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Seeking help with Windows + SDL2: Phantom Header Syndrome

Haskell on Reddit - Wed, 08/13/2014 - 2:54am
C:\dev>cabal install sdl2 Resolving dependencies... Configuring sdl2-1.1.0... Failed to install sdl2-1.1.0 Last 10 lines of the build log (C:\Users\tejon\AppData\Roaming\cabal\logs\sdl2-1.1.0.log ): setup-Cabal- Missing dependency on a foreign library: * Missing (or bad) header file: SDL.h This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. If the header file does exist, it may contain errors that are caught by the C compiler at the preprocessing stage. In this case you can re-run configure with the verbosity flag -v3 to see the error messages. cabal: Error: some packages failed to install: sdl2-1.1.0 failed during the configure step. The exception was: ExitFailure 1

I've checked and triple-checked the environment variables and tried the all the suggested flags. The header is there, the libs are there, and nothing complains except cabal itself. Any suggestions?

submitted by tejon
[link] [3 comments]
Categories: Incoming News

Stream Processing with a Spreadsheet

Lambda the Ultimate - Tue, 08/12/2014 - 10:10pm

ECOOP 2014 paper (distinguished) by Vaziri et. al, abstract:

Continuous data streams are ubiquitous and represent such a high volume of data that they cannot be stored to disk, yet it is often crucial for them to be analyzed in real-time. Stream processing is a programming paradigm that processes these immediately, and enables continuous analytics. Our objective is to make it easier for analysts, with little programming experience, to develop continuous analytics applications directly. We propose enhancing a spreadsheet, a pervasive tool, to obtain a programming platform for stream processing. We present the design and implementation of an enhanced spreadsheet that enables visualizing live streams, live programming to compute new streams, and exporting computations to be run on a server where they can be shared with other users, and persisted beyond the life of the spreadsheet. We formalize our core language, and present case studies that cover a range of stream processing applications.

Categories: Offsite Discussion

ANNOUNCE: wxHaskell

haskell-cafe - Tue, 08/12/2014 - 5:37pm
L.S., I am happy to announce a new version of wxHaskell. This version binds to both wxWidgets 2.9 and 3.0 [0]. As wxWidgets 3.0 is significantly better, I advice everyone, who is already using wxHaskell, to upgrade to wxWidgets 3.0.1 What is it? ----------- wxHaskell[1] is a portable and native GUI library for Haskell. The goal of the project is to provide an industrial strength GUI library for Haskell, but without the burden of developing (and maintaining) one ourselves. wxHaskell is therefore built on top of wxWidgets – a comprehensive C++ library that is portable across all major GUI platforms; including GTK, Windows, X11, and MacOS X. Furthermore, it is a mature library (in development since 1992) that supports a wide range of widgets with the native look-and-feel. What's new? ----------- - Accept both wxWidgets 2.9 and 3.0, with a preference for 3.0 - Added bindings for wxGraphicsContext::CreatePath - Use the same custom hook for `cabal copy` in wxc/Setup.hs - Many updates in definition
Categories: Offsite Discussion

Neil Mitchell: Safe library - generalising functions

Planet Haskell - Tue, 08/12/2014 - 3:08pm

Summary: The Safe library now has exact versions of take/drop, with twelve functions implemented on top of a generalised splitAt.

The Safe library is a simple Haskell library that provides versions of standard Prelude and Data.List functions that usually throw errors (e.g. tail), but wrapped to provide better error messages (e.g. tailNote), default values (e.g. tailDef) and Maybe results (e.g. tailMay).

I recently released version 0.3.5, which provides a new module Safe.Exact containing crashing versions of functions such as zip/zipWith (which error if the lists are not equal length) and take/drop/splitAt (which error if there are not enough elements), then wraps them to provide safe variants. As an example, the library provides:

takeExact :: Int -> [a] -> [a]
takeExactMay :: Int -> [a] -> Maybe [a]

These are like take, but if the Int is larger than the length of the list it will throw an error or return Nothing. Some sample evaluations:

takeExactMay 2 [1,2,3] == Just [1,2]
takeExact 2 [1,2,3] == [1,2]
takeExactMay 2 [1] == Nothing
takeExact 2 [1] ==
1:error "Safe.Exact.takeExact, index too large, index=2, length=1"
take 1 (takeExact 2 [1]) == [1]

So takeExactMay computes up-front whether the whole computation will succeed, and returns a Nothing if it will fail. In contrast, takeExact produces elements while they are present, but if you demand an additional element that is missing it will result in an error. All the exceptions in the Safe library are designed to provide the maximum level of detail about what went wrong, here telling us the index we were after and the length of the list.

