June 2016 - Haskell-Cafe

Monad Stack - State + Rand?
by Christopher Howard 18 Jun '16

18 Jun '16

Hi. I'm working through "Haskell Design Patterns" and got inspired to try to create my first monad stack. What I really wanted though (not shown in the book) was to combine State and Rand. I daresay I got something to compile: walk :: RandomGen g => StateT (Float, Float) (Rand g) (Float, Float) walk = do (x, y) <- get put (x + 1, y + 1) get >>= return However, the moment I try to insert a getRandomR or something in it, I get an error Could not deduce (MonadRandom (StateT (Float, Float) (Rand g))) arising from a use of `getRandomR' <...snip...> add an instance declaration for (MonadRandom (StateT (Float, Float) (Rand g))) I see there are instances MonadRandom m => MonadRandom (StateT s m) RandomGen g => MonadRandom (Rand g) in Control.Monad.Random.Class, so I am not quite sure what is expected of me. -- http://justonemoremathproblem.com To protect my privacy, please use PGP encryption. It's free and easy to use! My public key ID is 0x340EA95A (pgp.mit.edu)

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INLINABLE and SPECIALIZE, GHC 8.0.1
by Jyotirmoy Bhattacharya 18 Jun '16

18 Jun '16

I'm trying to write a mutable hash table library that would work both in the IO and ST monads, so I'm using the PrimMonad typeclass [see JmoyHash.hs in the attached archive]. For efficiency, I would like the functions to be specialized to the concrete monad at the call site. According to Section 9.31.9.2 of the GHC User's Guide <https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_ext…> The optimiser *also* considers each *imported* INLINABLE overloaded function, and specialises it for the different types at which it is called in M. So I marked all the functions in my library as INLINABLE. Yet adding a SPECIALIZE pragma in Main.hs (currently commented out) for an imported function improves runtime performance by 3x, which should not be happening since, if I understand the manual right, the function should have been specialized anyway since it is marked INLINABLE. I am writing to this list rather than filing a bug since I'm not sure if I'm reading the manual right and I have not explicitly verified that the specialization is not happening. I would greatly appreciate any help on both counts. I'm using GHC 8.0.1 with the -O2 flag. Marking the function as INLINE might solve the problem but that's something I don't want to do as it seems to me that specialization and not inlining is what's appropriate here. Regards, Jyotirmoy Bhattacharya

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Macros For Cabal Version
by aditya siram 18 Jun '16

18 Jun '16

Hi all, I have a non-trivial build script that I'm trying to make compatible with Cabal 1.22 and Cabal 1.24. So far the only macro I can find that helps me determine which version I'm using is __GLASGOW_HASKELL__. Is there a better one? Thanks! -deech

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partial solved problem installing postgreSQL-libpq on Windows?
by Phyx 17 Jun '16

17 Jun '16

Hi Kees, Would It be possible for you to try with GHC 7.10.3 or 8.0.1? A bunch of things have changed with the linker in these two versions which should make linking against packages easier. <command line>: can't load .so/.DLL for: > d:/PROGRA~2/POSTGR~1/9.3/lib\libpq.dll (addDLL: could not load DLL) ghc.exe: d:/PROGRA~2/POSTGR~1/9.3/lib\libpq: %1 is not a valid Win32 > application. This seems to be coming from the runtime linker, if indeed d:/PROGRA~2/POSTGR~1/9.3/lib\libpq.dll can be found then either it's the wrong bitness/image type or libpq.dll has dependencies on other dlls. 7.10.1 is not able to load these then as the search path won't be correct. To test this you can add d:/PROGRA~2/POSTGR~1/9.3/lib\ to your path and try loading it again. If that doesn't work and you want to know what's really going on, you can enable loader snaps in gflags for GHC.exe and GHCi.exe (or runHaskell, what ever it is you're using) and attach a debugger like gdb or windbg. https://msdn.microsoft.com/en-us/library/windows/hardware/ff556886(v=vs.85)… You'll then be able to see the OS's trace messages for the loader and it should tell you exactly what's wrong. >From the msys prompt: pexports "d:\program > files\postgreSQL\9.3\bin\libpq.dll" > libpq.def >From the msys prompt: dlltool -d libpq.def -l libpq.a Import libraries are supported by the GHC but not ghci or any other interpreted mode (until 8.0.2) So this would only work for the compiled program without Template Haskell. Regards, Tamar

