- Haskell-Cafe - Haskell.org

lightweight web interface for Haskell?
by Richard Eisenberg 27 Aug '14

27 Aug '14

Hi Cafe, Does anyone know of a website where I can write a few lines of Haskell and have them run? Ideally, there would be a place to write a module and then a place to load the module into GHCi. I seem to recall something that fit my needs at paste.hskll.org, but that site is no longer working. I'm asking because I'll be teaching an undergraduate course this fall in introductory Haskell. There are no prerequisites for the course, and my course roster suggests that I'll have several students with no programming background at all. (This is great, actually -- I love facilitating a new programmer's first steps!) It might be nice to get students off the ground quickly before bogging them down with installation details -- hence the website I'm looking for. Thanks! Richard

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[ANN] Wheb 0.3 release
by Kyle Hanson 27 Aug '14

27 Aug '14

Hello all, I have been working on a web framework that I use for my personal projects and I thought I would share my small progress. I started this project several months ago when I hit a big bump in difficulty going from a simple skeleton project to a more intricate project using alternative frameworks. Lately I've added three main features: WebSockets, Cache and Redis. WebSockets use the same WhebT interface as the standard HTTP handlers allowing you to share functions and middlware accross your application. Cache is a new plugin for simplistic Caching. It provides an interface to cache ByteStrings under Text keys. Like the other plugins, it is backend agnostic so you can write your own backend or use the prebuilt Redis cache (this requires you to configure your redis instance for caching). In addition to Redis acting as a Cache, you can also use it as a database and as a backend for the plugins Auth and Sessions. You can mix Redis with other plugins. For one of my projects I use the MongoDB plugin for auth and storage and Redis as a cache for the queries. Also included are typed web-routes, stateful cookie handling, and some other stuff. Feel free to check it out. I tried to include some documentation and examples that I refer to when I use it. Maybe it will help someone. Its far from perfect, so if you find any problems, let me know. https://hackage.haskell.org/package/Wheb https://github.com/hansonkd/Wheb-Framework Cheers, Kyle Hanson

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Re: [Haskell-cafe] Types depending on tuples.
by Mike Izbicki 26 Aug '14

26 Aug '14

This works just fine for me: class Vector a where toLists :: a -> [Double] instance Vector (Double,Double,Double) where toLists (a,b,c)=[a,b,c] On Tue, Aug 26, 2014 at 1:17 AM, Mike Meyer <mwm(a)mired.org> wrote: > It's there, but in the text: > > class Vector a where > toLists :: a -> [Double] > > > On Tue, Aug 26, 2014 at 2:59 AM, Mike Izbicki <mike(a)izbicki.me> wrote: >> >> You never actually provided the Vector type class, so it's difficult >> to tell what exactly you're trying to do. >> >> On Tue, Aug 26, 2014 at 12:46 AM, Mike Meyer <mwm(a)mired.org> wrote: >> > I'm doing some geometry in 2 and 3 dimensions. To do this, I've got >> > Vector2d >> > and Vector3d classes, which are basically just wrappers around 2 and >> > 3-tuples of doubles: >> > >> > type Vector2d = (Double, Double) >> > type Vector3d = (Double, Double, Double) >> > >> > I've also got a typeclass Vector so I can do polymorphic things on them: >> > >> > type Vector3d = (Double, Double, Double) >> > >> > (Ok, polymorphic thing). >> > >> > I now have a need to convert lists of Vector's to a list of lists of >> > Doubles. I'd like that to be part of the Vector typeclass, but - well, >> > declaring it is interesting. >> > >> > I can't just add a method "toLists :: a -> [Double]", because (according >> > to >> > ghc), there's no match between the expected type 'Double' and the actual >> > type '[t1]'. >> > >> > Can I declare this typeclass? >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > >

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Fwd: Locking of threads in one OS thread
by Nikolay Amiantov 26 Aug '14

26 Aug '14

On Tue, Aug 26, 2014 at 6:59 PM, Sylvain Henry <hsyl20(a)gmail.com> wrote: > AFAIK "errno" is handled explicitly in the RTS: > https://ghc.haskell.org/trac/ghc/browser/ghc/rts/Schedule.c#L484 > > -Sylvain > Oh, thanks for pointing this out! Now this behaviour makes sense.