The library provides takeExact, dropExact and splitAtExact, plus Def/May/Note versions, resulting in twelve similar functions. While the implementation of any one function is reasonably short (although not that short, once proper error messages are provided), I didn't want to write the same code twelve times. However, generalising over functions that check up-front and those that check on-demand requires a bit of thought. In the end I settled for:

splitAtExact_ :: (String -> r) -> ([a] -> r) -> (a -> r -> r) -> Int -> [a] -> r
splitAtExact_ err nil cons o xs
| o < 0 = err $ "index must not be negative, index=" ++ show o
| otherwise = f o xs
f 0 xs = nil xs
f i (x:xs) = x `cons` f (i-1) xs
f i [] = err $
"index too large, index=" ++ show o ++ ", length=" ++ show (o-i)

Here the splitAtExact_ function has a parameterised return type r, along with three functional arguments that construct and consume the r values. The functional arguments are:

  • err :: String -> r, says how to convert an error into a result value. For up-front checks this produces a Nothing, for on-demand checks this calls error.
  • nil :: [a] -> r, says what to do once we have consumed the full number of elements. For take we discard all the remaining elements, for drop we are only interested in the remaining elements.
  • cons :: a -> r -> r, says how to deal with one element before we reach the index. For take this will be (:), but for functions producing a Maybe we have to check the r parameter first.

With this generalisation, I was able to write all twelve variants. As a few examples:

addNote fun msg = error $ "Safe.Exact." ++ fun ++ ", " ++ msg

takeExact = splitAtExact_ (addNote "takeExact") (const []) (:)

dropExact = splitAtExact_ (addNote "dropExact") id (flip const)

takeExactMay = splitAtExact_ (const Nothing) (const $ Just []) (\a -> fmap (a:))

dropExactMay = splitAtExact_ (const Nothing) Just (flip const)

splitAtExact = splitAtExact_ (addNote "splitAtExact")
(\x -> ([], x)) (\a b -> first (a:) b)

splitAtExactMay = splitAtExact_ (const Nothing)
(\x -> Just ([], x)) (\a b -> fmap (first (a:)) b)

Normally I would have defined takeExact and dropExact in terms of fst/snd on top of splitAtExact. However, in the Safe library error messages are of paramount importance, so I go to additional effort to ensure the error says takeExact and not splitAtExact.

Categories: Offsite Blogs

What are some Haskell alternatives to Pandas/Numpy?

Haskell on Reddit - Tue, 08/12/2014 - 1:22pm

Title mostly says it all. I'm doing some data work at my job, and since we're a python shop we're using mostly pandas and numpy. They're great at what they do, but I would love to be able to do at least some of the same things in Haskell. It seems like making something like a pandas DataFrame would be possible in Haskell, and be quite useful. What are the best libraries for manipulating and operating on large matrices in Haskell, with efficient implementations of high-level tasks like time series, merge/join/groupby, parsing CSV and XLS, etc?

submitted by saikyou
[link] [26 comments]
Categories: Incoming News

A question about Functors.

Haskell on Reddit - Tue, 08/12/2014 - 1:01pm

If a functor a categorical homomorphism on objectes such as Maybe, then why do we call Maybe a Functor? Shouldn't fmap be the functor and the Maybe type constructor be the Category?

Forgive me if this is a stupid question, i'm reading "learn you a haskell". Thank you.

submitted by yyttr3
[link] [13 comments]
Categories: Incoming News

Help? Noob attempting to build hdevtools on Windows for Platform 2014.

Haskell on Reddit - Tue, 08/12/2014 - 10:52am

Haskell Sublime Text integration requires hdevtools.

hdevtools won't build right on Windows (requires unix, and even with MSYS it throws an unqualified error building that -- granted I haven't yet tried cygwin) which is fixed by this fork.

That fork fails to build with the newly-release Platform 2014 because it requires process==1.1.* and process is now at, with previous versions not installed.

Attempting to install process- fails because Not in scope: 'unsafePerformIO'.

Changing the requirement on the windows hdevtools fork to process==1.2.* allows me to configure, then gives me this on build:

src\Info.hs:130:59: Not in scope: data constructor `GHC.MatchGroup' Perhaps you meant `GHC.DocGroup' (imported from GHC) src\Info.hs:164:5: Not in scope: `Pretty.showDocWith' Perhaps you meant `Pretty.showDoc' (imported from Pretty) src\Info.hs:229:12: Not in scope: type constructor or class `PprTyThing.PrintExplicitForalls'

And the combination of my knowledge and Haskell documentation accessibility do not yet stack up to the task of hacking this out. :) Any pointers?

submitted by tejon
[link] [7 comments]
Categories: Incoming News

transformers: remove instance MonadFix ExceptT and ErrorT

libraries list - Tue, 08/12/2014 - 10:13am
I propose to remove the MonadFix instance for the ExceptT transformer, because it gives the illusion that it can handle exceptions, which it cannot. The current implementation is: instance (MonadFix m) => MonadFix (ExceptT e m) where mfix f = ExceptT $ mfix $ \ a -> runExceptT $ f $ case a of Right x -> x Left _ -> error "mfix ExceptT: Left" You see, that it cannot handle the exception case. Whenever someone needs an mfix on ExceptT I strongly suggest that he first handles the exception, thus turning (ExceptT e m a) into plain (m a) and then calls 'mfix' only on (m a). I further propose to declare the MonadFix instance as non-implementable in a way suggested in [1]: class NoMonadFix m where instance (NoMonadFix m) => MonadFix (ExceptT e m) where mfix = error "unimplementable" where no instance of NoMonadFix exist and NoMonadFix is not exported. Whenever someone tries to call 'mfix' on 'ExceptT e m' he will get the type error, that a NoMonadFix instance on 'm' is mi
Categories: Offsite Discussion

