January 2014 - Haskell-Cafe

How does one read complicated type signatures?
by Jakub Arnold 23 Jan '14

23 Jan '14

Hello world, I’ve just started playing with the http-conduit package and started inspecting type signatures of some of the functions, and I stumbled upon many things that look like this *Main Network.HTTP.Conduit> :t withManager withManager :: (monad-control-0.3.2.2:Control.Monad.Trans.Control.MonadBaseControl IO m, transformers-0.3.0.0:Control.Monad.IO.Class.MonadIO m) => (Manager -> resourcet-0.4.10:Control.Monad.Trans.Resource.Internal.ResourceT m a) -> m a The question is … how am I supposed to read these? It seems that they wrap around (maybe) 80 characters, which makes them unreadable … and the package prefixes also don’t really help. I’d like to point out here that I’m really a noob without any practical experience in Haskell, I’ve just read a few books. So I don’t have a problem understanding the concepts, but more and more often I find myself struggling just to read the type signature correctly Thanks for any tips, Jakub

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Sorting a Repa Array
by Hans Georg Schaathun 23 Jan '14

23 Jan '14

Hi, I am trying to parallellise genetic algorithms using haskell, but I get stuck at the stage where I need to sort the array of chromosomes based on their fitness. Using Repa, I am neither able to find a nice library implementation to sort an array, nor can I figure out how to generate a new array modifying only a few entries. I am not at a stage where I need to optimise execution time. It is still at the stage of learning exercise, wrt both to genetic algorithms, haskell, and splittable pseudo-random number generators. Thus, I would be grateful for both naïve and clever suggestions :-) Except for the sorting stage, the problem seems to well suited for Repa, with a chain of operations which can be mapped on the array. I am also planning to explore accelerate later for GPU work, and Repa seems to be a better step on the way than most other alternatives. My arrays are 1-D boxed arrays BTW. The entries are tuples including an Unboxed Vector (and a Double on which the sort order is defined). Doing it as a 2-D Unboxed Array is further down on the agenda; I need more practice before I get my head around using slices correctly. So, any advice? Whether it involves conversion to a data type where a library sort routine is available, a library sort routine I have missed, or how to create new deferred arrays by updating selected entries in the input array so that I can implement sorting myself. TIA -- :-- Hans Georg

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Re: [Haskell-cafe] off-topic question: how well do you think linguistic relativity applies to PLs and programming?
by Simon Yarde 23 Jan '14

23 Jan '14

> programming ... languages, and in particular the way that programmers use them, afford a > capacity for essentially limitless amounts of abstraction, unlike human > language and human communication. Au contraire! Humans use just such powerful abstractions in language, arts and sciences — what we call humanity is built on no less. They're called *memes*. The only difference is that humans communicate on a foundation of shared experience that is a little more fuzzy and infinitely richer than the result of a data-base lookup. Simon Yarde On 22 Jan 2014, at 23:35, Keshav Kini <keshav.kini(a)gmail.com> wrote: > Lucas Paul <reilithion(a)gmail.com> writes: >> This is my take, as a CS undergraduate. >> >> I'm not sure if we can say that a programmer's language of choice >> determines the way they think about programming (the strong version of >> linguistic relativity for programming, as I see it). But I think it's >> fairly obvious that the language we choose to use to solve a problem >> affects how we think about the solution. That's basically the entire >> raison d'être for domain-specific languages (DSLs)! >> >> DSLs are popular (and becoming more so) precisely because the right >> choice of DSL can make expressing the solution to a particular kind of >> problem almost trivial. A poor choice can almost doom an endeavor. >> Imagine trying to query a database in assembly language. No SQL. It >> would at the very least require some mental gymnastics that a SELECT >> statement simply obviates. > > I think this can be partially explained by noting that programming > languages, and in particular the way that programmers use them, afford a > capacity for essentially limitless amounts of abstraction, unlike human > language and human communication. For example, I might easily have a > magic library that does exactly what I want, with bindings for my > programming language of choice, in which case I don't need to think > about what to do, I just call the appropriate function. In human > communication, while someone might have perfectly formulated exactly the > idea I want to communicate, it is rarely sufficient for the purpose of > communication to say "insert pages 204 through 356 of _Foobar_ by John > Doe here" in the middle of a conversation :) > > -Keshav > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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off-topic question: how well do you think linguistic relativity applies to PLs and programming?
by Ömer Sinan Ağacan 23 Jan '14