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Locking of threads in one OS thread
by Nikolay Amiantov 26 Aug '14

26 Aug '14

Hello Cafe, I'm using FFI to interact with a library which calls, when fail, leave the reason in some kind of "errno"-like variable which is retrived via another call. AFAIU, this is not thread-safe in Haskell even if thread-local storage is used inside the library, because Haskell uses its own thread management and the Haskell thread in the same OS thread might be switched between the actual call and the retrival of errno value. This should be somehow handled already in Haskell (errno is widely used with syscalls in Linux, for example), but the source of Foreign.C.Error suggests that this is not handled in any way at all. For example, throwErrnoIf is implemented as such: throwErrno loc = do errno <- getErrno ioError (errnoToIOError loc errno Nothing Nothing) throwErrnoIf pred loc f = do res <- f if pred res then throwErrno loc else return res So, the question is: how is it ensured that this block is "atomic" in sense that at most one Haskell thread computes this whole function at every moment in every OS thread? Thanks for any explanations! (And sorry if my English is poor) Nikolay.

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Types depending on tuples.
by Mike Meyer 26 Aug '14

26 Aug '14

I'm doing some geometry in 2 and 3 dimensions. To do this, I've got Vector2d and Vector3d classes, which are basically just wrappers around 2 and 3-tuples of doubles: type Vector2d = (Double, Double) type Vector3d = (Double, Double, Double) I've also got a typeclass Vector so I can do polymorphic things on them: type Vector3d = (Double, Double, Double) (Ok, polymorphic thing). I now have a need to convert lists of Vector's to a list of lists of Doubles. I'd like that to be part of the Vector typeclass, but - well, declaring it is interesting. I can't just add a method "toLists :: a -> [Double]", because (according to ghc), there's no match between the expected type 'Double' and the actual type '[t1]'. Can I declare this typeclass?

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Undesired email
by felipe zapata 25 Aug '14

25 Aug '14

Dear List, I have a problem with me new smartphone. Sorry for the inconvenient. Best, FelipeZ

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Munich Haskell Meeting
by Heinrich Hördegen 24 Aug '14

24 Aug '14

Dear all, next Tuesday, 26th of August, Munich's Haskell enthusiasts will meet again at Cafe Puck around 19h30. Everyone is wellcome. Please note that we will meet at Cafe Puck. To help us reserve enough tables, please go to http://www.haskell-munich.de/dates and click the button. Heinrich

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ghc-mod v5.0.0
by Kazu Yamamoto 24 Aug '14

24 Aug '14

Hi cafe, We are happy to announce that ghc-mod v5.0.0 has been released. Highlights of this version is as follows: - ghc-mod v4.x.y is suffering from the space leak of GHC 7.8.x(*1). So, ghc-modi consumes huge memory. ghc-modi v5.0.0 consumes much less memory thanks to workaround. - Alejandro Serrano merges his results of Google Summer of Code. He implements case splitting and sophisticated type hole handling. I believe that he will write an article on his results but I also updated the manual of Emacs fronted. (*2) - Daniel Gröber brushed up the internal code by bringing the GhcMod monad. So, the API of ghc-mod drastically changed. If you are users of the ghc-mod library, you should check its document. The main purpose of this release is to show the results of GSoC to everyone. This is a snapshot release and its API would change in the future. I thank two guys above since they worked hard and did good jobs. We hope that everyone enjoys v5.0.0. (*1) https://ghc.haskell.org/trac/ghc/ticket/9314 (*2) http://www.mew.org/~kazu/proj/ghc-mod/en/emacs.html --Kazu

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How to write transform one set of ADTs to another with SYB?
by ducis 23 Aug '14

23 Aug '14

More specifically, I want to transform a tree of SomeExpr to a tree of (SomethingElse, SomeExpr), "prefixing" each node with a default value in the process. The result have essentially the same structure as the input. data ASTAttachment = AA {} deriving (Eq,Read,Show,Ord,Typeable,Data) type family AST a :: * type instance AST () = ASTExpr () type instance AST AA = (AA, ASTExpr AA) data ASTExpr a = ALiteral String | AApplication [AST a] (ASTOp a) | ARef String | ABind (AST a) String | AMany (ASTMany a) data ASTOp a = AOSym String | AOAlpha String | AOMany (ASTMany a) data ASTMany a = AMSimple [AST a] | AMAggeregate [AST a] deriving instance Typeable ASTMany deriving instance Typeable ASTOp deriving instance Typeable ASTExpr deriving instance Data (ASTMany ()) deriving instance Data (ASTOp ()) deriving instance Data (ASTExpr ()) Currently I have values of type (AST ()), and I need to transform them to (AST ASTAttachment) in the obvious way. Now I'm stuck at "derive instance Data (ASTExpr ASTAttachment)", where ghc reports "Multiple declarations of '$cSomeConstructor'". Even if I can get past this, I still don't know how to use SYB to transform between trees of different types.

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