November 2015 - Beginners

Multiline code in GHCi
by Shishir Srivastava 30 Aug '17

30 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 25 Jan '16

25 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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Anyone suggest .csv file parser?
by Jeon-Young Kang 30 Nov '15

30 Nov '15

Hi all. I am working on import *.csv file into haskell. As you guys recommended, I built code for comparison human's age as follows. data Person = Person {name:: String, age:: Int } deriving(Show) data Relations = Friend | Older | Younger class Comparison a where compare:: a -> a -> Relations instance Comparison Person where compare (Person a b) (Person a b) | b1 == b2 = Friend | b1 > b2 = Older | b1 < b2 = Younger Also, I have .csv file as follows. name age tom 19 jane 21 By using above code, I would like to apply the above code for .csv file. What is the best for this purpose? Sincerely, JY

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comment on this debugging trick?
by Dennis Raddle 29 Nov '15

29 Nov '15

When I design my code, I am aware of the properties I expect my internal data representations to have, or I want input data to conform to my expectations. For example, right now I'm writing code that reads MusicXML. MusicXML is a crazy language, way too complicated for what it does, and the music typesetters that export MusicXML all do their own idiosyncratic things with it. I'm only reading the output of one typesetter, Sibelius, and although I don't know the internals of Sibelius, I can make some assumptions about what it's going to produce by a few examples, plus my application doesn't use the full range of MusicXML. I don't need to handle every case, is what I'm getting at. However, if I should have been wrong about my assumptions when I wrote the code, I don't want my program to behave erratically. I want to find out exactly what went wrong as soon as possible. Previously, I was including a lot of exception throwing, with each exception having a unique message describing what had happened, and in particular describing where in the code it is located so I can find the problem spot If I throw generic error messages like "Error: problem parsing" and I have many of those located in the code, I need the debugger to find it. Same if I use things like "fromJust" or "head" I haven't had good experiences using the debugger. Maybe that's my fault, but I like that I can locate my unique exceptions and that they are descriptive. But a month ago I decided this system is pretty ugly. It bloats the code a lot, and requires writing a lot of messages for situations that will probably never occur. I am trying a different method now. I write the code so that surprising behavior will result in a case or pattern exhaustion, which produces a run time message with a file name and line number. I'm not sure why ghc gives the location for a case exhaustion, but not something like "head". For example, instead of using head xs I can write x = case xs of {y:_ -> y} This is a little bloat but not too bad. Most of the time I'm actually structuring cases and patterns, and it actually helps me to simplify code to think of how to write it with the fewest cases and so that a case exhaustion will indicate something pretty specific. Any comments welcome.

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REPL with visualisation
by Ondrej Nekola 29 Nov '15

29 Nov '15

Hi Just curious: is there some recommended "first choice" combination of REPL and libs for (Apple) Swift styled "playgrounds"? (I have just seen a fractalish coffee spill and remembered that I have not implemented Mandelbrot set since high school and Pascal days and maybe it's time to be a bit childish again). Thanks Ondra 'satai' Nekola ondra(a)nekola.cz

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is Haskell practical?
by Dennis Raddle 28 Nov '15

28 Nov '15

I've been using Haskell for several years, but only really got into the power of the typeclasses recently. It's marvelous how pretty abstract concepts like Monoid, Monad, Functor, etc. can be used. 1. First it's remarkable how many instances they have on useful data structures. So the concepts become less abstract and more applied. 2. Second it's remarkable how the use of these concepts leads to expressive power and conciseness. Haskell is therefore a very beautiful language that seems to either be exploiting some mighty amazing coincidences or on the other hand is actually plumbing beautiful truths about the universe. But what if someone came along and said, "Well, conciseness isn't all that important. Having to type more isn't much of a drawback -- it doesn't really increase the time it takes to write a program once you consider that there is a greater time spent in requirements collection, overall design, debugging, and documentation. The real drawback of concise/expressive Haskell is the difficulty in understanding and using it fluently. Ultimately Haskell is just mathematicians having fun, but not very practical." Now, I would be sad to see someone argue this point of view, as I find Haskell to be beautiful and fun. But the learning difficulties are very real... 16 years of using imperative languages have hardly prepared my brain at all for concise/expressive Haskell. I only use it for a single hobby project and will probably never get beyond learning the list and Maybe instances of a few typeclasses. (It blows my mind to read the docs and see a dozen instances for certain typeclasses.... yikes! When would I ever be able to learn and use those?) I would just wonder what you, the reader, would say in response to an argument against the practicality of Haskell for these specific reasons: (1) conciseness isn't that important (2) it requires too much of an advanced mathematician's brain to use well. I have read it has advantages in parallel computing. What else? Or maybe you would say -- "It's not practical. So EMBRACE it. Have fun." Dennis

