- Haskell-Cafe - Haskell.org

Cabal Config constraints vs. constraint
by Christopher Reichert 22 Dec '14

22 Dec '14

Hey everyone, I am trying to add a list of version constraints to my global cabal config (~/.cabal/config) using the `constraints` keyword. However, after adding the list to the end of the ~/.cabal/config file and running `cabal update`, I noticed this warning: $ cabal update Warning: /home/test/.cabal/config: Unrecognized field constraints on line 176 Downloading the latest package list from hackage.haskell.org I did find this issue annotated as a FIXME in the cabal source: https://github.com/haskell/cabal/blob/64761550bb617ee5d9449512bd3e500489cfd… It appears as though `constraints` are currently only supported in cabal.config (and maybe cabal.sandbox.config) Do individual constraints (e.g. `constraint:`) have the same effect as a list of constraints? Does the `constraint` setting work in both global (~/.cabal/config) and local cabal.config the exact same way (especially if I have a long list of them) Is there any reason I would use individual constraints over a list of constraints? My assumption is that they work the same way in both ~/.cabal/config && cabal.config. I'm considering patching the cabal source to support the `constraints` setting in the ~/.cabal/config file. Is there any reason this might have been passed up front? More discussion here: https://github.com/fpco/stackage-server/issues/67 Any help or advice would be greatly appreciated. Thanks, -Christopher -- Christopher Reichert irc: creichert gpg: C81D 18C8 862A 3618 1376 FFA5 6BFC A992 9955 929B

2 1

Could someone help me to understand funB?
by Magicloud Magiclouds 22 Dec '14

22 Dec '14

Hi, Following code is to get a list of primes. Now it is hard for me to understand funB. I mean I can see what it does. But I cannot see the detailed process by every language part. import Control.Monad isPrime :: Integer -> Bool isPrime i = ap funA funB i funA :: Integer -> [Integer] -> Bool funA x xs = all (\x' -> (mod x x') /= 0) xs funB :: Integer -> [Integer] funB = flip takeWhile primes . ( . join (*)) . flip (<=) primes :: [Integer] primes = 2 : filter isPrime [3, 5 ..] main :: IO () main = print $ take 10 primes -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

3 4

Template haskell: This pattern-binding binds no variables
by Michael Baikov 21 Dec '14

21 Dec '14

I'm trying to introduce a new local variable to some scope, in this example this scope is represented by print $( varE ohNoes) Everything works as expected, but I'm getting a warning message which I don't seem quite right to me. Am I doing anything wrong? ---- Blah.hs ---- {-# LANGUAGE TemplateHaskell #-} module Blah where import Language.Haskell.TH sample :: Q Exp sample = newName "ohNoes" >>= \ohNoes -> [| let $( varP ohNoes ) = "Teh warning!" in print $( varE ohNoes ) |] ---- Main.hs ---- {-# LANGUAGE TemplateHaskell #-} module Main where import Blah main :: IO () main = $( sample ) ---- output ---- Blah.hs:8:49: Warning: This pattern-binding binds no variables: $(varP ohNoes) = "Teh warning!" sample ======> let ohNoes_a2Al = "Teh warning!" in print ohNoes_a2Al

2 2

ANN: deepseq-bounded, seqaid, leaky
by Andrew Seniuk 21 Dec '14

21 Dec '14

This trio of related packages explores strictness control in a variety of ways. deepseq-bounded provides classes and generic functions to artificially force evaluation, to extents controlled by static or dynamic configuration. seqaid puts that into practise, providing a GHC plugin to auto-instrument your package with a strictness harness, which is dynamically optimisable during runtime. This is supported directly in the GHC compilation pipeline, without requiring (or performing!) any edits to your sources. leaky is a minimal, prototypic executable that leaks space under current state-of-the-art compilation (GHC 7.8.3 -O2, at the present time). deepseq-bounded hackage: https://hackage.haskell.org/package/deepseq-bounded homepage: http://www.fremissant.net/deepseq-bounded seqaid hackage: https://hackage.haskell.org/package/seqaid homepage: http://www.fremissant.net/seqaid leaky hackage: https://hackage.haskell.org/package/leaky homepage: http://www.fremissant.net/leaky Reddit discussion for the three together: http://www.reddit.com/r/haskell/comments/2ps8f5/ann_deepseqbounded_seqaid_l… Easiest way to try them all, is to install seqaid and run the demo: cabal install seqaid seqaid demo This tests seqaid on a local copy of the leaky source package. It turned out to be routine to extend deepseq-bounded and seqaid to dynamically configurable parallelisation (paraid?). Many other wrappers could be explored, too! Maybe seqaid should be renamed to koolaid or something... It's a pretty complicated system, and just first release, so there's bound to be lots of problems. I've not set up a bug tracker, but will maintain a casual list of bugs and feature requests at http://www.fremissant.net/seqaid/trac and will set up a proper tracker if there's interest. Any isssues (or comments), I'm here, or on the reddit discussion (or email). Andrew Seniuk rasfar on #haskell

