- Haskell-Cafe - Haskell.org

Vacancy: Functional Software Engineer
by Damian Nadales 21 Apr '17

21 Apr '17

Hello Cafe, I'm forwarding a job opening in Eindhoven, The Netherlands. """ We are looking for an experienced software developer with good GUI development skills. Proven knowledge on the principles and working of build, test and release management through experience with several tools is required. Part of the work will be in choosing an appropriate GUI build framework catering for the different partner environments. The Model Driven Development Tool is implemented in Haskell. However, no specific Haskell knowledge is required, but hands on experience with a modern functional language is. Please contact Koen.Schilders(a)enter-group.nl """ Best regards, Damian.

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Wow Monads!
by David McClain 21 Apr '17

21 Apr '17

It’s been about 15 years on/off since I first looked at Monads. This weekend I finally sat down and really learned what they are, how they work. I found what looks like the seminal paper on them by Phil Wadler: https://page.mi.fu-berlin.de/scravy/realworldhaskell/materialien/the-essenc… <https://page.mi.fu-berlin.de/scravy/realworldhaskell/materialien/the-essenc…> I’m a pretty heavy Common Lisp guy, going on 30 years with it. I also did tons of SML and OCaml programming. But I only dipped my toe into Haskell a few times. What I was looking for was a more in-depth understanding of Monads and how they work. I remember reading that Wadler paper many years ago, and I was intrigued by the conciseness of changing the interpreter to do different instrumentation. I was hoping to find a magic bullet like that for my Lisp code. And I noticed that Lisp almost never makes any mention of Monads. Surely there is a benefit that could be had… Anyone else have Lisp experience using Monads? Did it offer some major enhancements for you? - DM

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Why does `mzip /= liftM2 (,)`
by Mateusz Kowalczyk 20 Apr '17

20 Apr '17

Hi, Today after half a decade I have found `MonadZip` in `base`. The laws state: ``` liftM (f *** g) (mzip ma mb) = mzip (liftM f ma) (liftM g mb) liftM (const ()) ma = liftM (const ()) mb ==> munzip (mzip ma mb) = (ma, mb) ``` The question is why does ``` mzip /= liftM2 (,) ``` in plain words. Notably we can see lack of IO instance which I'm guessing has something to do asynchronous exceptions but I'm probably wrong…. Rather than investigating for next 30 minutes, I thought I'd just sample existing knowledge (that I could not find out from Google). In what circumstances can we not replace `liftM2 (,)` with `mzip`? -- Mateusz K.

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Problem with tuple with one element constrained
by Tyson Whitehead 20 Apr '17

20 Apr '17

I've been unable to get the following (simplified) code to be accepted by GHCi (8.0.1 and 8.0.2) Prelude> let (x, y) = (1, return 2) <interactive>:1:5: error: • Could not deduce (Monad m0) from the context: Num t bound by the inferred type for ‘x’: Num t => t at <interactive>:1:5-24 The type variable ‘m0’ is ambiguous • When checking that the inferred type x :: forall a (m :: * -> *) t. (Num a, Num t, Monad m) => t is as general as its inferred signature x :: forall t. Num t => t and I don't seem to be able to provide a type signatures that GHCi is okay with Prelude> let { x :: Int; y :: Monad m => m Int; (x, y) = (1, return 2) } <interactive>:5:40: error: • Ambiguous type variable ‘m0’ prevents the constraint ‘(Monad m0)’ from being solved. • When checking that the inferred type x :: forall a (m :: * -> *) t (m1 :: * -> *). (Num a, Num t, Monad m) => t is as general as its signature x :: Int Prelude> let (x, y) = (1, return 2) :: (Int, Monad m => m Int) <interactive>:4:31: error: • Illegal qualified type: Monad m => m Int GHC doesn't yet support impredicative polymorphism • In an expression type signature: (Int, Monad m => m Int) In the expression: (1, return 2) :: (Int, Monad m => m Int) In a pattern binding: (x, y) = (1, return 2) :: (Int, Monad m => m Int) Prelude> let (x,y) = (1,return 2) :: Monad m => (Int, m Int) <interactive>:7:5: error: • Ambiguous type variable ‘m0’ prevents the constraint ‘(Monad m0)’ from being solved. • When checking that the inferred type x :: forall (m :: * -> *). Monad m => Int is as general as its inferred signature x :: Int It seems the constraint on y leaks to x where it isn't well formed? Any help and/or explanations would be greatly appreciated. In my actual code the assignment is from the return value of a function so I can't just split it into two separate statements. Thanks! -Tyson

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Re: [Haskell-cafe] Problem with tuple with one element constrained
by Tyson Whitehead 20 Apr '17

20 Apr '17

On Tue, Apr 18, 2017, 16:02 Joshua Grosso <jgrosso1(a)asu.edu <mailto:jgrosso1@asu.edu>> wrote: /If/ I'm understanding correctly, GHCI's complaining about the fact that `return 2` requires it to create an instance of a `Monad`, but it doesn't know which `Monad` instance it should use... Does `let (x, y) = (1, return 2) :: (Int, Maybe Int)` work (where you can replace `Maybe` with any other monad)? Hi Joshua, Thanks for the input. You are correct that if I choose a particular Monad instance GHCi accepts the code. I believe it should be able to do the assignment without choosing one though. It is, for example, okay with let y = return 2. I'm thinking Sergiu is onto something about this having to do with special GHCi handling. Thanks! -Tyson

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Re: [Haskell-cafe] Haskell 2020: 'let' to be optional and with wider scope of visibility, like other Haskell functions
by Doug McIlroy 19 Apr '17

19 Apr '17

> It's just helping clean up a bit words that doesn't have much meaning. > Currently GHCi in GHC 8 is supporting this, so you could write both: > > x = 10 > x <- return 10 > > Which is great! I trust that "x = 10" above does NOT t mean the same as "let x = 10". If it did, then it wouldn't mean what it does in Haskell. Then code tried successfully in GHCI could fail in GHC. Compare this GHCI session > let x = 10 > :t x x :: Num a => a > x + 0.0 10.0 to this, in which x = 10 has its Haskell meaning > :! cat test.hs x = 10 > :load test.hs > :t x x :: Integer > x + 0.0 No instance for (Fractional Integer) arising from the literal ‘0.0’. Doug

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Re: [Haskell-cafe] Where do I start if I would like help improve GHC compilation times?
by Johannes Waldmann 19 Apr '17

19 Apr '17

> it would be quite helpful if `pretty` > gained a special-case for the infinite band-width case Absolutely +1 this. Not "pretty" but similar: https://github.com/jwaldmann/wl-pprint-text/commit/7843d96db57ab01c0be336a5… - J.W.

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Re: [Haskell-cafe] Wow Monads!
by David McClain 19 Apr '17

19 Apr '17

Hi Sergiu, Excellent point you make about type checking versus provably correct code. I probably allowed myself to be lulled by a false equivalence many times in the past, confusing one for the other under pressure to get something working. But then, how many programmers actually produce provably correct code? I can see that happening very easily in the small. But for overall large programs? - DM > On Apr 18, 2017, at 14:36, Sergiu Ivanov <sivanov(a)colimite.fr> wrote: > > > Hello David, > > Thanks a lot for sharing your opinions, I find it very interesting. > > Thus quoth David McClain at 19:26 on Mon, Apr 17 2017: >> >> That’s when I began migrating back over to my old standby Lisp >> system. I live inside of my Lisp all day long, for days on end. It is >> a whole ecosystem. There is not crisp boundary of edit / compile / >> debug. It is all incremental and extensional. I think that kind of >> environment, regardless of language, is the holy grail of computing. > > Just a small question: have you ever tried Smalltalk/Squeak/Pharo? (I'm > in no way affiliated.) > > These guys seem to be quite keen on blending the edit/compile/debug > boundaries. > > -- > Sergiu

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Re: [Haskell-cafe] Problem with tuple with one element constrained
by Justus Adam 18 Apr '17

