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Kind product
by Gautier DI FOLCO 01 Dec '14

01 Dec '14

Hello all, I have the following code: {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} data Branch = Left | Right type family Product (v :: Branch) (a :: k) (b :: k) :: k where Product Left l r = l Product Right l r = r My goal is to have a thing like that: type family Product (v :: Branch) (a :: k1) (b :: k2) :: (Either k1 k2) where Is it possible to do such a thing? If not I can find a solution with Kind families/Kind pattern matching, but I don't think that exists, am I wrong? Thanks in advance for your help, Regards.

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How to update Cabal manually?
by Jeffrey Brown 01 Dec '14

01 Dec '14

"cabal install cabal-install" is not working for me. I am not the first to find this problem; it is mentioned here <https://www.haskell.org/haskellwiki/Cabal/Survival>: "Notice, that in some cases (OS X, especially) the new cabal-install might not be installed on top of the previous one, and the old cabal-install must be replaced manually" I don't know how to manually replace Cabal. The closest thing I could find to instructions for that were here <https://www.haskell.org/pipermail/haskell-cafe/2010-February/073526.html>, but I don't understand them. The Haskell Platform installer is what installed Cabal for me. I have not found how to undo that, nor how to install Cabal in any other fashion. Running the Haskell Platform installer again seems dangerous, potentially conflicting with its earlier work. A likely unnecessary shell transcript demonstrating the problem appears below. I am running OS X 10.9 on a MacBook Pro, of which the decision to purchase I regret more daily. Thank you. ----- When I try to update cabal, it looks like it works: jefferys-mbp:~ jbb$ cabal install cabal-install Resolving dependencies... Configuring cabal-install-1.20.0.3... Building cabal-install-1.20.0.3... Installed cabal-install-1.20.0.3 Updating documentation index /Users/jbb/Library/Haskell/share/doc/index.html But the version number exhibits no change: jefferys-mbp:~ jbb$ cabal --version cabal-install version 1.18.0.5 using version 1.18.1.4 of the Cabal library

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Typeclass Show Question
by Yasuaki Kudo 01 Dec '14

01 Dec '14

Hi, When I launch GHCI with the option -XGADTs and load the following program, f = \x y -> (1, x,y) data S a b where S :: (Num a, Num b) => a -> b -> S a b I encounter this dialog: *Main> S 1 1 <interactive>:3:1: No instance for (Show (S a0 b0)) arising from a use of `print' Possible fix: add an instance declaration for (Show (S a0 b0)) In a stmt of an interactive GHCi command: print it I tried to follow the recommendation but still could not figure out how. For example, after adding: instance Show (S a b) where show (S x y) = show(x) I still get: conmath.hs:10:24: Could not deduce (Show a) arising from a use of `show' from the context (Num a, Num b) bound by a pattern with constructor S :: forall a b. (Num a, Num b) => a -> b -> S a b, in an equation for `show' at conmath.hs:10:15-19 Possible fix: add (Show a) to the context of the data constructor `S' or the instance declaration In the expression: show (x) In an equation for `show': show (S x y) = show (x) In the instance declaration for `Show (S a b)' Failed, modules loaded: none. What should I do to show S 1 1 ? Regards, Yasu

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Hosting a static site with Yesod
by Yifan Yu 30 Nov '14

30 Nov '14

Hi all, I'm experimenting with Yesod and I've created a simple scaffolding site with yesod. I've downloaded a bootstrap template site and wish to simply host this site with yesod. The template site has an index.html and a bunch of css and js files. This seemly simple task has baffled me. By my understanding, the site should be placed under the 'static' directory, I tried to use sendFile to send the index.html file in getHomeR, but only the content of the that file is displayed, without the css and js. Should I do this with a Subsite? Thank you

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multiple declarations error in Haskell - should it be relaxed?
by Andrew Butterfield 30 Nov '14

