August 2016 - Haskell-Cafe

ordering of structured values
by Paolino 19 Aug '16

19 Aug '16

Hello Cafè, I'd like to share a piece of code I've found useful to nest ordering on EQ. {-# language GADTs #-} import Data.Monoid (mconcat) import Data.Ord (comparing) data Orderings t where Ascending :: Ord a => (t -> a) -> Orderings t Descending :: Ord a => (t -> a) -> Orderings t comparings :: [Orderings t] -> t -> t -> Ordering comparings c x y = mconcat $ map g c where g (Ascending f) = comparing f x y g (Descending f) = comparing f y x I couldn't find a simpler solution, maybe somebody else has one. I've tried using comparings :: [forall a. Ord a => t -> a] -> t -> t -> Ordering which is not compiling for impredicativity and I'd like to understand better. An example on sorting (Ord a, Ord b, Ord c) => [(a,(b,c)] with 'a' in opposite order, then 'c' and 'b' in ascending order goes like sortBy (comparings [Descending fst, Ascending $ snd . snd, Ascending $ fst . snd]) Thanks for help/comments paolino

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[ANN] mysql-haskell 0.2.0.0
by 韩冬(基础平台部) 19 Aug '16

19 Aug '16

Hi all, I'm happy to announce mysql-haskell, a pure haskell mysql driver, is now available on package: https://github.com/winterland1989/mysql-haskell https://github.com/winterland1989/mysql-haskell The github page lists the motivation for creating this library, which are * prepared statment and binary protocol support. * better concurrency. * replication protocol support. All these targets are met. This release also includes quite complete tests and benchmarks, we think it’s stable enough to be used for pre-production environment, please fetch the code and testing, issues and bugs report are welcomed! Regards, Winterland@DIDI GROUP

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New online course "Functional Programming in Haskell" starts 19 Sept
by Wim Vanderbauwhede 18 Aug '16

18 Aug '16

We have created an introductory six-week online course (MOOC) on "Functional Programming in Haskell". The course will start on 19 September and is entirely free. We think it's a great way to get started with Haskell! https://www.futurelearn.com/courses/functional-programming-haskell The course is developed at Glasgow university's School of Computing Science and hosted by FutureLearn Enjoy! Wim Vanderbauwhede & Jeremy Singer @HaskellMOOC

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[ANN] ghc-typelits-knownnat-0.2: Solving GHCs KnownNat constraints
by Christiaan Baaij 17 Aug '16

17 Aug '16

Hi, If you have ever worked with `GHC.TypeLits`, then you have probably encountered an error very similar to: > • Could not deduce (KnownNat (n + 2)) > arising from a use of ‘natVal’ > from the context: KnownNat n i.e. where GHC cannot infer a `KnownNat (n+2)` constraint, even though a `KnownNat n` constraint is already given. Enter http://hackage.haskell.org/package/ghc-typelits-knownnat A type-checker plugin that can automatically derive "complex" KnownNat constraints from simpler ones. To use it, you simply add the {-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-} pragma to the top of your file. If you want to know more, you can also check out the corresponding blog posts: http://qbaylogic.com/blog/2016/08/10/solving-knownnat-constraints-plugin.ht… http://qbaylogic.com/blog/2016/08/17/solving-knownnat-custom-operations.html Regards, Christiaan

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(no subject)
by Corentin Dupont 17 Aug '16

17 Aug '16

Hi guys, is there some library with a Monad (+Applicative, Functor...) instance of the following type: data Todo a b = Todo [a] | Done b Thanks! Corentin

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[ANN] generic-random-0.2.0.0
by Li-yao Xia 16 Aug '16

16 Aug '16

Hello, This library aims to derive random generators for ADTs. The previous and first release was an implementation of Boltzmann generators, in response to this blogpost by Brent Yorgey. https://byorgey.wordpress.com/2016/03/23/boltzmann-sampling-for-generic-arb… It's nice to look at, but it is an opinionated choice about the resulting probability distribution which is not always suitable. In this release, using GHC.Generics, Generic.Random.Generic wraps common boilerplate to define "arbitrary" in particular, hiding from you long chains of the following: Constructor <$> arbitrary <*> arbitrary <*> arbitrary as well as frequency [(n1, C1 <$> arbitrary), (n2, C2 <$> arbitrary), ...] leaving just the weights [n1, n2, ...] for you to specify. data Tree a = Leaf a | Node (Tree a) (Tree a) deriving Generic instance Arbitrary a => Arbitrary (Tree a) where arbitrary = genericArbitraryFrequency [9, 8] -- equivalent to -- > arbitrary = frequency -- > [ (9, Leaf <$> arbitrary) -- > , (8, Node <$> arbitrary <*> arbitrary) -- > ] It comes with a simple opt-in strategy to bound the size of generated data, especially convenient for recursive data types. Indeed, even for a simple binary tree, the naive generator obtained by producing a leaf half of the times, and an inner node the other half, with two recursively generated children, results in a random tree of *infinite* expected size (though the tree is still "almost surely" finite). There is no theoretical reason the library uses two flavors of generics for different tasks. There are trade-offs, but I can't see impassable obstacles to use one in place of the other. If I had to be consistent now I'd choose GHC.Generics, though I was not fluent enough in that flavor at the time when I wrote the Data part. Property-based testing is the main application I have in mind for this library. Nevertheless, I welcome ideas, questions and suggestions from any domain or of general nature. Github: https://github.com/lysxia/generic-random Hackage: http://hackage.haskell.org/package/generic-random Li-yao