18 Apr '17

I am not sure really what you are trying to do, but here are a few approaches which I think may solve your problem. 1. Calculate the monadic value outside or fmap over the tuple constructor for instance do x <- functionCall return (y, x) or (y, ) <$> functionCall (uses the TupleSections extension) 2. Use explicit forall and scoped type variables If you really want to return a tuple with that monadic value from a function then the `Monad m` constraint must be on the function itself. Like so Compiled with ExplicitForAll and ScopedTypeVariables myFunction :: forall m. Monad m => m ... myFunction = let x :: m SomeType (y, x) = (..., return ...) in ... Which binds `m` to be of the same type as the outer `m`. This is necessary as there must be some way that even from outside the function we can figure out what concretely `m` actually is. Ergo it must be bound in some way to the functions type signature. 3. ExistentialTypes If you __really__ think you only need a `Monad` constraint on this type (which I doubt) you can use an existential to wrap things into generic monads. (Uses ExistentialTypes) data ExtMonad a = forall m. Monad m => ExtMonad m a The thing I don't quite understand is, if you have a function call, which produces the value, doesn't that call return a concrete monad? Rather than just `Monad m`? It sounds to me like you're actually barking up the wrong tree when you try and do what you are doing here so I suggest using something like approach 1 or 2 and perhaps rethinking if your problem may lay somewhere else. Regards Justus -- Justus Adam dev(a)justus.science On Tue, Apr 18, 2017, at 12:41 PM, Tyson Whitehead wrote: > I've been unable to get the following (simplified) code to be accepted by > GHCi (8.0.1 and 8.0.2) > > Prelude> let (x, y) = (1, return 2) > <interactive>:1:5: error: > • Could not deduce (Monad m0) > from the context: Num t > bound by the inferred type for ‘x’: > Num t => t > at <interactive>:1:5-24 > The type variable ‘m0’ is ambiguous > • When checking that the inferred type > x :: forall a (m :: * -> *) t. (Num a, Num t, Monad m) => t > is as general as its inferred signature > x :: forall t. Num t => t > > and I don't seem to be able to provide a type signatures that GHCi is > okay with > > Prelude> let { x :: Int; y :: Monad m => m Int; (x, y) = (1, return 2) } > <interactive>:5:40: error: > • Ambiguous type variable ‘m0’ > prevents the constraint ‘(Monad m0)’ from being solved. > • When checking that the inferred type > x :: forall a (m :: * -> *) t (m1 :: * -> *). > (Num a, Num t, Monad m) => > t > is as general as its signature > x :: Int > > Prelude> let (x, y) = (1, return 2) :: (Int, Monad m => m Int) > <interactive>:4:31: error: > • Illegal qualified type: Monad m => m Int > GHC doesn't yet support impredicative polymorphism > • In an expression type signature: (Int, Monad m => m Int) > In the expression: (1, return 2) :: (Int, Monad m => m Int) > In a pattern binding: > (x, y) = (1, return 2) :: (Int, Monad m => m Int) > > Prelude> let (x,y) = (1,return 2) :: Monad m => (Int, m Int) > <interactive>:7:5: error: > • Ambiguous type variable ‘m0’ > prevents the constraint ‘(Monad m0)’ from being solved. > • When checking that the inferred type > x :: forall (m :: * -> *). Monad m => Int > is as general as its inferred signature > x :: Int > > It seems the constraint on y leaks to x where it isn't well formed? Any > help and/or explanations would be greatly appreciated. In my actual code > the assignment is from the return value of a function so I can't just > split it into two separate statements. > > Thanks! -Tyson > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post.

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Cross-compiling GHC
by Jan von Löwenstein 18 Apr '17

18 Apr '17

Hi, I would like to have an integer-simple GHC for Mac and Linux. As there is no official download for that I should probably build it myself. My CI server is running Linux containers though. When https://ghc.haskell.org/trac/ghc/wiki/Building/CrossCompiling says "build must equal host", does that mean I cannot produce a mac ghc with Linux ci server? How do others produce ghc for mac or Windows? Best Jan

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slow type level function
by Baojun Wang 18 Apr '17

18 Apr '17

Hello cafe, I tried to play with some type level natural numbers, and it seems type level function is quite slow, for instance: (full source) https://gist.github.com/wangbj/5939aa7a30c3d756d98f5b5775e162a6 data Z data S n class KnownNat n where natSing :: n -> Integer instance KnownNat Z where natSing _ = 0 instance KnownNat n => KnownNat (S n) where natSing _ = 1 + natSing (undefined :: n) natVal :: KnownNat n => n -> Integer natVal = natSing natSing doesn't seems to know how to optimize when KnownNat is very big (i.e: 10000), I tried Peano Add/Mul, and they are very slow to be really useful. Is there any ways to improve this? How fully dependent typed language such as Adga/Idris handle this, do they have the same performance issue? Thanks baojun

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Re: [Haskell-cafe] Wow Monads!
by David McClain 18 Apr '17

18 Apr '17

Your’e welcome… > On Apr 18, 2017, at 10:20, Bryan Richter <b(a)chreekat.net> wrote: > > On Tue, Apr 18, 2017 at 09:57:10AM -0700, David McClain wrote: >> <...> > > These posts of yours have been incredibly interesting to me, and I just > wanted to thank you for sharing them! I think my point about ecosystem programming is really about the importance of immediate feedback when crafting software. You need a powerful language so that you don’t get torpedoed with your fresh ideas with lots of mandatory scaffolding. In C-like languages the goal becomes one of writing a correct line of code. The strongly typed languages, of the FPL variety, don’t force that upon you. But you do get sidetracked by the challenge of getting that last bit of code to type check successfully. And if you aren’t already in an ecosystem with the language, even more distracted by the demands of the edit / compile cycle. Any distractions from your idea means that, if you happened to have chosen the wrong approach, you aren’t thinking properly about that aspect because the distractions have set up a nearer term goal for you to reach. We become like hamsters on a wheel, and we feel good after doing battle successfully. But the problem is, after all that, did it work the way you hoped? And without immediate feedback, even if you have some distractions that you must put up with, you are using an extended tape measure to flip the light switch on the opposite wall. I think Haskell does offer some ecosystems. I have one that I installed on my Mac a couple months ago called “Haskell for Mac” by Chakravarty. It seems a pretty nifty little environment, but I have only been in there for about an hour or two. Part of my own push for polygot creds. Another one that looks possibly interesting is Shen, formerly Qi. I did a fair amount with the old Qi language, even adding / correcting the compiler system to fit my own needs. Mastering the logic system is another challenge ahead of me. I was impressed at the implementation, and the equational reasoning, which I first saw back in the 1990’s with the publication of the SML Language. But right off the bat, Haskell will present the bigger challenge to me. There is a huge scaffold of type class hierarchy that comes in the base environment. Reminds me a bit of trying to learn the old MSVC system from yore… - DM

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containers: Efficiently concatenating nested maps (or should I look at another data structure)
by Oliver Charles 18 Apr '17

18 Apr '17

Hi all, First, a little context. I'm currently working on [1] a little OpenGL project - a Quake 3 map viewer in Haskell. For the most part, things are working out great, but the way I construct the scene is costing me a lot of alloction, and I wonder if I can do better. The OpenGL system is essentially a giant state machine. Whenever you issue a draw call ("draw these triangles"), OpenGL will do different things depending on what the state is. For example, if you have a texture bound, then the triangle will be textured, and if you have blending enabled, then the triangle might be blended into whatever was drawn previously. While this can be tricky to manage, Haskell does a good job of giving us tools to model state. The real spanner is that state transitions are not free. In the above example, the state transitions associated with binding a texture or enabling blending both cost time - and this time is variable depending on the state change. In this case, setting the currently bound texture is a very expensive operation, whereas setting the blending mode is less costly. For this reason, it's common in graphics programming to want to sort draw calls to minimise state changes. For example, we might draw all triangles with texture 1, then all triangles with texture 2 - rather than interleaving those draw calls. To model this in Haskell, I'm using a tree-like structure to group and express each state change [2]. To give you an example of what this looks like, I have: class RenderNode a m where draw :: a -> m () instance RenderNode (IO ()) IO where draw = id newtype BindTexture a = BindTexture (MonoidalIntMap a) [3] deriving (Functor, Monoid) instance (MonadIO m, RenderNode a m) => RenderNode (BindTexture a) m where draw (BindTexture t) = IM.foldrWithKey (\texture0 m next -> do glActiveTexture GL_TEXTURE0 glBindTexture GL_TEXTURE_2D (fromIntegral texture0) draw m next) (return ()) (getMonoidalIntMap t) newtype BlendMode a = BlendMode (MonoidalMap (GLuint, GLuint) a) [3] deriving (Functor, Monoid) blendMode :: (GLuint, GLuint) -> a -> BlendMode a blendMode srcDst = BlendMode . MonoidalMap . Map.singleton srcDst instance (MonadIO m, RenderNode a m) => RenderNode (BlendMode a) m where draw (BlendMode m) = Map.foldrWithKey (\(srcBlend, destBlend) child next -> do glBlendFunc srcBlend destBlend draw child next) (return ()) (getMonoidalMap m) I can then stack these nodes together to form a "render graph": newtype RenderGraph = RenderGraph (BindTexture (BlendMode (IO ()))) deriving (Monoid) The IO action at the leaf corresponds to a draw call to draw some triangles. Now that I've explained the basics of my rendering plan, I just need to tell you a little bit about Quake 3 maps. A Quake 3 map is essentially a big soup of "faces", where each face is a collection of triangles (that can be drawn in one call) and a set of associated state. Furthermore, each face is assigned a "cluster" (an Int), and we have a function that given a cluster gives us back a list of other clusters that we can "see" - this is visibility determination, or occlusion culling. I am hence modelling the map as an IntMap RenderGraph - each cluster maps to a RenderGraph of how to draw all the faces in that cluster, organised to minimise draw calls. Then, given some sort of visibility check, when I render I filter this IntMap to only the clusters that can be seen, and combine all the render graphs: IntMap.foldlWithKey' (\scene clusterNumber clusterFaces -> scene <> clusterFaces) mempty clusters In reality, this results in combining a few hundred-thousad very deeply nested structures, and naturally that's resulting in a fair bit of allocation pressure. I need to do this almost every frame, because as the camera moves, we might be able to see different clusters, so I don't think there's really much that I can cache. My actual final RenderGraph is Sort |> Cull |> MultiplePasses |> BindTexture |> SetUniform Bool |> SetDynamicUniform (M33 Float) |> AlphaFunc |> BlendMode |> DepthFunc where (|>) is Functor composition, and each of those functors is either a Map or IntMap. It seems to deteriorate to pairwise `mappend`, as you can see from the definition of mappend for MonoidalMap or MonoidalIntMap in [3]. Can anyone think of any obvious ways to rewrite this to have less allocation pressure? I'm of course open to other ways to batch operations, but there is something I find quite beautiful about shoving everything into Maps and mappending them. In the C world, the approach is usually to combine all the states into a single bitmask, and then sort that. That same approach might be possible here, but I'd like to keep the pick-and-choose compositional nature that I've got above. Thanks for reading this far, I hope my post has made sense. If you need clarification on anything, the source is below, and don't hesitate to ask me any questions! -- ocharles [1]: http://github.com/ocharles/hs-quake-3 [2]: https://github.com/ocharles/hs-quake-3/blob/master/RenderGraph.hs [3]: newtype MonoidalMap k a = MonoidalMap { getMonoidalMap :: Map k a } instance Functor (MonoidalMap k) where fmap f (MonoidalMap a) = MonoidalMap (fmap f a) instance (Monoid a, Ord k) => Monoid (MonoidalMap k a) where mempty = MonoidalMap Map.empty mappend (MonoidalMap a) (MonoidalMap b) = MonoidalMap (Map.unionWith mappend a b) mconcat = coerce . Map.unionsWith mappend . (coerce :: [MonoidalMap k a] -> [Map k a]) newtype MonoidalIntMap a = MonoidalIntMap { getMonoidalIntMap :: IntMap a } deriving (Functor) instance Monoid a => Monoid (MonoidalIntMap a) where mempty = MonoidalIntMap IM.empty mappend (MonoidalIntMap a) (MonoidalIntMap b) = MonoidalIntMap (IM.unionWith mappend a b) mconcat = coerce . IM.unionsWith mappend . (coerce :: [MonoidalIntMap a] -> [IntMap a])

