May 2013 - Haskell-Cafe

An APL library for Haskell
by Simon Peyton-Jones 16 Sep '13

16 Sep '13

Friends Many of you will know the array language APL<http://en.wikipedia.org/wiki/APL_%28programming_language%29>. It focuses on arrays and in particular has a rich, carefully-thought-out array algebra. An obvious idea is: what would a Haskell library that embodies APL's array algebra look like? In conversation with John Scholes and some of his colleagues in the APL community a group of us developed some ideas for a possible API, which you can find on the Haskell wiki here: http://www.haskell.org/haskellwiki/APL However, we have all gone our separate ways, and I think it's entirely possible that that the idea will go no further. So this message is to ask: * Is anyone interested in an APL-style array library in Haskell? * If so, would you like to lead the process of developing the API? I think there are quite a few people who would be willing to contribute, including some core gurus from the APL community: John Scholes, Arthur Whitney, and Roger Hui. Simon

5 5

Haskell / Functional Programmers Group in Madrid (Spain)?
by Alejandro Serrano Mena 04 Sep '13

04 Sep '13

Hi, Is there any Haskell or functional programmers user group in the region of Madrid? If not, I think it could be a great idea to get to know each other, and share experiences. Furthermore, we could try to organize some meetings with presentations, hackatons and so on (my personal ambition is to be able to organize another "ekmett Workshop" as the Japanese one - http://comonad.com/reader/2013/japanese-workshop-1/) I know there is some locations along Madrid dedicated to open source promotion, like http://madridonrails.com/, and which we could use for these meetings. I'm really looking forward to hearing from you! PS: Maybe a bare "Haskell" group is not big enough (at least according to Haskellers.com) but a "functional programmers" one may well be.

5 4

Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
by Henk-Jan van Tuyl 03 Aug '13

03 Aug '13

On Mon, 16 Jul 2012 00:03:49 +0200, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> wrote: > > I am willing to do administrator tasks. > >> 4. ReCAPTCHA enabled for 'edits adding new, unrecognized external >> links' - which is all of the spam. > > This is already enabled. The HaskellWiki is still flooded with spam; we should take some measure to reduce the stream severely. Most spam seems to be created (semi-)automated; the pages do not contain links, the usernames end with two digits, most of the time. Some cures I have thought up: - Verify new wiki accounts, before granting them rights, based on e-mails in the Haskell mailing lists (or subscription of a Haskell mailing list) - Let new users only change pages, not create new pages - Block creation of usernames o ending with two or more digits o with more than one x or q o starting with "buy" o longer than 20 characters o with more than 4 consonants in a row - Block creation of pages with words in a certain list (Coach, Vuitton, Chanel, handbags, purses, outlet, luggage, "Nike Air Jordan") Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

9 13

Monad Transformer Space Leak
by Clark Gaebel 19 Jul '13

19 Jul '13

Hi everyone! For some reason, this leaks thunks: module Main where import Control.Monad import Control.Monad.MC -- from monte-carlo import Control.Monad.ST.Strict go :: Int -> MCT (ST s) () go k = replicateM_ k (return ()) main = print $ runST $ evalMCT (go 100000000) rng where rng = mt19937 0 while this does not: module Main where import Control.Monad import Control.Monad.MC go :: Int -> MC () go k = replicateM_ k (return ()) main = print $ evalMC (go 100000000) rng where rng = mt19937 0 Can anyone help me figure out what's going on here? Thanks, - Clark

5 11

Re: [Haskell-cafe] Optimization flag changing result of code execution
by Azeem -ul-Hasan 18 Jul '13

18 Jul '13

I am using GHC 7.6.1 mwc-random 0.12.0.1 vector 0.9.1 primitive 0.4.1 Azeem On 16.03.2013 13:31, Azeem -ul-Hasan wrote: > Nope that isn't the case either. Even if I make use of defaultSeed > through create the problem still remains. The problem seems to be in the > generation of a vector of (a,a) i.e in the part > > V.generateM ((round $ p*(fromIntegral $ l*z)) `div` 2) (\i-> R.uniformR > ((0,0) , (l-1,l-1)) gen) > > in line 16. Thanks again. > I've tried to run you program and I've got approximately same results regardless of optimization level. Which versions of GHC, mwc-random, vector and primitive do you use?

5 10

Non-deterministic behaviour of aeson's parser
by Roman Cheplyaka 10 Jun '13

10 Jun '13

I am observing a non-deterministic behaviour of aeson's parser. I'm writing here in addition to filing a bug report [1] to draw attention to this (pretty scary) problem. To try to reproduce this problem, do this: git clone https://gist.github.com/5604887.git aeson cd aeson ghc aeson.hs ./aeson | sort | uniq -c This is my result: 32 Left "key \"module\" not present" 55 Left "When parsing the record SymValue of type Main.SymValueInfo the key fixity was not present." 1913 Right () Can others reproduce this in their environments? Does anyone have ideas about where the bug may lie? Many aeson's dependencies do unsafe IO stuff that could lead to such consequences. Roman [1]: https://github.com/bos/aeson/issues/125

6 8

Re: [Haskell-cafe] [haskell.org Google Summer of Code 2013] Approved Projects
by Dominic Steinitz 05 Jun '13

05 Jun '13

Hi Edward, Although the project I am interested in (as a user) has been accepted :-), I can't help feeling the selection process is a bit opaque. Is it documented somewhere and I just missed it? Apologies if I did. BTW I appreciate all the hard work that goes into the selection process. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