23 Jan '14

Hi all, I recently got myself thinking about programming languages and their effects on programmers. I already knew that concept of "linguistic relativity" ( http://en.wikipedia.org/wiki/Linguistic_relativity ) and I was thinking that this may be relevant with programming too, although I don't have any concrete evidence. I was wondering if anyone else also find that idea of programming language's effect of the programmer interesting. Do we have any research on that kinds of things? Thanks,

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Why Maybe exists if there is Either?
by Vlatko Basic 22 Jan '14

22 Jan '14

Hello Cafe, With my current knowledge of Haskell, I do not see why is there Maybe if we have Either. For example, Functor and Monad instances (and many others) of Maybe and Either are the same (except for fail). In other words, this should hold: Maybe a = Either String a -- String or something else Nothing = Left "" Just a = Right a I'm curious to find out what was the reasoning to make Maybe? What is the added value with introducing it? In which situations the above substitution does not hold? Best regards, vlatko

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ANN: ZuriHac 2014
by Bas van Dijk 22 Jan '14

22 Jan '14

Dear Haskellers, After a very successful ZuriHac 2013 we are delighted to announce ZuriHac 2014! When: Friday 6 June 2014 - Sunday 8 June 2014 Where: Erudify offices, Zurich, Switzerland ZuriHac is an international Haskell hackathon: a grassroots, collaborative coding festival with a simple focus, to build and improve Haskell libraries, tools, and infrastructure. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools, or even start new ones! Registration ------------ If you wish to attend, please register by filling in this form: http://bit.ly/ZuriHac2014 Please note that we have a limited number of places -- first come, first served. Full details can be found on the wiki page: http://www.haskell.org/haskellwiki/ZuriHac2014 We look forward to seeing you there! -- The organisers of ZuriHac 2014 ZuriHac 2014 is sponsored by Erudify & Google

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Haddock Constructor parameter documentation without record syntax
by satvik chauhan 22 Jan '14

22 Jan '14

Hi, Does anybody know how to document parameters of a constructor in haddock. The following code gives parse error on the second parameter while generating haddock documentation. ``` data Foo = Foo Int -- ^ First Int -- ^ Second ``` -Satvik

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Help on simpleHttp in Network.HTTP.Conduit problem
by Stuart Mills 22 Jan '14

22 Jan '14

I copied and pasted some html parsing demo source from FP School. While the code works on the FP demo site (in the IDE), I get the following error on my Windows 7 64 bit: InternalIOException getAddrInfo: does not exist (error 10093). Thanks

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ANNOUNCE: Hac NYC: Haskell Hackathon in NYC, April 4-6
by MightyByte 21 Jan '14

21 Jan '14

Greetings, On behalf of the organizers, I am pleased to officially announce the first Hac NYC, a Haskell hackathon/get-together to be held April 4-6 in New York City. Space will be provided by Etsy on Saturday and Sunday. Details for Friday will be announced as they become finalized. We want to stress that everyone is welcome---you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, you must officially register by filling out our registration form [1]. Other details for travel, lodging, etc can be found on the Hac NYC wiki [2]. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in April! -The Hac NYC Team [1] https://docs.google.com/forms/d/1taZtjgYozFNebLt1TR2VnKv-ovD2Yv5sOdSZzmi_xF… [2] http://www.haskell.org/haskellwiki/Hac_NYC [3] http://www.haskell.org/haskellwiki/Hac_NYC/Talks

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Constraint on data type
by Vlatko Basic 21 Jan '14

21 Jan '14

Hi Cafe, I have these typeclasses defined: class UA a where ... class RA a where ... and this data definition data (UA u, RA r) => AD u r = AD { ... } I have another data definition data ACT aur c = ACT { x :: aur, ... } but the type 'aur' should be restricted to (AD u r) only. (I would like to keep 'aur' as 1 parameter, not as data ACT u r c = ACT { x :: AD u r, ... } ) How can such constraint be defined? Best regards, vlatko

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