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VSTTE 2017 - Second Call for Papers
by Andrei Paskevich 18 Apr '17

18 Apr '17

9th Working Conference on Verified Software: Theories, Tools, and Experiments (VSTTE) https://vstte17.lri.fr July 22-23, 2017, Heidelberg, Germany Co-located with the 29th International Conference on Computer-Aided Verification, CAV 2017 Important Dates * Abstract submission: Mon, Apr 24, 2017 * Full paper submission: Mon, May 1, 2017 * Notification: Mon, Jun 5, 2017 * VSTTE: Sat-Sun, Jul 22-23, 2017 * Camera-ready: Mon, Aug 21, 2017 Overview The goal of the VSTTE conference series is to advance the state of the art in the science and technology of software verification, through the interaction of theory development, tool evolution, and experimental validation. We welcome submissions describing significant advances in the production of verified software, i.e., software that has been proved to meet its functional specifications. Submissions of theoretical, practical, and experimental contributions are equally encouraged, including those that focus on specific problems or problem domains. We are especially interested in submissions describing large-scale verification efforts that involve collaboration, theory unification, tool integration, and formalized domain knowledge. We also welcome papers describing novel experiments and case studies evaluating verification techniques and technologies. Topics of interest for VSTTE include education, requirements modeling, specification languages, specification/verification/certification case studies, formal calculi, software design methods, automatic code generation, refinement methodologies, compositional analysis, verification tools (e.g., static analysis, dynamic analysis, model checking, theorem proving, satisfiability), tool integration, benchmarks, challenge problems, and integrated verification environments. Paper Submissions We accept both long (limited to 16 pages) and short (limited to 10 pages) paper submissions. Short submissions also cover Verification Pearls describing an elegant proof or proof technique. Submitted research papers and system descriptions must be original and not submitted for publication elsewhere. Each submission will be evaluated by at least three members of the Program Committee. We expect that one author of every accepted paper will present their work at the conference. Paper submissions must be written in English using the LNCS LaTeX format (http://www.springer.com/gp/computer-science/lncs/conference-proceedings-gui…) and must include a cogent and self-contained description of the ideas, methods, results, and comparison to existing work. Papers will be submitted via EasyChair at the VSTTE 2017 conference page (https://www.easychair.org/conferences/?conf=vstte2017) The post-conference proceedings of VSTTE 2017 will be published in the LNCS series. Authors of accepted papers will be requested to sign the copyright transfer form. A selection of best papers will be invited for publication in the Journal of Automated Reasoning. Invited Speakers * Christoph Weidenbach (Max Planck Institute for Informatics, Germany) * Santiago Zanella-Beguelin (Microsoft Research, UK) Program Committee * June Andronick (University of New South Wales, Australia) * Christel Baier (TU Dresden, Germany) * Sandrine Blazy (Université de Rennes 1, France) * Arthur Charguéraud (Inria, France) * Ernie Cohen (Amazon Web Services, USA) * Rayna Dimitrova (MPI-SWS, Germany) * Carlo A. Furia (Chalmers University of Technology, Sweden) * Arie Gurfinkel (University of Waterloo, Canada) * Hossein Hojjat (Rochester Institute of Technology, USA) * Marieke Huisman (University of Twente, Netherlands) * Bart Jacobs (KU Leuven, Belgium) * Rajeev Joshi (NASA Jet Propulsion Laboratory, USA) * Zachary Kincaid (Princeton University, USA) * Akash Lal (Microsoft Research, India) * Shuvendu Lahiri (Microsoft Research, USA) * Francesco Logozzo (Facebook, USA) * Peter Müller (ETH Zürich, Switzerland) * Jorge A. Navas (SRI International, USA) * Scott Owens (University of Kent, UK) * Andrei Paskevich (Université Paris-Sud, France), co-chair * Gerhard Schellhorn (Universität Augsburg, Germany) * Peter Schrammel (University of Sussex, UK) * Natarajan Shankar (SRI International, USA) * Mihaela Sighireanu (Université Paris-Diderot, France) * Julien Signoles (CEA LIST, France) * Michael Tautschnig (Queen Mary University of London, UK) * Tachio Terauchi (JAIST, Japan) * Oksana Tkachuk (NASA Ames Research Center, USA) * Mattias Ulbrich (Karlsruhe Institute of Technology, Germany) * Thomas Wies (New York University, USA), co-chair

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Req. for co-maintaining hs-libhdf5 and bindings-hdf5
by Marco Zocca 18 Apr '17

18 Apr '17

Dear all, I would like to help taking care of two libraries for HDF5. The author, James Cook (mokus0 on GitHub, CC'd here), didn't reply to a private email I sent about this on April 12. I hope he's doing well, by the way. HDF5 is a popular format for scientific datasets, and it would be a pity to see the Haskell bindings bitrot. I have previous experience building Haskell C bindings with HSC, c2hs and inline-c, and as co-maintainer I would, as first steps, add memory brackets and user-facing `exceptions`, enable continuous integration (Travis) and "stack-ify" the libraries. Hoping to hear soon from James and/or the Hackage trustees; All the best, Marco (ocramz)

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Call for participation: PLDI 2017 and co-located events
by Tobias Grosser 17 Apr '17

17 Apr '17

Call for participation: PLDI 2017 and co-located events PLDI is the premier forum in the field of programming languages and programming systems research, covering the areas of design, implementation, theory, applications, and performance. The co-located conferences take place in Barcelona, June 18-23, 2017. This year, PLDI is co-located with ECOOP, LCTES, DEBS, ISMM, Curry On and others. The conferences will take place at the Universitat Polytècnica de Catalunya in Barcelona, Spain. http://conf.researchr.org/home/pldi-2017 Registration is now open, please visit: https://regmaster4.com/2017conf/BARC17/register.php to register. The early registration rate ends on May 26th. The tentative program is available at: http://pldi17.sigplan.org/program/program-pldi-2017 PLDI will also hold an ACM Student Research Competition: http://pldi17.sigplan.org/track/pldi-2017-student-research-competition Co-located events: + ECOOP: European Conference on Object-Oriented Programming + DEBS: annual conference on Distributed Event-Based Systems + Curry On: conference on programming languages and emerging challenges in industry. + ISMM: International Symposium on Memory Management + LCTES: Languages, Compilers, and Tools for Embedded Systems Co-located workshops include: + ARRAY: Workshop on Libraries, Languages and Compilers for Array Programming + DSW: Deep Specifications in the Wild + FMS: Formal Methods for Security + IC: Workshop on Incremental Computing + MAPL: Machine Learning and Programming Languages + PLMW: Programming Languages Mentoring Workshop + SOAP: International Workshop on the State Of the Art in Java Program Analysis + WCIRE: Workshop for Compiler Infrastructure for Research and Education Additionally, there will be eight co-located tutorials: + Bug detection in JavaScript web apps using the SAFE framework + Building your own modular static analyzer with Facebook Infer + Engineering Static Analyzers with Soufflé + Graal: High Performance Compilation for Managed Languages + P4: Programming the Network Data Plane + Polyhedral Compilation + Refinement Types for Program Verification and Synthesis + Scala, LMS and Delite for High-Performance DSLs and Program Generators + WALA Hack-A-Thon + Writing Verified Programs in CakeML See the web site for a schedule and further details and links. For further updates, follow PLDI on the social media: Facebook: https://www.facebook.com/PLDIConf Twitter: https://twitter.com/PLDI Albert Cohen, PLDI 2017 General Chair Tobias Grosser, PLDI 2017 Publicity Chair

