to get TyCon information in TcPluginM monad and I wrote as follows: ```haskell do md <- lookupModule (mkModuleName "Proof.Propositional.Empty") (fsLit "equational-reasoning") classTyCon <$> (tcLookupClass =<< lookupOrig md (mkTcOcc "Empty")) ``` But this code doesn't work as expected. For example, the code ```haskell {-# LANGUAGE DataKinds, TypeOperators, GADTs, TypeFamilies, ExplicitForAll, FlexibleContexts #-} import Data.Type.Equality import GHC.TypeLits (type (+), type (<=), type (<=?)) import Proof.Propositional (Empty(..)) predSucc :: Empty (n :~: 0) => proxy n -> (n + 1 <=? n + n) :~: 'True predSucc _ = Witness ``` resulted in the following compile-time error: ``` Can't find interface-file declaration for type constructor or class equational-reasoning-0.4.0.0:Proof.Propositional.Empty Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error ``` The probem is, Empty class is provided by separate existing package [^2], but is once defined in hidden module and re-exported from exposed from exposed module. If I expose the module where Empty is originally defined and change the module name passed to lookupModule appropriately, then the first code becomes working as expected. So the question: is there any way to retrieve TyCon information defined in hidden module but exported? [^1]: https://github.com/konn/ghc-typelits-presburger/ [^2]: https://github.com/konn/equational-reasoning-in-haskell -- Hiromi ISHII konn.jinro(a)gmail.com Doctoral program in Mathematics, University of Tsukuba

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linking dlls with relative paths on Windows
by Phyx 25 Jul '16

25 Jul '16

Hi Peter, I forgot to respond to this, if you haven't received an answer to this yet then feel free to open a ticket on the ghc trac and I will take a look as soon as I have working Internet again. I don't know what the status is of this for now, the linked to stackoverfow article should work though. Also 7.10.3 has some issues when loading dependent dlls in the same folder as the main one being loaded when not on the search path. 8.0.1 should be fine. Kind regards, Tamar

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Munich Haskell Meeting, 2016-07-27 @ 19:30 Augustiner Bierhalle
by Christian Neukirchen 25 Jul '16

25 Jul '16

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Wednesday, July 27 at Augustiner Bierhalle (Neuhauser Str.) 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-2016/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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pattern match on forall'ed data
by Corentin Dupont 22 Jul '16

22 Jul '16

Hi all, I'm surprised this doesn't work: data SomeData = forall e. (Typeable e, Eq e) => SomeData e (===) :: (Typeable a, Typeable b, Eq a, Eq b) => a -> b -> Bool (===) x y = cast x == Just y test :: SomeData' -> Bool test (SomeData' e) | e === Nothing = True test _ = False It says Could not deduce (Eq a1) arising from a use of ‘===’ How can I achieve something of the same effect? Thanks Corentin

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Re: [Haskell-cafe] Practical use of Stream's monad instance?
by Olaf Klinke 20 Jul '16

20 Jul '16

As someone else on this list whose name I don't recall put it, there is no choice on whether to make Stream a monad or not. It simply _is_ a monad. Anyone with some CS education hearing 'diagonal of inifinite list of infinite lists' should immediately think of Georg Cantor. import Data.Stream import Data.Ratio type Real = Stream Rational -- approximate a real number by an ascending stream -- of lower bounds. supremum :: Stream Real -> Real supremum = join -- If a stream of reals is index-wise ascending, -- the monad instance for Stream computes its supremum. -- Use this e.g. to compute any mathematical quantity -- defined as a supremum. -- Olaf

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Re: [Haskell-cafe] [Haskell] Function to replace given element in list
by amindfv＠gmail.com 20 Jul '16

20 Jul '16

(Moving to haskell-cafe@ -- haskell@ is only for announcements) > El 19 jul 2016, a las 19:44, David Feuer <david.feuer(a)gmail.com> escribió: > > A zipper is a good way to separate the search from the replacement. But the problem at hand does not require such a separation! If you don't need that, a zipper seems like overkill. > > > On Jul 19, 2016 7:39 PM, "Carl Folke Henschen Edman" <carledman(a)gmail.com> wrote: > On Tue, Jul 19, 2016 at 6:13 PM, David Feuer <david.feuer(a)gmail.com> wrote: >> Using a zipper will not get you very far here. The best way would >> likely be to replace the list with a balanced search tree. > > > That depends on the pattern of access and usage. For some a zippered list will outperform a self-balancing tree and vice versa. For others a zippered tree, or something else, will beat either. But when seeing the pattern of changing a single element in the middle of a list, a zippered list is the first improved data structure that comes to mind. > > > Carl Edman > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell

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Local Haskell talk in Leipzig, 27 July 2016 at 3:30pm in HTWK
by Heinrich Apfelmus 20 Jul '16

20 Jul '16

Dear (filter (`livesInOrCloseTo` Leipzig_Germany) haskellers), we would like to invite you to a talk by Stephen Blackheath "They why and how of Functional Reactive Programming with Java/Sodium" on Wednesday, 27 July 2016, at 15:30 Uhr in Room Gu 102, Gutenberg-Bau, HTWK Leipzig Abstract: <https://portal.imn.htwk-leipzig.de/termine/vortrag-the-why-and-how-of-funct… Geographical coordinates: <http://www.openstreetmap.org/?mlat=51.31248&mlon=12.37498#map=18/51.31248/1…> Afterwards (~ 17:00 Uhr) there will be a get-together in the Cafe Suedbrause <http://suedbrause.de>. Stop Listening! Johannes Waldmann and Heinrich Apfelmus

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