December 2015 - Beginners

Multiline code in GHCi
by Shishir Srivastava 29 Aug '17

29 Aug '17

Hi, I am trying to run this basic multiline code in GHCi. I am not sure if this is even possible without writing it in a file and compiling it in a traditional way. ------- Prelude> :set +m Prelude> let main1 = do Prelude| a <- readLn Prelude| return a Prelude| Prelude> Prelude> main1 <interactive>:75:1: No instance for (Show (IO b0)) arising from a use of `print' In the first argument of `print', namely `it' In a stmt of an interactive GHCi command: print it ------- Could someone please confirm if the function can be defined and called like this in GHCi or point to where am going wrong. Thanks, Shishir Srivastava

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Functional Programming for the Object Oriented
by Øystein Kolsrud 24 Jan '16

24 Jan '16

I gave a presentation on functional programming a couple of weeks ago that I thought might be of interest to this community as well: https://www.youtube.com/watch?v=I2tMmsZC1ZU It's target audience is programmers familiar with object oriented programming, and it presents how concepts from the functional paradigm are relevant also in most modern OO languages. For more information about the presentation, please refer to the following site: http://www.foocafe.org/previous_event/functional-programming-for-the-object… -- Mvh Øystein Kolsrud

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Parametrizing [] as an instance of the Functor type class
by Olumide 08 Jan '16

08 Jan '16

According to LYH, the list is an instance of the Functor type class: instance Functor [] where fmap = map Why does 'f' not appear in this expression?, considering that Functor is defined as class Functor f where fmap :: (a -> b) -> f a -> f b Overall, I'm a bit confused about the relationship between the type constructor f and []. Thanks, - Olumide

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Why must inflix constructors begin with a colon?
by Olumide 31 Dec '15

31 Dec '15

I'm new to Haskell and wondering why this restriction? For example (from LYH): infixr 5 :-: data List a = Empty | a :-: (List a) deriving (Show, Read, Eq, Ord) Thanks, - Olumide

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Doubts about functional programming paradigm
by Abhishek Kumar 29 Dec '15

29 Dec '15

I am a beginner in haskell.I have heard a lot about haskell being great for parallel programming and concurrency but couldn't understand why?Aren't iterative algorithms like MapReduce more suitable to run parallely?Also how immutable data structures add to speed?I'm having trouble understanding very philosophy of functional programming, how do we gain by writing everything as functions and pure code(without side effects)? Any links or references will be a great help. Thanks Abhishek Kumar

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How to show a predicate
by martin 29 Dec '15

29 Dec '15

Hello all, in my program, I do stuff with predicates (a->Bool). For the most part this representation is just fine, but at the very end I need to convert a resulting predicate into a String so I can write it to a file. Wenn I represent my predicates as Lists or Sets, then this is doable and I am tempted to do it this way. The only other option I could come up with was to have a representation of "everything", which would in my case be large (10^8) but finite. Then I could construct a List or a Set at the very end, as [x | x<-everything, p x] without having explicit sets in the intermediate steps. I cannot see any other option, but I thought I better ask.

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find an element in a list
by Fabien R 26 Dec '15

26 Dec '15

As a newbie, I'm studying the pdf 'the Haskell road to logic, math and programming' and I'm stuck with one exercise. I want to extract x if x is at the beginning of a list. I thought to use something like this: extractIfBegins x [xs] | [xs] == (x:ys) = [ys] | otherwise = [xs] But ghci complains that ys is not defined. Without giving the answer, can someone give a hint about the approach to follow ? -- Fabien

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warning in making instances of Functor class!
by Abhijit Patel 25 Dec '15

25 Dec '15

why am i getting warning in the following code?? though it works perfectly!!! Prelude> newtype Pair1 c b a=Pair1 {getPair1 :: (a,b,c)} Prelude> :{ Prelude| instance Functor (Pair1 m n) where Prelude| fmap f (Pair1 (x,y,z))=Pair1 (f x,y,z) Prelude| :} <interactive>:55:10: Warning: No explicit implementation for ‘Prelude.fmap’ In the instance declaration for ‘Functor (Pair1 m n)’ -- The below shows it is working fine! Prelude> getPair1 $ fmap (*100) (Pair1 (2,3,1)) (200,3,1)

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FW: Initial startup of GHCi (Windows) v. 7.10.3
by pmcilroy＠gmail.com 25 Dec '15

25 Dec '15

Running on windows 10, GHC interactive fails to load any standard packages. The startup message does not include the prompt about initializing packages do, as shown commonly on Haskell getting started tutorials. Instead I see: GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help Prelude> :l Data.Text <no location info>: module ‘Data.Text’ is a package module Failed, modules loaded: none. Prelude> Any pointers on what to do next?

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Re: [Haskell-beginners] Doubts about functional programming paradigm
by Doug McIlroy 22 Dec '15

22 Dec '15

Though this post was prompted by beginners(a)haskell.org, it seemed pertinent enough for cross-posting to haskell-prime. > I just wanted to make a point that learning haskell is *much* > harder than learning most other programming languages I would not put it that way. One gets so much farther in capability almost instantly with a functional language. I know. At the dawn of functional programming, John McCarthy could teach in one hour everything from cons up to symbol manipulation that would have been a month's project in Fortran. (My account of that hour is at http://www.paulgraham.com/mcilroy.html.) What's hard about Haskell is that its landscape extends into terrains not imagined elsewhere. As long as you stay in the flat Floridian landscape of Fortran et al, you don't need to explore the Himalayas of Haskell. But of course the very exhilaration and inspiration of the high peaks draws one into the "*much* harder" training necessary to surmount them. > the learning aids that are available are not yet cohesive > enough to present a clear path ahead for the average programmer. I agree with this. Alas, there does not exist a definition of the Haskell one reads about on the haskell-cafe mailing list. I treasure my hardcover Haskell 98 report--so rare you can't even find it in the used-book marketplace. Haskell 2010 was obsolete as soon as it was promulgated. Contemporary Haskell as practiced by cognoscenti flaps in a gale of language pragmas. There is no authoritative source about these pragmas. They are listed and described in the GHC User Guide, but that source all too often defines solely by example, not even bolstered by a formal syntax specification. I earnestly hope the newly revived Haskell-prime committee can rectify this state of affairs, and that GHC will provide a compliant implementation. Doug McIlroy

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