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Re: [Haskell-cafe] Wow Monads!
by David McClain 17 Apr '17

17 Apr '17

That was absolutely excellent !! Great video presentation by Douglas Crockford. It took me about 2 hours to slowly advance through it, stopping along the way to translate his JS into Lisp to try out the Monads. This is by far the clearest explanation of Monads that I have seen, from among at least 2 dozen different videos and blogs. Even clearer than that Wadler paper. I highly recommend viewing this presentation. As for the rest of them, I hearken to the words of another great scientist who once stated that: “If you can’t explain a problem to your grandmother so that she understands it, then you don’t really understand the problem yourself…" - DM >> On Apr 16, 2017, at 15:43, MarLinn <monkleyon(a)gmail.com> wrote: >> >> On 2017-04-16 03:13, John Wiegley wrote: >>> MD> Anyone else have Lisp experience using Monads? Did it offer some major enhancements for you? >>> >>> In Lisp, I don't think the abstraction buys you much, because all the functionality you need (state, exceptions, runtime, etc) are always available to all expressions. They become an excellent way of representing "extensible composition" in a pure language that otherwise could not allow those things, but I don't see that you need it for an untyped language like Lisp. >> >> May I present to you the lecture "Monads and Gonads" by Douglas Crockford (https://www.youtube.com/watch?v=b0EF0VTs9Dc) He shows that monads can make even a language like JavaScript useful (not his words). And the talk is one of the better monad tutorials out there to boot, not least because it's different from all the others. >> >> My own conclusion from it is that I would maybe even turn your argument around and claim that especially a language that has as few tools to keep programs sensible as JS can benefit greatly from the structure a monad provides. The benefits will be different for Lisp, but I imagine there might be some nice use cases as well. After all, structures are a great tool even if you're not forced to use them. ;) >> >> On another note: the more I work with monads and their brethren, the more I find myself thinking in terms of (a -> f b) functions instead of things like bind. Not only is it closer to the mathematical basis, but there's also the close relationship to lenses. I mention this because my feeling is that this type of function is a more usable puzzle piece in languages with limited syntax support. Especially if you also implement functors. But that's just unsubstantiated gut feeling. >> >> Cheers, >> MarLinn >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >

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Ensuring that blank-state has been handled in UIs?
by Saurabh Nanda 16 Apr '17

16 Apr '17

Hi Everyone, In most user-interfaces we need special-treatment for blank states (zero-item state). For example: * In case there are no customers, don't show an empty table, instead show the message "You seem to have no customers, why not start by creating one..." * In case there are no bookings, don't show an empty table, instead show the message, "No bookings here. Want to create one?" * In case there are no search results, don't show an empty table, instead show the message, "No items found. Undo your last filter?" In some cases, the one-item case also needs special treatment, but I'm unable to come with relatable examples at the moment. A lot of times the dev simply forgets about the blank state, and it is caught during QA, which results in a quick-fix on the following lines: if length item > 0 then displayTable else showBlankState if Data.Map.Strict.size map > 0 then displayTable else showBlankState if Data.Set.Ordered.size set > 0 then displayTable else showBlankState Is there a way to prevent this bug from the get-go? Can we use the type-system (or anything else) to enforce the dev to at least _think_ about the blank state? Obviously, the type system can't help us with _what_ needs to be done with the blank state, but at least it can _remind_ the dev at compile-time about handling blank states. -- Saurabh.

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ANN: projects available for adoption
by Simon Michael 16 Apr '17

16 Apr '17

Hi all, to increase focus on other things, I am offering the following projects for adoption. Are you interested in one of these ? Would you like to pick it up and make it shine ? Ideally you are a regular user of the project yourself, and will be a more active maintainer than I have been. Or, you might like to try being co-maintainer or release manager for a time. Let's talk! http://joyful.com/shelltestrunner - compares command-line programs or shell commands with expected output. http://hackage.haskell.org/package/quickbench - produces quick, readable comparative benchmarks of program invocations. http://hackage.haskell.org/package/rss2irc - an IRC bot that announces any RSS/Atom feed to IRC. Formerly used for hackagebot in #haskell. http://joyful.com/fungen - the oldest and easiest way to build cross platform, easy to install 2D games in haskell. Also: http://planet.darcs.net <http://planet.darcs.net/> - Darcs-related feed aggregator (Planet Venus) http://planet.squeak.org <http://planet.squeak.org/> - Squeak Smalltalk-related feed aggregators (Planet Venus) http://hub.darcs.net/simon/darcsum <http://hub.darcs.net/simon/darcsum> - emacs UI for darcs (think mini-magit) (elisp) http://zwiki.org <http://zwiki.org/> - a powerful wiki engine built on the Zope 2 platform. (Long shot, just putting it out there.) (python) Best! -Simon

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Jane Street Question
by Michael Litchard 15 Apr '17

15 Apr '17

Could those of you who have worked at Jane Street comment on how often a hiring round occurs? Once a quarter? Twice a year?

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Re: [Haskell-cafe] Downsides of the Prompt Monad
by Oleg 15 Apr '17

15 Apr '17

David Menendez wrote > It's worth noting that Prompt p is the free monad for a type constructor p, > just as Free f is the free monad for a Functor f. As such, when p is a > Functor, they are isomorphic. > data Free f a = Var a | Wrap (f (Free f a)) > newtype Prompt p a = > Prompt { unPrompt :: forall b. (forall i. p i -> (i -> b) -> b) -> (a -> b) -> b } > -- equivalently, Prompt p a = Done a | forall i. Prompt (p i) (i -> Prompt > p a) Indeed. The Freer monad paper, Sec 2.4, describes this correspondence in general: type FFree p = Free (Lan p) (FFree p is the alternative Prompt p in David's message) where Lan p is a (simple version) of the left Kan extension that turns any p :: * -> * into a functor. Once (Lan p) is a Functor, the standard free monad construction applies. The Freer monad paper then goes on to say that since the continuation is made explicit, it can be represented in a better way than just a function.

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Haskell plugin dynamic linking issue
by Lana Black 14 Apr '17

14 Apr '17

Hello, I'm working on a Haskell plugin (a shared library) for a C program. When I load the plugin, I get the following error: > ./open dlopen() failed: /usr/lib64/ghc-8.0.2/ghc-prim-0.5.0.0/libHSghc-prim-0.5.0.0-ghc8.0.2.so: undefined symbol: stg_thawArrayzh open is just a testing wrapper around dlopen(). I found out that stg_thawArrayzh symbol is exported by the rts, and my library isn't linked to it, and neither is ghc-prim. How do I fix that? Adding -lHSrts-ghc8.0.2 to ld-options doesn't seem to have any effect. Thanks.

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ANN: Nomyx 1.0, the only game where you can change the rules
by Corentin Dupont 14 Apr '17

14 Apr '17

After a loooooong wait... I am finally proud to announce Nomyx <http://www.nomyx.net> V1.0! Nomyx is a unique game where you can change the rules of the game itself while playing it. The major novelty in this version is that you can play even without knowing how to program. There is a library of rules that are ready to be proposed. Let's start a new game! You can login here: http://www.nomyx.net:8000/Nomyx The games are discussed on the mailing list: nomyx-game(a)googlegroups.com Here is a video introduction of the game: http://vimeo.com/58265498 I created a tutorial to learn how to play: http:// corentindupont.info/blog/posts/Programming/2014-09-23-Nomyx-Language.html Some background: this is an implementation of a Nomic [1] game in Haskell (I believe the first complete implementation of a Nomic game on a computer). At the beginning, the initial rules are describing: - how to add new rules and change existing ones. For example a unanimity vote is necessary to have a new rule accepted. - how to win the game. For example you win the game if you have 5 rules accepted. But of course even that can be changed! Cheers, Corentin [1] www.nomic.net

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looking for a file mentioned on haskell.org
by Michael Litchard 12 Apr '17

12 Apr '17

Can someone post/send a copy of the file p83.dat mentioned in the below link? https://wiki.haskell.org/99_questions/80_to_89