ಠ_ಠ :: String -> a

Haskell on Reddit - Tue, 08/12/2014 - 9:51am
Categories: Incoming News

HGamer3D - version 0.4

haskell-cafe - Tue, 08/12/2014 - 7:36am
You will find version 0.4 of HGamer3D on github and hackage now. It also comes with some updated introductions on . best regards Peter (uotbw) _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe< at >
Categories: Offsite Discussion

[code review] Strive: a client for the Strava API

Haskell on Reddit - Tue, 08/12/2014 - 7:24am

I've spent the last couple months tinkering on a client for Strava's API. I ended up using http-conduit for making requests, aeson for decoding JSON, and a lens-based interface for manipulating records. I'd love some feedback about the decisions I've made and the code I wrote. In particular, I'd like some feedback about:

  • Template Haskell. It cleaned up my code, but the TH code I wrote is gross. I don't yet know enough to make it better. (See issue #69 for details.)
  • Template Haskell again. Is it recommended to use something like zeroth to avoid a compile-time dependency on TH? (See issue 76 for details.)
  • Internals. Should I export stuff in the Strive.Internal module?

Thanks in advance for any input!

submitted by taylorfausak
[link] [3 comments]
Categories: Incoming News

What's your opinion on "Beginning Haskell: A Project-Based Approach"?

Haskell on Reddit - Tue, 08/12/2014 - 3:57am

I've just finished chapter 11, and I don't really know whether I should continue. /u/ReinH already stated that there's something wrong with it:

[...] unfortunately I don't know of a good, modern, pragmatic guide to Haskell. Beginning Haskell: a Project Based Approach could have been that but the book is so poorly put together that I worry that one might become a worse Haskeller after reading it, which is a real shame (I suspect that a lot of this is the publisher's fault).

The book is riddled with wrong types, constraints and typos. While typos are usually OK, wrong types and constraints are a personal no-go. An example:

Functors, Applicatives, and Monads

There are some interesting things going on with the use of (<$>) in the preceding section. The signature of (<$>) is equal to that of fmap of a Functor:

(<$>) :: Applicative f => (a -> b) -> f a -> f b fmap :: Functor f => (a -> b) -> f a -> f b

At some point before that section (<$>) has type (a ->b) -> f a -> f b (missing constraint). Stuff like this prevents me from recommending the book to anyone asking about Haskell. However, it also prevents me from going on.

So how reliable are chapters 11-16 (and appendix)? Are they also riddled with wrong types and/or missing constraints? And what is your general opinion on BH:APBA?

submitted by qZeta
[link] [24 comments]
Categories: Incoming News

strictness in Data.Map.Strict with Text

Haskell on Reddit - Tue, 08/12/2014 - 2:40am

I was watching the Udacity course on "Design of Computer Programs" and thought it would be kindof fun to redo the stuff in Haskell.

So I more or less 1:1 the python stuff there into Haskell (you can find it here: Gist )

Now to get the stats for the hand-ranks I thought it a good idea to draw 1mio. random hands and classify them using this function:

randomStats :: Int -> IO [(Text, Float)] randomStats n = percentOf . sortWith (negate . snd) . M.toList <$> drawHands n M.empty where drawHands n' m = if n' == 0 then return m else do !r <- classifyRank . head <$> randomHands defaultDeck 1 let m' = M.insertWith (+) r 1 m drawHands (n'-1) m' percentOf :: [(Text, Int)] -> [(Text, Float)] percentOf = map (\ (x,c) -> (x, 100 * fromIntegral c / fromIntegral n))

Where this is using Data.Map.Strict and Data.Text.

Now as far as I understand the strict maps they should eval the keys into WHNF - but when I remove the bang from r I can watch the program eating memory till I run out of RAM/Swap ;) ... with it it runs fine (not really quick but ok)

Can someone explain to me why? Should not insertWith do basically the same?

Thank you for your help

submitted by CKoenig
[link] [8 comments]
Categories: Incoming News

The Good, the Bad and the GUI

haskell-cafe - Mon, 08/11/2014 - 11:16pm
Dear All, Haskell is great for great many areas, let me name just two: - parsers, translators, interpreters, compilers; highly concurrent systems. Haskell is however not great for GUIs. I've been thinking a little why this is so. I think one of the reasons might be that in Haskell it is unusual to deal with data that is incomplete or otherwise erroneous. Let me try to explain, what I mean, by example. If you declare Person class in Java, you automatically get a thingy that you can readily use in UI construction, because all the fields can temporarily be null, even the required ones. In Haskell you'd need two data types: the usual proper Haskell data type, and another which wraps every field in Maybe, facilitates editing, validation, etc. Perhaps it would be possible to generate one data type from the other, or generate both from a common specification. Let me write the same thing in other words. It is not controversial to say on this list that specifying what is correct means, is a good idea.
Categories: Offsite Discussion