1 2

Parametric kinds
by Gautier DI FOLCO 21 Dec '14

21 Dec '14

Hello all, I'm trying to port typeclasses to kind-level, here are my data types: -- newtype Fix f a = Fix (f (Fix f a)) -- data Mu f = In (f (Mu f)) data Fix (f :: k1 -> k0) (n :: k1) = Fx data Maybe a = Nothing | Just a If I try to create type families to represent Functors, I end up with this: -- type family MapFix (f :: (* -> k) -> * -> k) a :: * where -- type family MapFix f a where type family MapFix (f :: (k0 -> k1) -> k0 -> k1) (a :: k2) :: * where MapFix f (Fix a n) = Fix (f a) (MapFix f n) -- type family MapMaybe (f :: k -> k) (a :: Maybe k) :: Maybe k where -- type family MapMaybe f a where type family MapMaybe (f :: k0 -> k1) (a :: Maybe k0) :: Maybe k1 where MapMaybe f ('Just a) = 'Just (f a) MapMaybe f 'Nothing = 'Nothing But I can't defined a single open type family for these two data types. I think I lack of parametric kinds, for example: `*k p*` could let me fit `*Maybe k*`. Currently, in the way I see kinds, on `*k0 -> k1*` allow me to parameterize kinds, but `*Maybe k*` doesn't fit. I know that this is certainly unclear, don't hesitate to ask me for clarifications. I also know that my `*Fix*` sucks, any good idea on how to do it is welcome :) Also, if you have any links to have a deeper insight of Kinds it will save me from mental illness. Thanks in advance for your help, Regards.

1 1

How to avoid keeping old data formats in your code?
by Corentin Dupont 21 Dec '14

21 Dec '14

Hi, I didn't had so much luck with this question. The question was: -> "How to avoid keeping old data formats in your code?" Indeed those old data formats are often needed for retro-compatibility with previous versions of your software. Say you have a data structure that is serialized in a file, and then you add a field in a later version. You are obliged to keep both versions of the data structure if you want to be able to read both versions of the file. Potentially, if you update very often your structure between releases, you are obliged to keep N versions of the data structure in your code. See the exemple at: http://acid-state.seize.it/safecopy On Sun, Nov 30, 2014 at 1:04 PM, Corentin Dupont <corentin.dupont(a)gmail.com> wrote: > Hi the list, > I have some question relative to data migration. > > Say you have a software in version A which save data in a file with format > FA. Later, you update your software to version B, with data format FB. > Now, if you want your version B of the software to be able to read data > saved by version A, you're obliged to include FA in version B, together > with some functions to translate from FA to FB. > > This is what I don't find elegant: you're obliged to keep old code (data > formats) in your software. > -> do you know of any way to avoid keeping old data formats (for > retro-compatiblilty) in your code? > > In practice I use acid-state with safecopy: > http://acid-state.seize.it/safecopy > In this example you can see the problem: the author is obliged to keep old > code (data structures) to maintain compatibility. Even worth, you might be > obliged to suffix your data structure with a version number: > > data MyType_V1 = MyType_V1 Intdata MyType_V2 = MyType_V2 Integer > > > Instead, I'm thinking of a process using GIT, or Cabal as a back-end. > The idea would be to have an additional program (or library) specialized > in the data migration of your main software. It would extract both version > A and B from the repo, and then would compile an application capable of > handling migrations from FA to FB. > > Does something like this exists (even outside of Haskell)? > > Cheers, > Corentin >

4 5

[ANN]Haskell Rhythm game development tutorial
by Fumiaki Kinoshita 21 Dec '14

21 Dec '14

Hello, I've published a tutorial[0] to create a rhythm game. This tutorial focuses on the design of interactive, real-time application using lens, objective, Call package. Feel free to point out infelicities or offer suggestion. Cheers [0] http://fumieval.github.io/rhythm-game-tutorial/ -- Fumiaki Kinoshita

