August 2010 - Haskell-Cafe

cabal-install profiling libraries
by Johannes Waldmann 20 Aug '10

20 Aug '10

Here's another instance of the machine (*) telling me what to do, instead of doing it (or am I missing something): I have a large set of cabal packages installed with ghc. Then suddenly I need some package Foo with profiling. So I switch to library-profiling: True in my .cabal/config, and then "cabal install Foo" - failing with the message: Perhaps you haven't installed the profiling libraries for package `Bar' for some package Bar that Foo depends upon. - Dear Cabal: Yes! I know that I haven't installed them! I want you to install them for me! But it isn't listening ... (*) "machine" = everything in that metal box that was so expensive and has a lot of cables coming out, and ventilators running. Of course you know that I have the highest respect for the work of the cabal authors. I'm just suggesting that the above feature (auto-re-install dependencies) would be helpful. Perhaps it's already there? If not - would it be hard to specify? To build? Or would it have bad consequences? Is it "cabal upgrade --reinstall"? But that was deprecated? Here I really want "reinstall with exactly the same versions". Is it the problem that their sources may have vanished, meanwhile? Could it be solved by having "cabal install" storing a copy of the source package that it used? Thanks - J.W.

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ANNOUNCE: text-json-qq 0.2.0 (now with haskell-src-meta goodness)
by Oscar Finnsson 20 Aug '10

20 Aug '10

Hi, I've just uploaded a new version of text-json-qq, the json quasiquoter. Now it's possible (thanks to haskell-src-meta) to insert Haskell code inside the qq-code: > myCode = [$jsonQQ| {age: <| age + 34 :: Integer |>, name: <| map toUpper name |>} |] > where age = 34 :: Integer > name = "Pelle" For further info read the documentation at http://hackage.haskell.org/package/text-json-qq or read/fork the source code at http://github.com/finnsson/text-json-qq -- Oscar

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ANNOUNCE: binary-generic-0.2, generic binary serialisation using binary and syb.
by Lars Petersen 20 Aug '10

20 Aug '10

Hello cafe, although there was no announcement for version 0.1 there is one for the current 0.2: binary-generic allows to perform binary serialisation without explicitly defining every type specific case. If an algebraic type instantiates the 'Data' class the library is able to serialize it in a canonical way. Unfortunately version 0.2 is not binary compatible with 0.1. I decided to break this for the sake of simplicity: In 0.2 all multibyte values are encoded big-endian. For further version I'll try to supply compatibility functions even if something changes, but since 0.1 is not even a week old, I think it's not necessary this time. Okay, features: Common primitive types are supported out of the box: * Char, Word, Int are serialised as big-endian, taken from Data.Binary * Float and Double are serialised big-endian according to binary-ieee754 * Integer is serialized as in Data.Binary, but consistently big-endian * Data.ByteString as it is * Data.Text as Utf8 For types that are not supported yet, there is described an easy way of extension in 'Data.Binary.Extensions'. You are also free to ovveride certain choices and supply your own. If you think there are more types that should be supported without explicit extension, drop me a line. Cheers, Lars

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Creating binary distributions with Cabal
by Christopher Done 20 Aug '10

20 Aug '10

Does Cabal have a way to produce binary distributions from a package? I need to create a binary distribution of my project which does not depend on GHC or any development tools. The package should include all required data files and configuration files. I've got the latter covered with Data-Files and getDataFileName, but not sure about what to do regarding configuration files -- read/write to $HOME/.myproject/config or $HOME/.myprojectrc, etc., or what? I'm specifically targeting Redhat because that's the production server, but I'm wondering if there is or will be a way to agnostically access data files and configuration files without having to think about what OS it will be running on, in the same way I can use sockets or file access without worrying about the particular OS. Something like cabal sdist --binary --rpm/deb/arch/win/etc? How does everyone else package up their Haskell programs for binary distribution? Cheers!

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GtkImageView release!
by Andy Stewart 20 Aug '10

20 Aug '10

Hi all, I have release gtkimageview, it's a Gtk APIs for build image viewer. Here is screenshot : http://www.flickr.com/photos/48809572@N02/4909785139/ You can click right-bottom to popup navigate window to drag area in image. Here is demo : https://patch-tag.com/r/AndyStewart/gtkimageview/snapshot/current/content/p… Enjoy! -- Andy

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Academic Haskell Course
by Eyal Lotem 20 Aug '10

20 Aug '10

I was invited to give a course on Haskell (More in the lines of "Advanced FP in Haskell") for graduate students who already know a bit of ML, in an academic institution. I could lay out my own syllabus and course, but finding academic sources for all of the ideas is difficult - and I am probably duplicating work. Can anyone point me towards existing work I could use? Open course material and syllabuses I could use, with the necessary references? Thanks! Eyal

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philosophy of Haskell
by Michael Mossey 20 Aug '10

20 Aug '10

When I started to study Haskell, I was surprised that so much emphasis was placed on simple things. Monads were introduced to me as basically a wrapper, and a bind function that unwrapped something and wrapped something else back up again. I didn't understand what the fuss was about. Later I saw the amazing feats of expressiveness that were possible. I scratched my head in confusion---"Wait, say that again?" Here's a quote from Bertrand Russell about philosophy (read: Haskell). He's actually being humorous, but it applies, in a way: "The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical no one will believe it."