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RFC: Handling marginally bad data via logging "soft-errors"
by Saurabh Nanda 12 Apr '17

12 Apr '17

Cross-posted from https://www.reddit.com/r/haskell/comments/64ymup/rfc_handling_marginally_ba… Hi Everyone, I would like to hear some perspectives from people who have more experience in handling production code in Haskell. SCENARIO: You're loading data from a DB (or any other external source) and are expecting certain fields to be present. You end-up loading a record/row which has all the strictly necessary fields, but a not-so-necessary field is missing. Example: loading a full name, stored as the following HSTORE in PG: {'title' => '...', 'first_name' => '...', 'last_name' => '...'} For whatever legacy reasons, there is a possibility that some records may not have the title field, or may not have the last_name field. Now, the skies are not going to fall if you display the name with some components missing, so, you don't want to throw an error and terminate the entire operation. It's a "soft error" which you want to log, and proceed with a default value. OPTION 1: Use unsafePerformIO to avoid making an otherwise pure function, "impure", i.e. parseName :: HStore -> FullName -- uses unsafePerformIO to log soft-errors OPTION 2: Put the data transformation function in IO or the app-wide monad AppM because it will call the logging function, i.e. parseName :: HStore -> AppM FullName OPTION 3: Use a wrapper over Writer monad, called ErrorCollector, that forces the calling function to log while accessing the underlying value, i.e. parseName :: HStore -> ErrorCollector FullName logErrorAndExtract extraLoggingContext (parseName hstoreVal) THOUGHTS: -- The downside of Option 1 is that (a) it's unsafe, and (b) the log won't have surrounding context, for example it won't have the primary key of the row that was being loaded within which this error occurred. -- The downside of Option 2 is that it introduces AppM into the function signature, and even that log won't have enough surrounding context to help in debugging. -- The downside of Option 3 is that it pushes some logging responsibility to the calling functions, but gives the ability to put extra context when a soft-error occurs. How do others deal with this problem? -- Saurabh.

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ANN: HacFreiburg 2017, Friday 4 August - Sunday 6 August, Freiburg, Germany
by Stefan Wehr 12 Apr '17

12 Apr '17

Dear Haskellers, there is another Haskell Hackathon, taking place in Freiburg, Germany this year! When: Friday 4 August 2017 - Sunday 6 August 2017 Where: Faculty of Engineering, University of Freiburg, Germany (https://goo.gl/maps/ZcHWLpMPhtj) Info page: https://wiki.haskell.org/HacFreiburg2017 Organizers: Peter Thiemann, Alexander Thiemann, Stefan Wehr HacFreiburg is another Haskell Hackathon, where Haskell programmers from all around the world meet in Freiburg, Germany, to discuss, hack together and improve the Haskell infrastructure. We welcome all programmers interested in Haskell, beginners and experts! The Haskell Hackathon in Freiburg is part of the "Sommercampus 2017" (https://sommercampus.fachschaft.tf/en/) organized by the Faculty of Engineering at the University of Freiburg. Hope to see you there, Stefan (with Peter and Alex)

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Semantic Synchrony: some open-source knowledge graph software
by Jeffrey Brown 12 Apr '17

12 Apr '17

Esteemed Haskellers, Semantic Synchrony is an open-source knowledge graph server with an Emacs front end. It integrates with Chrome and Git. Git allows a graph to be selectively shared, as a collection of repositories. Subgraphs can be created and edited using indented plain text or Markdown; the latter allows nodes to serve double-duty as edge labels, by linking to them in section or subsection headings. The front end and server communicate using simple JSON. We are open to taking it in new directions. For hacking it, check out our invitation to coders[1]. For learning more about it, check out the wiki[2], which is thorough and friendly -- it even including videos. [1] github.com/synchrony/smsn-why/blob/master/invitations/to-coders.md [2] github.com/synchrony/smsn/wiki -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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install problems
by Gregory Guthrie 12 Apr '17

12 Apr '17

I tried to install HSBC and HSQL-sqlite3, Bot fail, And the error report is: >cabal install HDBC Resolving dependencies... Configuring old-locale-1.0.0.7... Configuring utf8-string-1.0.1.1... Building old-locale-1.0.0.7... Building utf8-string-1.0.1.1... Installed old-locale-1.0.0.7 Configuring old-time-1.1.0.3... Installed utf8-string-1.0.1.1 Failed to install old-time-1.1.0.3 Build log ( C:\Users\guthrie\AppData\Roaming\cabal\logs\old-time-1.1.0.3.log ): Configuring old-time-1.1.0.3... configure: WARNING: unrecognized options: --with-compiler checking for gcc... e:\Plang\Haskell Platform\8.0.2\mingw\bin\gcc.exe checking whether the C compiler works... no configure: error: in `/tmp/cabal-tmp-21488/old-time-1.1.0.3': configure: error: C compiler cannot create executables See `config.log' for more details But the path to gcc is correct, and gc works fine. I had a similar problem once, and the issue was that the autoconf script didn't allow paths with spaces, Odd since Haskell installs into "Haskell Package" on windows. That time I had to reconfigure my whole installation (eliminating the space character), and then install it, and then restore the paths to what they were. Is this an ongoing issue? Should I reconfigure Haskell to install differently?

3 2

Help with hspec-wai
by Ricardo Boccato Alves 12 Apr '17

12 Apr '17

Hello, I have been struggling with hspec-wai with what seems to me to be a simple problem: I want to create an Application using a parameter and access that parameter inside the test. The closer I found was this article: https://begriffs.com/posts/2014-10-19-warp-server-controller-test.html The part I am struggling is to access the connection inside the test, this gist is the closer I got. https://gist.github.com/boccato/e151d3a4e4f5d925b904126f80d8cb65 I know I am doing something very stupid and / or didn't understand something very basic, but I just can't see it by myself. Tks, Ricardo Boccato Alves.

1 0

[ANN] Karps-0.2.0 Haskell frontend and optimizer for Apache Spark's dataframes and datasets
by Karps 11 Apr '17

11 Apr '17

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How does GHC avoid "<stderr>: hPutChar: invalid argument (invalid character)"?
by Yuji Yamamoto 11 Apr '17

11 Apr '17

Hello Haskellers, Currently, I’m working on this issue <https://github.com/haskell/haddock/pull/566>, where haddock crashes when printing the Unicode “bullet character” <http://www.fileformat.info/info/unicode/char/2022/index.htm> on stderr whose character encoding is not UTF-8. In the beforementioned pull request, I just added hSetEncoding stderr utf8 as a quick-and-dirty workaround. But GHC actually doesn’t do so: GHC prints “?” instead of the bullet character when stderr is not Unicode-compatible. So, I believe there’s a better way to handle the case, and GHC knows it. Then, how does GHC detect the handle’s character encoding and convert incompatible characters (such as the bullet character) into “?” to avoid the error? I couldn’t get it by reading the source of GHC a bit. Thanks in advance!

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 11 Apr '17

11 Apr '17

Dear all, If you or one of your students recently completed a PhD in the area of functional programming, please submit the dissertation abstract for publication in JFP: simple process, no refereeing, deadline 30th April 2017. Please share! Best wishes, Graham Hutton ============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 30th April 2017 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year. As a service to the community, the Journal of Functional Programming publishes the abstracts from PhD dissertations completed during the previous year. The abstracts are made freely available on the JFP website, i.e. not behind any paywall. They do not require any transfer of copyright, merely a license from the author. A dissertation is eligible for inclusion if parts of it have or could have appeared in JFP, that is, if it is in the general area of functional programming. The abstracts are not reviewed. Please submit dissertation abstracts according to the instructions below. We welcome submissions from both the PhD student and PhD advisor/supervisor although we encourage them to coordinate. ============================================================ SUBMISSION: Please submit the following information to Graham Hutton <graham.hutton(a)nottingham.ac.uk> by 30th April 2017. o Dissertation title: (including any subtitle) o Student: (full name) o Awarding institution: (full name and country) o Date of PhD award: (month and year; depending on the institution, this may be the date of the viva, corrections being approved, graduation ceremony, or otherwise) o Advisor/supervisor: (full names) o Dissertation URL: (please provide a permanently accessible link to the dissertation if you have one, such as to an institutional repository or other public archive; links to personal web pages should be considered a last resort) o Dissertation abstract: (plain text, maximum 1000 words; you may use \emph{...} for emphasis, but we prefer no other markup or formatting in the abstract, but do get in touch if this causes significant problems) Please do not submit a copy of the dissertation itself, as this is not required. JFP reserves the right to decline to publish abstracts that are not deemed appropriate. ============================================================ PHD ABSTRACT EDITOR: Graham Hutton School of Computer Science University of Nottingham Nottingham NG8 1BB United Kingdom ============================================================ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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2nd CFP - Only Two Weeks Left: International Workshop on OCL and Textual Modeling (OCL 2017)
by Achim D. Brucker 10 Apr '17

10 Apr '17

(Apologies for duplicates) -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.ch | https://logicalhacking.com/blog @adbrucker | @logicalhacking