2 2

<*> for Data.Sequence: mission accomplished
by David Feuer 20 Dec '14

20 Dec '14

All right! It's taken a long time, but I finally managed to write a nice implementation of `<*>` for `Data.Sequence`! It looks nothing like my original concept, but it accomplishes my incremental performance goals while also being a little better than the current implementation when forcing the whole result. Anyone interested can see the code in the `ap` branch of treeowl/containers on GitHub. There are still some ugly special cases for sufficiently small arguments--any ideas for changing that would be most welcome. Many thanks to Joachim Breitner for helping implement an earlier approach, to Ross Paterson for coming up with a solution to a previous formulation of the problem (thus helping me understand that I had asked the wrong question), and to Carter Schonwald for putting up with my stream of consciousness rambling. On Sun, Nov 23, 2014 at 12:07 AM, David Feuer <david.feuer(a)gmail.com> wrote: > OK, sorry for the flood of posts, but I think I've found a way to make that > work. Specifically, I think I can write a three-Seq append that takes the > total size and uses it to be as lazy as possible in the second of the three > Seqs. I'm still working out the details, but I think it will work. It does > the (possibly avoidable) rebuilding, but I *think* it's at least > asymptotically optimal. Of course, if Ross Paterson can find something more > efficient, that'd be even better. > > On Sat, Nov 22, 2014 at 10:10 PM, David Feuer <david.feuer(a)gmail.com> wrote: >> >> OK, so I've thought about this some more. I think the essential *concept* >> I want is close to this, but it won't quite work this way: >> >> fs <*> xs = equalJoin $ fmap (<$> xs) fs >> >> equalJoin :: Int -> Seq (Seq a) -> Seq a >> equalJoin n s >> | length s <= 2*n = simpleJoin s >> | otherwise = simpleJoin pref >< >> equalJoin (2*n) mid >< >> simpleJoin suff >> where (pref, s') = splitAt n s >> (mid, suff) = splitAt (length s - 2*n) s' >> >> simpleJoin :: Seq (Seq a) -> Seq a >> simpleJoin s >> | null s = empty >> | length s == 1 = index s 0 >> | otherwise = simpleJoin front >< simpleJoin back >> where >> (front,back) = splitAt (length s `quot` 2) s >> >> I think the reason this doesn't work is that >< is too strict. I believe >> the only potential way around this is to dig into the FingerTree >> representation and build the thing top-down. I still don't understand how >> (if at all) this can be done. >> >> >> On Sat, Nov 22, 2014 at 12:57 PM, David Feuer <david.feuer(a)gmail.com> >> wrote: >>> >>> The ideal goal, which has taken me forever to identify and which may well >>> be unattainable, is to get O(log(min{i,mn-i})) access to each element of the >>> result, while maintaining O(mn) time to force it entirely. Each of these is >>> possible separately, of course. To get them both, if it's possible, we need >>> to give up on the list-like approach and start splitting Seqs instead of >>> lists. As we descend, we want to pass a single thunk to each element of each >>> Digit to give it just enough to do its thing. Representing the splits >>> efficiently and/or memoizing them could be a bit of a challenge. >>> >>> On Fri, Nov 21, 2014 at 02:00:16PM -0500, David Feuer wrote: >>> > To be precise, I *think* using the fromList approach for <*> makes us >>> > create O >>> > (n) thunks in order to extract the last element of the result. If we >>> > build the >>> > result inward, I *think* we can avoid this, getting the last element of >>> > the >>> > result in O(1) time and space. But my understanding of this data >>> > structure >>> > remains primitive. >>> >>> This modification of the previous should do that. >>> >>> mult :: Seq (a -> b) -> Seq a -> Seq b >>> mult sfs sxs = fromTwoLists (length sfs * length sxs) ys rev_ys >>> where >>> fs = toList sfs >>> rev_fs = toRevList sfs >>> xs = toList sxs >>> rev_xs = toRevList sxs >>> ys = [f x | f <- fs, x <- xs] >>> rev_ys = [f x | f <- rev_fs, x <- rev_xs] >>> >>> -- toRevList xs = toList (reverse xs) >>> toRevList :: Seq a -> [a] >>> toRevList = foldl (flip (:)) [] >>> >>> -- Build a tree lazy in the middle, from a list and its reverse. >>> -- >>> -- fromTwoLists (length xs) xs (reverse xs) = fromList xs >>> -- >>> -- Getting the kth element from either end involves forcing the lists >>> -- to length k. >>> fromTwoLists :: Int -> [a] -> [a] -> Seq a >>> fromTwoLists len_xs xs rev_xs = >>> Seq $ mkTree2 len_xs 1 (map Elem xs) (map Elem rev_xs) >>> >>> -- Construct a fingertree from the first n elements of xs. >>> -- The arguments must satisfy n <= length xs && rev_xs = reverse xs. >>> -- Each element of xs has the same size, provided as an argument. >>> mkTree2 :: Int -> Int -> [a] -> [a] -> FingerTree a >>> mkTree2 n size xs rev_xs >>> | n == 0 = Empty >>> | n == 1 = let [x1] = xs in Single x1 >>> | n < 6 = let >>> nl = n `div` 2 >>> l = Data.List.take nl xs >>> r = Data.List.take (n - nl) rev_xs >>> in Deep totalSize (mkDigit l) Empty (mkRevDigit r) >>> | otherwise = let >>> size' = 3*size >>> n' = (n-4) `div` 3 >>> digits = n - n'*3 >>> nl = digits `div` 2 >>> (l, xs') = Data.List.splitAt nl xs >>> (r, rev_xs') = Data.List.splitAt (digits - nl) rev_xs >>> nodes = mkNodes size' xs' >>> rev_nodes = mkRevNodes size' rev_xs' >>> in Deep totalSize (mkDigit l) (mkTree2 n' size' nodes rev_nodes) >>> (mkRevDigit r) >>> where >>> totalSize = n*size >>> >>> mkDigit :: [a] -> Digit a >>> mkDigit [x1] = One x1 >>> mkDigit [x1, x2] = Two x1 x2 >>> mkDigit [x1, x2, x3] = Three x1 x2 x3 >>> mkDigit [x1, x2, x3, x4] = Four x1 x2 x3 x4 >>> >>> -- length xs <= 4 => mkRevDigit xs = mkDigit (reverse xs) >>> mkRevDigit :: [a] -> Digit a >>> mkRevDigit [x1] = One x1 >>> mkRevDigit [x2, x1] = Two x1 x2 >>> mkRevDigit [x3, x2, x1] = Three x1 x2 x3 >>> mkRevDigit [x4, x3, x2, x1] = Four x1 x2 x3 x4 >>> >>> mkNodes :: Int -> [a] -> [Node a] >>> mkNodes size (x1:x2:x3:xs) = Node3 size x1 x2 x3:mkNodes size xs >>> >>> -- length xs `mod` 3 == 0 => >>> -- mkRevNodes size xs = reverse (mkNodes size (reverse xs)) >>> mkRevNodes :: Int -> [a] -> [Node a] >>> mkRevNodes size (x3:x2:x1:xs) = Node3 size x1 x2 x3:mkRevNodes size xs >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