30 Nov '14

Condsider the following Haskell program (fragment): f 0 = “zero” g 0 = “NULL” f n = “non-zero” g n = “PRESENT” This will result in two “Multiple Declaration” errors. There is a good motivation for this - disallowing such an interleaving of declarations makes it easy for the compiler to capture a common typo, namely errors of the following form - here an attempt to define a single function called myFun. myFun 0 = “zero” myfun 1 = “one” myFun n = “too big!” However I have use-cases where it would be nice to interleave as per the first example above - with markedly different function names. It invokes a large case analysis, where I have other auxiliary functions associated with each case, but which I’d like to (1) have at the top-level for testability (2) keep textually local to the case with which they are associated. I don’t think there is a language extension to disable the multiple declaration check - but would such a feature we possible. I’d see it as one which still performs the check, but issues a warning rather than an error - particularly if it notices that the interleaved names are very similar. Is this a reasonable suggestion, or are there other reasons for not doing this that I’ve missed? Maybe there is a better way to satisfy (1) and (2) above? Regards, Andrew -------------------------------------------------------------------- Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations & Methods Research Group School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ --------------------------------------------------------------------

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[Announce] Music Suite v1.8
by Hans Höglund 30 Nov '14

30 Nov '14

I am happy to announce the release of Music Suite version 1.8. This release in short: Cleaner API for dealing with Voices, Notes and time structures in general. New equal-temperament pitch representation. Improved handling of diatonic vs. chromatic transposition in the Common pitch representation. Better support for clef and staff line positions. Better support for handling instrument ambitus etc. See full notes at https://github.com/music-suite/music-docs/blob/master/releases/Notes.md#18 The Music Suite is a language for description, analysis, composition and manipulation of music embedded in Haskell. For more information see http://music-suite.github.io or my talk at FARM2014 http://functional-art.org/2014/. Regards, Hans Hoeglund - https://twitter.com/hanshogl https://soundcloud.com/hanshoglund http://github.com/hanshoglund

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Getting Haskell in the top 20
by Henk-Jan van Tuyl 30 Nov '14

30 Nov '14

L.S., In the Tiobe Index[0], F# jumped from position 43 in June to 13 in July and is since than in the top twenty. What happened in July and how can we get Haskell in the top 20? Regards, Henk-Jan van Tuyl [0] http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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looking for ongoing Haskell projects ..
by Vasili I. Galchin 30 Nov '14

30 Nov '14

Hi Fellow Haskellers, I was just reading at http://www.haskell.org. I am looking for a list of currently active Haskell projects needing help .. :-) Vasili

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Data migration and software versioning
by Corentin Dupont 30 Nov '14

30 Nov '14

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

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Kind signatures and closed type families syntax
by Gautier DI FOLCO 30 Nov '14

30 Nov '14

Hello all, I have the following code: {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} data Branch = Left | Right type family Product (v :: Branch) (a :: k) (b :: k) :: k where Product Left l r = l Product Right l r = r He is doing what I expect it to do and Product has the right Kind: *Main> :kind Product Product :: Branch -> k -> k -> k But when I change the Kind signature syntax, its Kind changes: type family Product v a b :: Branch -> k -> k -> k where Its Kind become: *Main> :kind Product Product :: Branch -> (Branch -> k -> k -> k) -> (Branch -> k1 -> k1 -> k1) -> Branch -> k2 -> k2 -> k2 It's even worse with this syntax: type family Product :: Branch -> k -> k -> k where Produces: *Main> :r [1 of 1] Compiling Main ( product-highkind.hs, interpreted ) product-highkind.hs:13:3: Number of parameters must match family declaration; expected 0 In the equations for closed type family ‘Product’ In the type family declaration for ‘Product’ Failed, modules loaded: none. I don't know if I'm tired or not but in the documentation [1] these syntaxes should be equivalent. If not, I'll take any explanations. Thanks in advance for your help. [1] https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/kind-polymor…

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