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Same code, system, but different arch using Win32 for reading registry.
by Akos Marton 10 Apr '17

10 Apr '17

Dear Haskellers, The setup: Having a binary compiled on either x86 or x64 system (same installation, utilizing stack) in order to preserve compatibility against x64 systems. Actually, not to have 2 separate executable for each, that would be the overall goal. However it surprises me when running the x86 .exe utilizing Win32-2.5.4.1 package with ghc-8.0.2, reading out a registry key fails on x64 system with the following:/ / /me.exe: RegOpenKey: invalid argument (The system cannot find the file specified.)/ That would be fine, however the key does exists. When same code, same system, but the .exe built to be x64 it runs like a charm. A couple of question, which some of them eventually will not make sense, but still: - Can it be ghc code optimization issue? - but this is a runtime check in IO, if so, how? - Yes, I could use a built-in windows system command and parse the input of that; unless absolutely necessary I would not introduce another dependency (system package). Would love to solve it with the currently utilized weapons. - the issue just puzzles me... I would know the answer if possible. - Is it more library (Win32), ghc, binary I generate, issue? What library can I use to detect a system's architecture which works in this scenario? Another thing which convoluted in the issue... The function, /getSystemInfo :: IO SYSTEM_INFO, /can read out the underlying architecture. When compiled on x86 and run on x64 it would tell me: "I am running on x86". That's failure. Most importantly: what is the obvious I am missing? Thank you for your insights! Best, Akos Ps.: Would you/we need sample code to puzzle about I can quickly weld one. - not sure if necessary.

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Call for Presentations: CUFP 2017, September 7-9, Oxford, UK
by Jasper Van der Jeugt 10 Apr '17

10 Apr '17

This CFP and the form for submitting presentations proposals can be found at: http://cufp.org/2017/call-for-presentations.html ------------------------------------------------------------------------ 2017 Call for Presentations Workshop for Commercial Users of Functional Programming 2017 Sponsored by SIGPLAN CUFP 2017 Co-located with ICFP 2017 Oxford, UK September 7-9 Talk Proposal Submission Deadline: 9 June 2017 The annual CUFP event is a place where people can see how others are using functional programming to solve real world problems; where practitioners meet and collaborate; where language designers and users can share ideas about the future of their favorite language; and where one can learn practical techniques and approaches for putting functional programming to work. ------------------------------------------------------------------------ Giving a CUFP Talk If you have experience using functional languages in a practical setting, we invite you to submit a proposal to give a talk at the event. We're looking for two kinds of talks: Retrospective reports are typically 25 minutes long. Now that CUFP has run for more than a decade, we intend to invite past speakers to share what they’ve learned after a decade spent as commercial users of functional programming. We will favour experience reports that include technical content. Technical talks are also 25 minutes long, and should focus on teaching the audience something about a particular technique or methodology, from the point of view of someone who has seen it play out in practice. These talks could cover anything from techniques for building functional concurrent applications, to managing dynamic reconfigurations, to design recipes for using types effectively in large-scale applications. While these talks will often be based on a particular language, they should be accessible to a broad range of programmers. We strongly encourage submissions from people in communities that are underrepresented in functional programming, including but not limited to women; people of color; people in gender, sexual and romantic minorities; people with disabilities; people residing in Asia, Africa, or Latin America; and people who have never presented at a conference before. We recognize that inclusion is an important part of our ission to promote functional programming. So that CUFP can be a safe environment in which participants openly exchange ideas, we abide by the SIGPLAN Conference Anti-Harassment Policy: http://www.sigplan.org/Resources/Policies/Anti-harassment If you are interested in offering a talk, or nominating someone to do so, please submit your presentation before 09 June 2017 via the CUFP 2017 Presentation Submission Form: https://goo.gl/forms/KPloANxHHwdiaoVj2 You do not need to submit a paper, just a short proposal for your talk. There will be a short scribe's report of the presentations and discussions but not of the details of individual talks, as the meeting is intended to be more of a discussion forum than a technical interchange. Nevertheless, presentations will be recorded and presenters will be expected to sign an ACM copyright release form. Note that we will need presenters to register for the CUFP workshop and travel to Oxford at their own expense. There are some funds available to would-be presenters who require assistance in this respect. ------------------------------------------------------------------------ Program Committee Alex Lang (Tsuru Capital), co-chair Rachel Reese (Mulberry Labs), co-chair Garrett Smith (Guild AI) Danielle Sucher (Jane Street) Jasper Van der Jeugt (Fugue) Yukitoshi Suzuki (Ziosoft) Evelina Gabasova (University of Cambridge) Brian Mitchell (Jet.com) ------------------------------------------------------------------------ More information For more information on CUFP, including videos of presentations from previous years, take a look at the CUFP website at http://cufp.org. Note that presenters, like other attendees, will need to register for the event. Acceptance and rejection letters will be sent out by July 15th. Guidance on giving a great CUFP talk Focus on the interesting bits: Think about what will distinguish your talk, and what will engage the audience, and focus there. There are a number of places to look for those interesting bits. Setting: FP is pretty well-established in some areas, including formal verification, financial processing, and server-side web services. An unusual setting can be a source of interest. If you're deploying FP-based mobile UIs or building servers on oil rigs, then the challenges of that scenario are worth focusing on. Did FP help or hinder in adapting to the setting? Technology: The CUFP audience is hungry to learn about how FP techniques work in practice. What design patterns have you applied, and to what areas? Did you use functional reactive programming for user interfaces, or DSLs for playing chess, or fault-tolerant actors for large-scale geological data processing? Teach us something about the techniques you used, and why we should consider using them ourselves. Getting things done: How did you deal with large-scale software development in the absence of pre-existing support tools that are often expected in larger commercial environments (IDEs, coverage tools, debuggers, profilers) and without larger, proven bodies of libraries? Did you hit any brick walls that required support from the community? Don't just be a cheerleader: It's easy to write a rah-rah talk about how well FP worked for you, but CUFP is more interesting when the talks also cover what doesn't work. Even when the results were all great, you should spend more time on the challenges along the way than on the parts that went smoothly.

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Does this function have a name?
by Paul Brauner 10 Apr '17

10 Apr '17

Hello Haskell Cafe, I had to write the following function and I was wondering if it (or its generalization to Alternative or Traversable) was exposed by some library under a different name: first [] = Nothing first (t:ts) = fmap (:ts) (s t) <|> fmap (t:) (first ts) Somehow it is to "traverse" as "any" is to "all". Cheers, Paul

3 5

Eliminating phantom type
by Timotej Tomandl 09 Apr '17

09 Apr '17

Hi, I have stumbled upon the Control-Monad-ST2 package by Kevin Backhouse, I have decided to expand the code with a run function and replace IO with ST s; and as of now I have removed the functions working on arrays and conversion to IO for the sake of simplicity. Here is the code in its modified form with newtype wrappers removed: {-# LANGUAGE RankNTypes #-} {-# LANGUAGE DeriveFunctor #-} -- Copyright 2013 Kevin Backhouse. -- Copyright 2017 Timotej Tomandl. Modifications import Data.STRef import Control.Monad.ST import Control.Applicative import Control.Monad newtype ST2 r w s a = ST2 { unwrapST2 :: ST s a } deriving (Functor) instance Monad (ST2 r w s) where return x = ST2 $ return x (ST2 m) >>= f = ST2 $ do x <- m unwrapST2 (f x) instance Applicative (ST2 r w s) where pure = return (<*>) = ap -- | This function checks that the sub-computation is polymorphic in -- both type parameters. This means that the sub-computation does not -- read or write any state from the enclosing context. {-# INLINE pureST2 #-} pureST2 :: (forall r w. ST2 r w s a) -> ST2 r' w' s a pureST2 m = m -- | This function checks that the computation is polymorphic in -- both parameters and then returns a pure value {-# INLINE runST2 #-} runST2 :: (forall r w s. ST2 r w s a) -> a runST2 m=runST $ unwrapST2 m -- | This function checks that the sub-computation is polymorphic in -- the @w@ type parameter. This means that the sub-computation does -- not write any state from the enclosing context (but read-only -- operations are permitted). {-# inline readOnlyST2 #-} readOnlyST2 :: (forall w. ST2 r w s a) -> ST2 r w' s a readOnlyST2 m = m -- | Mutable reference. 'ST2Ref' is actually just a newtype of an -- 'STRef', but the @r@ and @w@ type parameters allow the read and -- write dependencies to be tracked by the type system. newtype ST2Ref r w s a = ST2Ref (STRef s a) -- | Create a new reference. The @r@ and @w@ type parameters of the -- reference are unified with the 'ST2' monad to indicate that new -- state is created in the enclosing context. {-# INLINE newST2Ref #-} newST2Ref :: a -> ST2 r w s (ST2Ref r w s a) newST2Ref x = ST2 $ do r <- newSTRef x >>= \ var -> return var return (ST2Ref r) -- | Read a reference. The @w@ type parameter of the reference is not -- unified with the 'ST2' monad to indicate that this access is -- read-only. {-# INLINE readST2Ref #-} readST2Ref :: ST2Ref r w s a -> ST2 r w' s a readST2Ref (ST2Ref r) = ST2 $ readSTRef r -- | Write to a reference. The @w@ type parameter of the reference is -- unified with the 'ST2' monad to indicate that state is written in -- the enclosing context. {-# INLINE writeST2Ref #-} writeST2Ref :: ST2Ref r w s a -> a -> ST2 r w s () writeST2Ref (ST2Ref r) x = ST2 $ writeSTRef r x -- | Modify a reference. {-# INLINE modifyST2Ref #-} modifyST2Ref :: ST2Ref r w s a -> (a -> a) -> ST2 r w s () modifyST2Ref (ST2Ref r) f = ST2 $ modifySTRef r f But as you can see, now all my types got tainted by s even though sharing the same r or w implies sharing the same s. Is there a way how to hide s from the type signatures, but still preserve an ability to write runST2 without resorting to IO/RealWorld? I have tried writing such a function and failed. Timotej Tomandl

