November 2014 - Haskell-Cafe

Typesafe multi-table PostgreSQL queries in Yesod with schema names?
by Manuel Gómez 12 Dec '14

12 Dec '14

Hi list, I’m looking for a rather specific point in the design space for database access libraries in Haskell and I’m having some trouble figuring out whether any existing library covers my use cases. Specifically, I need to 1. get a Yesod application to access a PostgreSQL database without having to reinvent the universe to get them decently integrated, 2. write queries in an EDSL with a good approach to composability and decent static verification against some declared form of the database objects, 3. be able to perform joins 4. be able to specify the schema name for each database object used in queries These are the closest solutions I’ve found: * HaskellDB: seems like the best approach to the EDSL issue and it’s certainly capable of performing joins, and I haven’t looked into schema names, but it seems possible — but it seems to be poorly maintained and somewhat abandoned, and comfortable Yesod integration doesn’t sound like a lot of fun. * Persistent: it’s perfectly integrated into Yesod, and together with Esqueleto, it provides a nice, typesafe and fully expressive EDSL — but, as far as I can tell, there is no support for specifying schema names, which is essential for my use case. * Groundhog: Yesod integration is reasonable since Persistent borrowed a good bit of its design, and after a few weeks of experimenting, I’ve got it pretty much worked out by now, but while it certainly takes an interesting and solid approach to the EDSL issue, it seems completely incapable of performing joins, and hence its usability is very heavily limited — I don’t fancy creating SQL views for each and every join I need. * postgresql-simple: minimal Yesod integration shouldn’t be too uncomfortable, since Persistent uses it internally anyway, and it can certainly do joins and use whatever fancy SQL magic I could ever want to write, certainly including schema names — but it doesn’t really do anything for statically checking queries against a schema specification or for composability and code reuse, and I might as well just use Persistent to do raw SQL. I’ve breifly looked at modifying Persistent to add support for schema names, but I got lost in the source code very quickly, and I worry about the fact that it requires serial primary keys — Groundhog is *very* nice in that it has a great approach to DBMS-specific syntax and other things I’ll probably need (non-numeric and composite keys), but it’s really a shame that it can’t do joins, and the workaround with views sort of defeats the purpose of SQL abstraction. I’m not sure what to do about this. Is there some grand unified relational database access library I just haven’t found, or is a pull request the only solution?

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Music made with Haskell
by Anton Kholomiov 08 Dec '14

08 Dec '14

I wrote two tracks completely with Haskell. You can listen to them on the soundcloud: https://soundcloud.com/anton-kho/celtic https://soundcloud.com/anton-kho/invisible-ocean The music is based on samples but a single track uses no more than 6 samples. The code is under 100 lines of code. I'm using my libs csound-sampler and csound-expression. You can check out the source code at: http://ge.tt/3MjwF852/v/0 Please turn off the AdBlock for this page to download the code. Happy haskelling! Anton

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Haskell 2014
by Дмитрий 02 Dec '14

02 Dec '14

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Using ghc-mod with FFI
by Dan Piponi 01 Dec '14

01 Dec '14

In my code I have this line: foreign import ccall "set_num_states" setNumStates :: Int -> IO () I can build my project using cabal and have an appropriate "c-sources:" line. But when I use ghc-mod check (eg. from " ghc-mod-5.2.0.0", and launched from Syntastic) it complains: "ghc-mod: ByteCodeLink: can't find label During interactive linking, GHCi couldn't find the following symbol: set_num_states" How should I be checking my code? -- Dan

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Re: [Haskell-cafe] Haskell 2014
by Roman Cheplyaka 01 Dec '14

01 Dec '14

The language («GHC Haskell») is evolving quite rapidly, it's just no-one is really interested in maintaining the standard anymore. I don't think it should disappoint you, unless you're a language researcher or compiler writer. On 30/11/14 14:29, Dmitry wrote: > Thanks for the answer, > > it's sad. > Even C++ now gets new releases every several years. > > Does anybody feel like the language is surpassed by other languages, for > example F#? > > > 30.11.2014, 17:46, "Roman Cheplyaka" <roma(a)ro-che.info>: >> >> On 30/11/14 04:03, Дмитрий wrote: >> >> Hi everyone, >> >> I couldn't find when haskell 2014 standard will come out. Is >> there any >> information about it? >> Somewhere I read that a new standard should be released each year >> but it >> seems not to be true nowadays >> >> There's no-one working on the Haskell standard, to the best of my >> knowledge. So no, it probably won't come out. >> >> Roman >>

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Kind product
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 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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Typeclass Show Question
by Yasuaki Kudo 30 Nov '14

30 Nov '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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