2 3

LambdaConf 2015 - CFP
by John A. De Goes 20 Dec '14

20 Dec '14

Hi all, LambdaConf 2015 has officially opened its Call for Proposals. LambdaConf 2015 is being held in the beautiful foothills of the Rocky Mountains, in Boulder, Colorado (home to many companies using FP). The conference is devoted to helping people become better programmers through functional and declarative programming. We're looking for lightning talks, workshops, and presentations on the following themes: FP 101. Introductory material for the FP curious or those just getting their feet wet. Languages. New languages (functional / logic), functional ways to use existing languages, language extensions, etc. Theory. Type theory, category theory, abstract algebra, ongoing or new research, and anything useful or interesting to functional programmers or academics working in the field. Libraries. Cool libraries that demonstrate the power of functional programming in helping programmers get stuff done. Case Studies. Case studies showing how using a functional language or library proved to be game-changing in solving real world problems. We currently have talks or workshops lined up for Haskell, Purescript, Scala, and Rust. If you'd like to submit a proposal, just send a private email to me or visit the official website for more information. We are not yet open for registrations, but we are actively looking for a couple more volunteers, as well as sponsors. If you know someone who might be interested, please spread the word! Regards, John

1 0

Random pattern generation with list comprehensions
by Mike Jones 20 Dec '14

20 Dec '14

I am looking for a way to generate random patterns for hardware validation. The goal is to separate patten generation from execution in hardware, and from pure code used to compute expected behavior from a model, detection of bad behavior, in the form of predicates, etc. The purpose of random patterns is to explore the behavior space in less time, as well as improve time to discovery by preventing long runs that focus in narrow areas for long periods. Patterns are to be infinite lists with the ability to specify run length for any run, which becomes a quality dial of sorts. I initially started with finite sequences expressed as list comprehensions, that generate tuples of ints, floats, structured data, and lists of attributes (bit combinations of hardware registers). I believe the list comprehension evaluates each generator (a | a <- ...) in a strict order, such that an infinite list will cause infinite evaluation of a single generator, such that other generators are not evaluated. Even if each generator is constrained (take n), results will not not be completely random. What I need is for each generator to supply one value each, and combine them into a tuple. I could probable express this in a monadic loop construct, as long as each generator is random and holds state, like the article I saw using the State Monad to generate random numbers. Or perhaps a State monad that holds multiple generators. Is there a better way to express the solution or a helpful module others use for this? Mike Sent from my iPad

5 5