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State monad exit
by p75213＠gmail.com 08 Apr '17

08 Apr '17

Hi, I am playing around with the State monad and queues. At the moment I have the following code: {-# LANGUAGE ViewPatterns, FlexibleContexts #-} module Main where import Criterion.Main import Control.Monad.State.Lazy import Data.Maybe (fromJust) import Data.Sequence ((<|), ViewR ((:>))) import qualified Data.Sequence as S -------------------------------------------------------- data Queue a = Queue { enqueue :: [a], dequeue :: [a] } deriving (Eq, Show) -- adds an item push :: a -> Queue a -> Queue a push a q = Queue (a:enqueue q) (dequeue q) pop :: Queue a -> Maybe (a, Queue a) pop q = if null (dequeue q) then go $ Queue [] (reverse (enqueue q)) else go q where go (Queue _ []) = Nothing go (Queue en (x:de)) = Just (x, Queue en de) queueTst :: Int -> Queue Int -> Queue Int queueTst 0 q = q queueTst n q | even n = queueTst (n - 1) (push (100 + n) q) | otherwise = queueTst (n - 1) (if popped == Nothing then q else snd (fromJust popped)) where popped = pop q ------------------------------------------------------------- pushS :: a -> S.Seq a -> S.Seq a pushS a s = a <| s pushS' :: a -> State (S.Seq a) (Maybe a) pushS' a = do s <- get put (a <| s) return Nothing pushS'' :: a -> State (S.Seq a) (Maybe a) pushS'' a = get >>= (\g -> put (a <| g)) >> return Nothing popS :: S.Seq a -> Maybe (a, S.Seq a) popS (S.viewr -> S.EmptyR) = Nothing popS (S.viewr -> s:>r) = Just (r,s) popS' :: State (S.Seq a) (Maybe a) popS' = do se <- get let sl = popS'' se put $ snd sl return $ fst sl where popS'' (S.viewr -> S.EmptyR) = (Nothing, S.empty) popS'' (S.viewr -> beg:>r) = (Just r, beg) queueTstS :: Int -> S.Seq Int -> S.Seq Int queueTstS 0 s = s queueTstS n s | even n = queueTstS (n - 1) (pushS (100 + n) s) | otherwise = queueTstS (n - 1) (if popped == Nothing then s else snd (fromJust popped)) where popped = popS s queueTstST :: Int -> State (S.Seq Int) (Maybe Int) queueTstST n = if (n > 0) then if even n then pushS' (100 + n) >> queueTstST (n - 1) else popS' >> queueTstST (n - 1) else return Nothing main1 :: IO () main1 = defaultMain [ bench "Twin Queue" $ whnf (queueTst 550) (Queue [500,499..1] []) , bench "Sequence Queue" $ whnf (queueTstS 550) (S.fromList [500,499..1]) , bench "State Queue" $ whnf (runState (queueTstST 550)) (S.fromList [500,499..1]) ] -------------------------------------------------------------------------------------------------------------------------------------------------- In the function "queueTstST" is there a way to exit while retaining the last "Maybe value" rather than with "Nothing"?

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LambdaConf standard registration opens
by Salas Sanchez-Bennasar 07 Apr '17

07 Apr '17

http://lambdaconf.us/events/2017/lcusc.html LambdaConf is one of the largest functional programming conferences in the world, focused on foundational concepts (such as lambda calculus, type theory, category theory), functional programming languages (Haskell, Scala, PureScript, F#, Clojure, Elixir, and more), new languages and approaches in functional programming, functional programming libraries, and related areas such as dependent types, proof systems, parallel array programming, functional GPU processing, and more. Standard registration is now open at Eventbrite: https://www.eventbrite.com/e/lambdaconf-2017-registration-29839799644 A complete list of sessions is online, including the following highlights: * 6 keynotes, including one on using Haskell to model life on the blockchain, one on how programmers brains work, and one on the axes of abstraction; * 8 leap workshops, including an in-depth workshop on category theory, one on optics, and one on building front-ends using PureScript Halogen; * 12 hop workshops, including one introducing Coq, another diving into recursion schemes, and another teaching functional data processing; * 9 de novo sessions, including on one formally specified digital logic, one on algebraic and monadic composability for distributed computing, and one on type-level REST programming; * 21 educational sessions, including one on Free monads, one on high-performance Haskell, one on codata and corecursion, one on FP on Android using Frege, and another on dependently-typed programming in Haskell; * 21 inspire sessions, including one on dependent-pairs, one on generative design, and one on type singletons. This is more than 110 hours of content, with enough variety and difficulty level for everyone. Speakers are book authors, contributors to open source libraries, researchers, and practicing functional programmers. The conference takes place in Boulder, Colorado, from May 25 - 27, at the University of Colorado Boulder in the foothills of the Rocky Mountains. Conference registration includes all sessions (including workshops), locally-catered breakfast and lunch on Thursday, Friday, and Saturday; a conference dinner on Friday night; a choice from a variety of networking activities on Sunday; free childcare and STEM workshops for children; all-gender restrooms and handicapped-accessible venues. Videos from some previous events can be found here: https://www.youtube.com/ channel/UCEtohQeDqMSebi2yvLMUItg; and here: https://www.youtube.com/ watch?v=JxC1ExlLjgw&list=PLE7tQUdRKcybh21_zOg8_y4f2oMKDHpUS The conference is preceded by commercial training opportunities on Monday - Tuesday, including: * Up & Running with Elixir & Phoenix, by Brooklyn Zelenka * Applied Haskell, by Michael Snoyman * Advanced FP in Scala, by John A. De Goes * Introduction to FP, by David Koontz * Mastering Apache Spark, by Pawel Szulc * Mastering Elm, by Isaac Shapira On Wednesday, several free mini-conferences are co-located with LambdaConf: * PureScript Conf 2017 * Introduction to FP with Haskell For more information on LambdaConf, the commercial training workshops, or the mini-conferences, please visit the website: http://lambdaconf.us/events/2017/lcusc.html

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Downsides of the Prompt Monad
by Michael Litchard 07 Apr '17

07 Apr '17

Could someone outline for me the downsides of using the Prompt monad?

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[ANN] etc-0.0.0 Declarative configuration spec for Haskell projects
by Román González 07 Apr '17

07 Apr '17

Is my pleasure to announce a new library for managing configuration values on Haskell projects. Some features: * Have a versioned spec of all values your application can accept * Provides an API for gathering values from multiple sources (files, overwrite files, cli arguments, environment variables) and then composing them into a single configuration map * Gives a sane precedence over sources of the configuration value sources * Provides inspection utilities to understand why the configuration is the way it is * Provides an API that abstracts away the source of configuration values and allows easy casting into record types your application or other libraries understand * Supports both configuration files in JSON or YAML (when cabal flags are used) * Dynamically generates CLI inputs from spec configuration file Any feedback, or comments feel free to create a ticket or send me a tweet over at @romanandreg Cheers. Roman Gonzalez.-

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Re: [Haskell-cafe] What is a safe Haskell data type to store and manipulate Money values
by Doug McIlroy 07 Apr '17

07 Apr '17

> REFERENCES TO LITERATURE PLEASE Another domain with pertinent literature is proportional representation in legislative bodies. Sorry, I have no particular references to offer. Doug

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Package maintainership takeover: quickcheck-instances
by Oleg Grenrus 07 Apr '17

07 Apr '17

The quickcheck-instances provides QuickCheck type-classes' instances for Haskell Platform packages. Unfortunately, author and current maintainer Antoine Latter is quite unresponsive. There is handful of PR open [2], and I had to use Hackage Trustee powers to allow new versions of dependencies (to keep e.g. Stackage nightly going on). I have tried to contact him to offer a co-maintainership help (at 14.12.2016). Cheers, Oleg Grenrus - [1]: http://hackage.haskell.org/package/quickcheck-instances - [2]: https://github.com/aslatter/qc-instances/pulls

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What is a safe Haskell data type to store and manipulate Money values?
by Sandeep Cr 06 Apr '17

06 Apr '17

Hello all, What is a good data type for storing and manipulating monetary values in a typical web application with payment processing? What are the pros and cons of various options like Data.Scientific, Rational and Decimal?