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Having a connection between kind * and kind * -> *
by Ivan Lazar Miljenovic 19 Aug '10

19 Aug '10

I'm trying to update container-classes to duplicate the pre-existing classes defined in the Prelude (Functor, etc.) and am trying to get my approach on how to have functions/classes that work on types of kind * (e.g. Bytestring) as well as kind * -> * (e.g. lists), as my previous approach didn't work. To define the connection, I've copied the style set out by Ganesh's rmonad ( http://hackage.haskell.org/package/rmonad ) package: ,---- | -- | Indicates what kind of value may be stored within a type. Once | -- superclass constraints are available, the @v@ parameter will | -- become an associated type. | class Stores c v | c -> v | | data family Constraints :: (* -> *) -> * -> * | | class (Stores (c v) v) => Suitable c v where | constraints :: Constraints c v `---- This works. However, what doesn't work is when I try to use these classes to re-define Functor: ,---- | class (Stores c v) => Mappable c v where | rigidMap :: (v -> v) -> c -> c | | class (Mappable (c v) v) => Functor c where | fmap :: (Suitable c a, Suitable c b) => (a -> b) -> c a -> c b `---- GHC doesn't like this definition of Functor; the only way to get it to work is to make `v' a parameter of the Functor class as well, even though it doesn't actually get used. Why is this? Is there any way around it? (Limitations like this are making me more and more consider dumping this whole concept as it's just becoming a pain, but more importantly rather boring :s) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Sifflet dependencies
by Don Stewart 19 Aug '10

19 Aug '10

$ cabal update Downloading the latest package list from hackage.haskell.org $ cabal install sifflet Resolving dependencies... cabal: cannot configure pango-0.11.1. It requires cairo >=0.11.1 && <0.12 and glib >=0.11.1 && <0.12 For the dependency on cairo >=0.11.1 && <0.12 there are these packages: cairo-0.11.1. However none of them are available. cairo-0.11.1 was excluded because sifflet-lib-1.0 requires cairo ==0.11.0 cairo-0.11.1 was excluded because cairo-0.11.0 was selected instead cairo-0.11.1 was excluded because sifflet-1.0 requires cairo ==0.11.0 For the dependency on glib >=0.11.1 && <0.12 there are these packages: glib-0.11.1. However none of them are available. glib-0.11.1 was excluded because sifflet-lib-1.0 requires glib ==0.11.0 glib-0.11.1 was excluded because glib-0.11.0 was selected instead glib-0.11.1 was excluded because sifflet-1.0 requires glib ==0.11.0 So that's pretty simple. 'sifflet' requires cairo ==0.11.0 and pango. But since pango doesn't work with cairo 0.11.0, the package can't be built. The solution is to ask the sifflet author to adjust the dependencies to be more flexible. http://hackage.haskell.org/package/sifflet Those specific versions of packages are overly constrained. They should follow the PVP, and be thus, 0.11.* -- Don

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Re: Specific to General to Specific
by oleg＠okmij.org 19 Aug '10

19 Aug '10

Given a widely parameterized type class > class Monad (m g n) => GenericNetworkMonad g m n where > ret :: a -> m g n a > ret = return the question seems to be about defining a specialized alias of it, instantiating the parameters g m n in some way. The hope is that the alias has fewer parameters and so is more convenient to use. Would not the standard approach of defining aliases work then? For example, > class NetworkMonad mg n where > instance GenericNetworkMonad g m n => NetworkMonad (m g) n where Granted, we would have to write boilerplate as we have to re-direct specialized methods to the general ones. Here is the complete code > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FlexibleContexts, FlexibleInstances #-} > {-# LANGUAGE UndecidableInstances #-} > > data family GNT g :: * -> * -> * > data family GNQ g :: * -> * -> * > > class Monad (m g n) => GenericNetworkMonad g m n where > ret :: a -> m g n a > ret = return > > -- A few instances > data instance GNT Int n a = GNT_Int a -- n is phantom > data instance GNQ Int n a = GNQ_Int a > > instance Monad (GNT Int n) where > return = GNT_Int > GNT_Int x >>= f = f x > > instance Monad (GNQ Int n) where > return = GNQ_Int > GNQ_Int x >>= f = f x > > type NT n a = GNT Int n a > type NQ n a = GNQ Int n a > > instance GenericNetworkMonad Int GNT n > instance GenericNetworkMonad Int GNQ n > > genericMonadicValue :: GenericNetworkMonad g m n => m g n () > genericMonadicValue = ret () > > monadicValue :: GenericNetworkMonad Int m n => m Int n () > monadicValue = ret () > > class NetworkMonad mg n where > mv'' :: mg n () > > instance GenericNetworkMonad g m n => NetworkMonad (m g) n where > mv'' = genericMonadicValue > > monadicValue'' :: NetworkMonad m n => m n () > monadicValue'' = mv'' In general, the Apply class trick should work for defining arbitrary aliases to class constraints. One Apply constraint is all one ever needs in Haskell: http://okmij.org/ftp/ftp/Haskell/types.html#Haskell1 Here is the instance of this trick, adopted to use type families rather than functional dependencies (no UndecidableInstances is required now): > class Apply label where > type Typ label :: * > apply :: label -> Typ label > > data NM (mg :: * -> * -> *) n = NM > instance GenericNetworkMonad g m n => Apply (NM (m g) n) where > type Typ (NM (m g) n) = m g n () > apply _ = genericMonadicValue > > monadicValue''' :: > forall mg n m. (Apply (NM mg n), > Typ (NM mg n) ~ m n ()) => > m n () > monadicValue''' = apply (NM :: NM mg n)

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