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Instances for (->) a (b :: * -> *)?
by lists+haskell-cafe＠jimpryor.net 17 Jun '16

17 Jun '16

If one wants to declare instances for a type schema `a -> _`, one writes something like this: instance Context => Class parameters ... ((->) a) where { ... } But what if you want to declare instances for a type schema `a -> b _`, that is, where the slot to be filled is not just the result of (->) a, but rather the composition of that together with another type operation b :: * -> *? I'd like to write something like this: instance Context => Class parameters ... ((->) a (b :: *->*)) where { ...} or: instance Context => Class parameters ... ((->) a (b *)) where { ...} but these don't work. Is it possible to do this? Concretely, here's what I'm trying to achieve. I wrote a typeclass for MonadReaders that are embedded inside (one level) of other MonadReaders, like this: {-# LANGUAGE FunctionalDependencies FlexibleInstances UndecidableInstances #-} class MonadReader w m => DeepMonadReader w r m | m -> r where { deepask :: m r ; deepask = deepreader id ; deeplocal :: (r -> r) -> m a -> m a ; deepreader :: (r -> a) -> m a ; deepreader f = do { r <- deepask; return (f r) } } instance MonadReader r m => DeepMonadReader w r (ReaderT w m) where { deepask = lift ask ; deeplocal = mapReaderT . local ; deepreader = lift . reader } It'd be nice to also provide an instance something like this: instance MonadReader r m => DeepMonadReader w r ((->) w (m :: * -> *)) where { deepask = \w -> ask ; deeplocal f xx = \w -> local f (xx w) ; deepreader xx = \w -> reader xx } -- Jim Pryor jim(a)jimpryor.net

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Trying to test natural transformations, in Haskell.
by David Banas 17 Jun '16

17 Jun '16

Hi all, In doing the challenge problems at the end of chapter 10 (Natural Transformations) in Bartosz Milewski’s “Category Theory for Programmers”, I’m trying to write a generic naturality checker: {-# LANGUAGE Rank2Types AllowAmbiguousTypes #-} type NatTran a = (Functor f, Functor f') => f a -> f' a to_assert :: (Functor f, Eq b) => (a -> b) -> NatTran a -> NatTran b -> f a -> Bool to_assert g h h' f = (fmap g . h) f == (h' . fmap g) f which is later made specific to a particular natural transformation: maybe_to_list :: Maybe a -> [a] maybe_to_list Nothing = [] maybe_to_list (Just x) = [x] test_func :: Num a => a -> (a, a) test_func x = (x, x + 1) assertions = map (to_assert test_func maybe_to_list) [Nothing, Just 1] but I’m getting this from ghc: Could not deduce (Functor f0) arising from a use of ‘fmap’ from the context (Functor f, Eq b) bound by the type signature for interactive:IHaskell465.to_assert :: (Functor f, Eq b) => (a -> b) -> interactive:IHaskell465.NatTran a -> interactive:IHaskell465.NatTran b -> f a -> Bool at :2:14-83 The type variable ‘f0’ is ambiguous Note: there are several potential instances: instance Monad m => Functor (Data.Vector.Fusion.Bundle.Monadic.Bundle m v) -- Defined in ‘Data.Vector.Fusion.Bundle.Monadic’ instance Functor Data.Vector.Fusion.Util.Box -- Defined in ‘Data.Vector.Fusion.Util’ instance Functor Data.Vector.Fusion.Util.Id -- Defined in ‘Data.Vector.Fusion.Util’ ...plus 27 others In the first argument of ‘(.)’, namely ‘fmap g’ In the expression: fmap g . h In the first argument of ‘(==)’, namely ‘(fmap g . h) f’ Can anyone offer some advice? Thanks, -db

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($) and ApplicativeDo
by amindfv＠gmail.com 16 Jun '16

16 Jun '16

foo :: Applicative f => f String foo = do x <- pure "this works" pure x ... but replace "pure x" with "pure $ x" and it doesn't typecheck: a monad instance is required! Tom

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Testing Msgpack system
by Jonn Mostovoy 16 Jun '16