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Re: [Haskell-cafe] [Haskell] Haskell Tools
by amindfv＠gmail.com 06 Apr '17

06 Apr '17

Moving this conversation to haskell-cafe! > El 6 abr 2017, a las 04:15, Boldizsár Németh <nboldi(a)caesar.elte.hu> escribió: > > Thanks for asking, I just added a note that you need to install the haskell-tools-daemon package to use the Atom plugin for Haskell Tools. > > If you want to run refactorings from a script or just without an editor, you can use haskell-tools-cli as well. > >> On 2017.04.06. 10:40, Ivan Lazar Miljenovic wrote: >> Is there support for anything except Atom at this time? >> >> I'm also unsure of what I need to install on my local machine if I >> want to try it out (do I need the daemon? or is the cli tool >> sufficient?). >> >>> On 6 April 2017 at 17:03, Boldizsár Németh <nboldi(a)caesar.elte.hu> wrote: >>> Dear Haskellers, >>> >>> I'm happy to announce a new tool for refactoring Haskell source code. >>> Check it out: http://haskelltools.org/ >>> >>> Best Regards, >>> Boldizsár Németh >>> _______________________________________________ >>> Haskell mailing list >>> Haskell(a)haskell.org >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell

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Re: [Haskell-cafe] [ANN] etc-0.0.0 Declarative configuration spec for Haskell projects
by Geraldus 06 Apr '17

06 Apr '17

Román González: > Was too excited delivering the newspaper that I forgot to add it to the > email: > > Github repo: https://github.com/roman/Haskell-etc >

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2nd call for papers: Trends in Functional Programming, 19-21 june 2017, University of Kent, Canterbury
by Peter Achten 05 Apr '17

05 Apr '17

----------------------------- C A L L F O R P A P E R S ----------------------------- ======== TFP 2017 =========== 18th Symposium on Trends in Functional Programming 19-21 June, 2017 University of Kent, Canterbury https://www.cs.kent.ac.uk/events/tfp17/index.html The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions (see below). Authors of draft papers will be invited to submit revised papers based on the feedback receive at the symposium. A post-symposium refereeing process will then select a subset of these articles for formal publication. TFP 2017 will be the main event of a pair of functional programming events. TFP 2017 will be accompanied by the International Workshop on Trends in Functional Programming in Education (TFPIE), which will take place on 22 June. The TFP symposium is the heir of the successful series of Scottish Functional Programming Workshops. Previous TFP symposia were held in * Edinburgh (Scotland) in 2003; * Munich (Germany) in 2004; * Tallinn (Estonia) in 2005; * Nottingham (UK) in 2006; * New York (USA) in 2007; * Nijmegen (The Netherlands) in 2008; * Komarno (Slovakia) in 2009; * Oklahoma (USA) in 2010; * Madrid (Spain) in 2011; * St. Andrews (UK) in 2012; * Provo (Utah, USA) in 2013; * Soesterberg (The Netherlands) in 2014; * Inria Sophia-Antipolis (France) in 2015; * and Maryland (USA) in 2016. For further general information about TFP please see the TFP homepage. (http://www.tifp.org/) == SCOPE == The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles: leading-edge, previously unpublished research work Position Articles: on what new trends should or should not be Project Articles: descriptions of recently started new projects Evaluation Articles: what lessons can be drawn from a finished project Overview Articles: summarizing work with respect to a trendy subject Articles must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include, but are not limited to: Functional programming and multicore/manycore computing Functional programming in the cloud High performance functional computing Extra-functional (behavioural) properties of functional programs Dependently typed functional programming Validation and verification of functional programs Debugging and profiling for functional languages Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. Interoperability with imperative programming languages Novel memory management techniques Program analysis and transformation techniques Empirical performance studies Abstract/virtual machines and compilers for functional languages (Embedded) domain specific languages New implementation strategies Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2017 program chairs, Scott Owens and Meng Wang. == BEST PAPER AWARDS == To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. == PAPER SUBMISSIONS == Acceptance of articles for presentation at the symposium is based on a lightweight peer review process of extended abstracts (4 to 10 pages in length) or full papers (20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which ALL authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. We use EasyChair for the refereeing process. Papers must be submitted at: https://easychair.org/conferences/?conf=tfp17 Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS web site: http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0 == INVITED SPEAKERS == Conor McBride University of Strathclyde (UK) Cătălin Hriţcu INRIA Paris (FR) == IMPORTANT DATES == Submission of draft papers: 5 May, 2017 Notification: 12 May, 2017 Registration: 11 June, 2017 TFP Symposium: 19-21 June, 2017 Student papers feedback: 29 June, 2017 Submission for formal review: 2 August, 2017 Notification of acceptance: 3 November, 2017 Camera ready paper: 2 December, 2017 == PROGRAM COMMITTEE == Co-Chairs Meng Wang University of Kent (UK) Scott Owens University of Kent (UK) PC Jeremy Yallop University of Cambridge (UK) Nicolas Wu University of Bristol (UK) Laura Castro University of A Coruña (ES) Gabriel Scherer Northeastern University (US) Edwin Brady University of St Andrews (UK) Janis Voigtländer Radboud University Nijmegen (NL) Peter Achten Radboud University Nijmegen (NL) Tom Schrijvers KU Leuven (BE) Matthew Fluet Rochester Institute of Technology (US) Mauro Jaskelioff CIFASIS/Universidad Nacional de Rosario (AG) Patricia Johann Appalachian State University (US) Bruno Oliveira The University of Hong Kong (HK) Rita Loogen Philipps-Universität Marburg (GE) David Van Horn University of Marylan (US) Soichiro Hidaka Hosei University (JP) Michał Pałka Chalmers University of Technology (SE) Sandrine Blazy University of Rennes 1 - IRISA (FR)

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Re: [Haskell-cafe] What is a safe Haskell data type to store and manipulate Money values?
by David Turner 04 Apr '17

04 Apr '17

On 4 Apr 2017 13:57, "David Turner" <dct25-561bs(a)mythic-beasts.com> wrote: > It's the currency conversions that will provide the most interesting > challenges in that list. If you were only working in a single currency then > something based on `Integer` would, I think, be fine. Perhaps `Int` or > `Int64` if you needed the extra speed. > > There are a number of different ways to implement multiple currencies > depending on your desired semantics and applicable accounting rules and > regulations. Peter Selinger has a good tutorial covering this at > http://www.mathstat.dal.ca/~selinger/accounting/tutorial.html > > One thing to watch out for: do you want to be able to do arithmetic on > amounts from different currencies? As in, do you want £5 + $5 to have a > meaningful result? If you can avoid this, life will be much simpler. > > > > On 4 Apr 2017 12:22, "Saurabh Nanda" <saurabhnanda(a)gmail.com> wrote: > >> Hi Manuel, >> >> Thank you for your reply. Some clarifications below... >> >> >>> It depends on what sort of computation you'll be doing upon those >>> monetary values and on the expectations of your users. It's typical >>> that payment processing applications sacrifice precision and instead >>> prefer to deal in exact quantities >> >> >> Is there a document which explains common use-cases for monetary values >> and best-practices for dealing with them? Any guidelines for which >> **Haskell** data-types to use for what kind of monetary calculations? >> >> Here is what we are doing in our app: >> >> * Powering web-based e-commerce checkout flows >> * Allowing store owners to input tax rates >> * Allowing end-customers to see product prices in different currencies >> (so, currency conversion) >> * Various reports to see total sales, total receivables, and total >> payables (basically a **very** small subset of small-business accounting) >> * Exporting data to full-fledged accounting systems, like Quickbooks and >> Xero. >> >> Based on this use-case, which Haskell data-type would you suggest? >> >> -- Saurabh. >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> >

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stack ignores my config.yaml?
by Alexey Egorov 03 Apr '17

03 Apr '17

Hello haskellers, I'm trying to use stack scripting capabilities, but it gives me an error (and suggestion to fix it by adding some lines to extra-deps in config.yaml). However, adding this lines to config.yaml doesn't change anything. My script and config are here - https://pastebin.com/PyRMQtqw Any help would be appreciated.

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HSpec & Monads
by Jan von Löwenstein 03 Apr '17

03 Apr '17

Hi, I have got a typeclass (MonadResource) that represents a client side rest resource and encapsulates http interaction with a server. Inside a `runResource` I want to do arbitrary calls to the server. I have a separate implementation running in `runResourceStub` that is a fake implementation allowing for tests that don't need a real server. I am unsure how to (best) integrate that with HSpec. ``` it "can create related backend resources" $ do runResourceStub $ do pod <- createPod "my-pod" ... secret <- createSecret... (getPod secret) `shouldBe` pod ``` That is what I would like to do. How do I do it? What else should I consider doing? Best Jan

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