3 6

Two GHC-related GSoC Proposals
by Patrick Palka 02 Jun '13

02 Jun '13

Hi, I'm interested in participating in the GSoC by improving GHC with one of these two features: 1) Implement native support for compiling modules in parallel (see #910<http://hackage.haskell.org/trac/ghc/ticket/910>). This will involve making the compilation pipeline thread-safe, implementing the logic for building modules in parallel (with an emphasis on keeping compiler output deterministic), and lots of testing and benchmarking. Being able to seamlessly build modules in parallel will shorten the time it takes to recompile a project and will therefore improve the life of every GHC user. 2) Improve existing constant folding, strength reduction and peephole optimizations on arithmetic and logical expressions, and optionally implement a core-to-core pass for optimizing nested comparisons (relevant tickets include #2132 <http://hackage.haskell.org/trac/ghc/ticket/2132>, #5615 <http://hackage.haskell.org/trac/ghc/ticket/5615>,#4101<http://hackage.haskell.org/trac/ghc/ticket/4101>). GHC currently performs some of these simplifications (via its BuiltinRule framework), but there is a lot of room for improvement. For instance, the core for this snippet is essentially identical to the Haskell source: foo :: Int -> Int -> Int -> Int foo a b c = 10*((b+7+a+12+b+9)+4) + 5*(a+7+b+12+a+9) + 7 + b + c Yet the RHS is actually equivalent to 20*a + 26*b + c + 467 And: bar :: Int -> Int -> Int bar a b = a + b - a - b -- the RHS is should be optimized away to 0 Other optimizations include: multiplication and division by powers of two should be converted to shifts; multiple plusAddr calls with constant operands should be coalesced into a single plusAddr call; floating point functions should be constant folded, etc.. GHC should be able to perform all these algebraic simplifications. Of course, emphasis should be placed on the correctness of such transformations. A flag for performing unsafe optimizations like assuming floating point arithmetic is associative and distributive should be added. This proposal will benefit anybody writing or using numerically intensive code. I'm wondering what the community thinks of these projects. Which project is a better fit for GSoC, or are both a good fit? Is a mentor willing to supervise one of these projects? Thanks for your time. Patrick (A little about myself: I'm a Mathematics student in the US, and I've been programming in Haskell for about 3.5 years. Having a keen interest in Haskell and compilers, I began studying the GHC source about 1 year ago and I've since gotten a good understanding of its internals, contributing a few patches along the way.)

6 9

question about "singletons"
by TP 02 Jun '13

02 Jun '13

Hi all, Following a discussion on Haskell Cafe some time ago (1), Roman C. wrote: """ On the other hand, the type 'Succ 'Zero has kind 'Nat and doesn't have any inhabitants — only types of kind * do. """ Indeed, this is what seems to occur in the following example: ----------------------- {-# LANGUAGE DataKinds #-} data Nat = Zero | Succ Nat deriving ( Show, Eq, Ord ) main = do let one = undefined :: 'Succ 'Zero print one ----------------------- We obtain an error (ghc 7.6.2): ----------------------- Kind mis-match Expected kind `OpenKind', but `Succ Zero' has kind `Nat' In an expression type signature: Succ Zero In the expression: undefined :: Succ Zero In an equation for `one': one = undefined :: Succ Zero ----------------------- (note that the error is slightly different in the HEAD version of GHC (2)). Is it related to the sentence "As bottom is an inhabitant of every type (though with some caveats concerning types of unboxed kind), bottoms can be used wherever a value of that type would be. " found at address (3)? Here we have a non-* kind, such that bottom would not be an inhabitant of ('Succ 'Zero). Why is this so? This seems to be an assumed choice (4), but I would like to understand the reason for this design, which makes necessary to use singletons, as explained at (4). Now, if I use a singleton to make my example work, I obtain an error when trying to make the type (Sing a) an instance of Show: ----------------------- {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE StandaloneDeriving #-} data Nat = Zero | Succ Nat deriving ( Show, Eq, Ord ) data Sing :: a -> * deriving instance Show (Sing a) main = do let one = undefined :: Sing ('Succ 'Zero) print one ----------------------- The error is simply: ----------------------- test_noinhabitant_corrected.hs: Void showsPrec ----------------------- Why? Thanks, TP References: ---------- (1): https://groups.google.com/forum/?fromgroups#!topic/haskell-cafe/AltmX5iCFi8 (2): ----------------------- Expected a type, but ‛Succ Zero’ has kind ‛Nat’ In an expression type signature: Succ Zero In the expression: undefined :: Succ Zero In an equation for ‛one’: one = undefined :: Succ Zero ----------------------- (3): http://www.haskell.org/haskellwiki/Bottom (4): http://hackage.haskell.org/trac/ghc/wiki/TypeNats/Basics """ Both numeric and symbol literal types are empty---they have no inhabitants. However, they may be used as parameters to other type constructors, which makes them useful. """ """ A singleton type is simply a type that has only one interesting inhabitant. """

2 4

mapFst and mapSnd
by Dominique Devriese 01 Jun '13

01 Jun '13

Hi all, I often find myself needing the following definitions: mapPair :: (a -> b) -> (c -> d) -> (a,c) -> (b,d) mapPair f g (x,y) = (f x, g y) mapFst :: (a -> b) -> (a,c) -> (b,c) mapFst f = mapPair f id mapSnd :: (b -> c) -> (a,b) -> (a,c) mapSnd = mapPair id But they seem missing from the prelude and Hoogle or Hayoo only turn up versions of them in packages like scion or fgl. Has anyone else felt the need for these functions? Am I missing some generalisation of them perhaps? Regards, Dominique

12 13