16 Jun '16

Dear friends, we have a distributed system written in Haskell, consisting of three types of nodes with dozen of instances of each of two types and a central node of the third type. Each node is started by executing a binary which sets up acid-state persistence layer and sockets over which msgpack messages are sent around. It is trivial to write low level functionality quickcheck test suites, which test properties of functions. We would like, however, to have a quickcheck-esque suite which sets up the network, then gets it to an arbitrary valid state (sending correct messages between nodes), then rigorously testing it for three high-level properties: 1. Chaos monkey test (disable random node, see if certain invariants hold); 2. Evil node test (make several nodes work against the system, see if certain properties hold); 3. Rigorous testing of network-wide invariants, if all the nodes operate correctly. The problem we're facing is the following — if we want to inspect state of nodes in Haskell-land, we have to write a huge machinery which emulates every launch of node via thread. There will be bugs in this machinery, so we won't be able to get quality testing information before we fix those; if we want to run things as processes, then the best thing we can do is to inspect either acid-state dbs of each node (it poses resource locking problems and forces us to dump the state on every change, which is undesirable), or make an observer node, which dumps the consensus as Text and then parsing the data into Haskell terms, making decisions about the required properties based on that (so far, it feels like the best option). Am I missing something? How is something like this achieved in culture? How would you approach such a problem? Links to source files of test suites which do something similar are highly appreciated.

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base version numbers
by Tom Ellis 16 Jun '16

16 Jun '16

Since each version of GHC depends precisely on one version of base, would it not be more clear to name them after their version of GHC, rather than 4.x.y.z? Could this be done? Tom

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WST 2016, 2nd call for papers
by Salvador Lucas 16 Jun '16

16 Jun '16

========================================================================== WST 2016 - 2nd Call for Papers 15th International Workshop on Termination September 5-7, 2016, Obergurgl, Austria http://cl-informatik.uibk.ac.at/events/wst-2016/ ========================================================================== The Workshop on Termination (WST) traditionally brings together, in an informal setting, researchers interested in all aspects of termination, whether this interest be practical or theoretical, primary or derived. The workshop also provides a ground for cross-fertilization of ideas from term rewriting and from the different programming language communities. The friendly atmosphere enables fruitful exchanges leading to joint research and subsequent publications. The event is held as part of CLA 2016 http://cl-informatik.uibk.ac.at/events/cla-2016/ IMPORTANT DATES: * submission June 22, 2016 * notification July 12, 2016 * final version August 3, 2016 * workshop September 5-7, 2016 TOPICS: The 15th International Workshop on Termination welcomes contributions on all aspects of termination and termination analysis. Contributions from the imperative, constraint, functional, and logic programming communities, and papers investigating applications of complexity or termination (for example in program transformation or theorem proving) are particularly welcome. Topics of interest include all aspects of termination. This includes (but is not limited to): * certification of termination and complexity proofs * challenging termination problems * comparison and classification of termination methods * complexity analysis in any domain * implementation of termination and complexity methods * implicit computational complexity * infinitary normalization * non-termination analysis and loop detection * normalization in lambda calculi * operational termination of conditional rewrite systems * ordinal notation and subrecursive hierarchies * SAT, SMT, and constraint solving for (non-)termination analysis * scalability and modularity of termination methods * termination analysis in any domain * well-founded relations and well-quasi-orders COMPETITION: There will be a live complexity and termination competition during the workshop, including time to present both the results and the tools of the participants. More details will be provided in a dedicated announcement on the competition. PROGRAM COMMITTEE: * Ugo Dal Lago Bologna University * JÃ¶rg Endrullis VU University Amsterdam * Yukiyoshi Kameyama University of Tsukuba * Salvador Lucas Universidad PolitÃ©cnica de Valencia * Aart Middeldorp University of Innsbruck, co-chair * Andrey Rybalchenko Microsoft Research * Thomas StrÃ¶der RWTH Aachen * RenÃ© Thiemann University of Innsbruck, co-chair * Andreas Weiermann Ghent University INVITED SPEAKERS: * Reiner HÃ¤hnle TU Darmstadt SUBMISSION: Submissions are short papers/extended abstract which should not exceed 5 pages. There will be no formal reviewing. In particular, we welcome short versions of recently published articles and papers submitted elsewhere. The program committee checks relevance and provides additional feedback for each submission. The accepted papers will be made available electronically before the workshop. Papers should be submitted electronically via the submission page: http://www.easychair.org/conferences/?conf=wst2016 Final versions should be created using LaTeX and the LIPIcs style file http://drops.dagstuhl.de/styles/lipics/lipics-authors.tgz

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