- Haskell-Cafe - Haskell.org

Using stringize and string concatenation in ghc preprocessing
by Harendra Kumar 21 Aug '16

21 Aug '16

Hi, To reduce boilerplate code in an FFI implementation file I am trying to use the stringizing and string concatenation features of the C preprocessor. Since ghc passes '-traditional' to the preprocessor which disables these features I thought I can pass my own flags to the preprocessor like this: {-# OPTIONS_GHC -optP -E -optP -undef #-} But "-optP" seems to only append to the flags that GHC already passes and gcc has no "-no-traditional" option to undo the effect of the "-traditional" that GHC has already passed. I think "-optP" should override the flags passed by ghc rather than appending to them. Is there a reason not to do that? Is there any other better way to achieve this? What is the standard way of doing this if any? -harendra

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GHC-related project for undergraduate thesis
by Eugene Akentyev 21 Aug '16

21 Aug '16

Hello, I am an undergraduate student and would like to do something useful for GHC as undergraduate thesis. This list https://ghc.haskell.org/trac/ghc/wiki/ProjectSuggestions is outdated (last modified 6 years ago) and some projects are already done (?). Do you have any ideas/projects? Thanks.

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ordering of structured values
by Paolino 20 Aug '16

20 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 18 Aug '16

18 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 17 Aug '16

17 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 17 Aug '16

17 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 17 Aug '16

17 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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