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Proposal: Rational read
by amindfv＠gmail.com 16 Aug '16

16 Aug '16

Is there a reason to not allow '.' as a separator for Rational's Read instance, so that both "1%2" and "0.5" can be read? Tom

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[ANN] aeson 1.0.0.0
by Adam Bergmark 16 Aug '16

16 Aug '16

Hi haskellers, I'm happy to announce that aeson v1.0.0.0 is now available on hackage! While 0.11.x was mostly regression fixes 1.x adds a lot of improvements that had been in the pipeline for a long time. We decided to move to 1.x since there are a lot of changes. Fortunately most of them are backwards-compatible so hopefully migration will be simple for you. Still, we recommend reading the changelog (https://github.com/bos/aeson/blob/master/changelog.md#1000) Thanks to all contributors of this release: * Adam Bergmark * Andrew Martin * Artyom * Ewout * Matthías Páll Gissurarson * Oleg Grenrus * Ryan Scott * lyxia Regards, Adam

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Help! Haskell Platform install broke System.Random.TF
by Jeremy Gibbons 16 Aug '16

16 Aug '16

I've just installed GHC 8.0.1, and it seems that my QuickCheck is now broken. I'm presuming that these two facts are connected. I have a trivially small QuickCheck example: > import Test.QuickCheck > prop_PlusAssociative :: Integer -> Integer -> Integer -> Bool > prop_PlusAssociative x y z = (x+y)+z == x+(y+z) This loads fine, but when I try to test the property I get a mysterious error: jg$ ghci quickcheck.lhs GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( quickcheck.lhs, interpreted ) Ok, modules loaded: Main. *Main> quickCheck prop_PlusAssociative can't load .so/.DLL for: /Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib (dlopen(/Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib, 5): Library not loaded: @rpath/libHSrandom-1.1-1z8Ujelqc6aKgvPnbRUKkP-ghc7.10.2.dylib Referenced from: /Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib Reason: image not found) I don't think it's anything to do with QuickCheck itself; I get the same error just trying to use the System.Random.TF library directly: jg$ ghci GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help Prelude> import System.Random.TF.Gen Prelude System.Random.TF.Gen> seedTFGen (1,1,1,1) can't load .so/.DLL for: /Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib (dlopen(/Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib, 5): Library not loaded: @rpath/libHSrandom-1.1-1z8Ujelqc6aKgvPnbRUKkP-ghc7.10.2.dylib Referenced from: /Library/Haskell/ghc-7.10.2-x86_64/lib/tf-random-0.5/libHStf-random-0.5-926BwEbiHzi7pBkp4dTxOe-ghc7.10.2.dylib Reason: image not found) The "7.10.2" is because I deactivated 8.0.1, then installed 7.10.3, then uninstalled both, then uninstalled all GHCs and deleted /Library/Haskell and reinstalled GHC 7.10.2 (my last working version) from what I hoped would be a clean slate. All to no avail. However, I do appear still to have a ghc-7.4.2, overlooked by uninstall-hs; and that does work fine - apparently not using the same System.Random.TF library. Can anyone suggest what I'm doing wrong, and more importantly, how to fix it? Jeremy Jeremy.Gibbons(a)cs.ox.ac.uk Oxford University Department of Computer Science, Wolfson Building, Parks Road, Oxford OX1 3QD, UK. +44 1865 283521 http://www.cs.ox.ac.uk/people/jeremy.gibbons/

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Proposal: Rational read
by Doug McIlroy 16 Aug '16

16 Aug '16

It has been suggested that (read "0.5" :: Rational) need not work because (show (0.5::Rational)) will never yield the string "0.5". In contrast, (read "1" :: Double) does work, even though (show (1::Double)) will never yield the string "1". The inconsistency is stark. Doug

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