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ffi array and peekArray
by PICCA Frederic-Emmanuel 26 Nov '15

26 Nov '15

Hello, Here the signature of one of my C function -- hkl.h -- typedef struct _HklFactory HklFactory; HKLAPI HklFactory **hkl_factory_get_all(size_t *n) HKL_ARG_NONNULL(1); HKLAPI HklFactory *hkl_factory_get_by_name(const char *name, GError **error) HKL_ARG_NONNULL(1) HKL_WARN_UNUSED_RESULT; HKLAPI const char *hkl_factory_name_get(const HklFactory *self) HKL_ARG_NONNULL(1); HKLAPI HklGeometry *hkl_factory_create_new_geometry(const HklFactory *self) HKL_ARG_NONNULL(1); HKLAPI HklEngineList *hkl_factory_create_new_engine_list(const HklFactory *self) HKL_ARG_NONNULL(1); I will focuss for now only the get_all method. As you can see even for the C client of my API HklFactory is an opac struct This get_all method return an array of (HklFactory *) and get the size of the array via the n parameter. So I am trying to use this API from haskell with the foreign system and return a [HklFactory] import Foreign (Ptr, peek) import Foreign.Marshal.Alloc (alloca) import Foreign.Marshal.Array (peekArray) data HklFactory foreign import ccall safe "hkl.h hkl_factory_get_all" c_hkl_factory_get_all :: Ptr Int -> IO (Ptr HklFactory) hklFactoryGetAll :: IO [HklFactory] hklFactoryGetAll = alloca $ \n -> do factories <- c_hkl_factory_get_all n peekArray n factories but indeed it doesn not work, peekArray complain that n is Ptr Int instead of Int. what should I do in order to get my array of HklFactory thanks for your help. Frederic

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Re: [Haskell-beginners] Simple function comparison
by MJ Williams 26 Nov '15

26 Nov '15

That's great, thanks. Matt At 11:53 26/11/2015, you wrote: >On Thu, Nov 26, 2015 at 6:20 PM, MJ Williams ><<mailto:matthewjwilliams101@gmail.com>matthewjwilliams101(a)gmail.com> wrote: > > >therefore f ( g a ) = g ( f a ) . (transitivity) > > >If you're interested in getting a firm grasp of airtight proofs -- >with a view toward Haskell (and Agda and Idris) mastery -- you might >want to pick and choose your way through the web-based proof exercises here: > ><https://www.coursera.org/course/intrologic>https://www.coursera.org/course/intrologic > >This is a course firmly in the American analytic philosophy >tradition, so Logic here is Symbolic Logic. Bonus: the course keeps >AI and Machine Learning applications in the backdrop. > >It's unfortunate that it's some kind of well-kept secret. > >Those who don't need it -- because they've obtained the knowledge >elsewhere -- won't know about it. And those who do want that >knowledge also won't know about it. > >Did I mention the exercises are web-based? Yes, you get >instantaneous feedback on whether you've got a correct proof or not. > >-- Kim-Ee >_______________________________________________ >Beginners mailing list >Beginners(a)haskell.org >http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Simple function comparison
by Stephen Renehan 26 Nov '15

26 Nov '15

Hi, I'm looking to do a comparison between 2 simple functions to see if they are equivalent but I appear to be running into some problems if anyone can help. The two functions I want to compare are f (g a) and g (f a). I have f defined and g defined as f :: a -> a f = undefined g :: a -> a. g = undefined The compiler was complaining about the type signature lacking a binding so added undefined to get around the error. I understand that I must be missing something very basic as they look incomparable in this form. Any suggestions of what changes I should make such a comparison practical? Many thanks, Stephen -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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Comparison between fields of each record
by Jeon-Young Kang 25 Nov '15

25 Nov '15

Dear All. I'd like to compare fields of each record. here is my record. data Person = Person {name:: String, age:: Int } deriving(Show) data Relations = Friend | Older | Younger class Comparison a where compare:: a -> a -> Relations instance Comparison Person where compare Person a b Person a b | b1 == b2 = Friend | b1 > b2 = Older | b1 < b2 = Younger How can I fit it? Sincerely, Jeon-Young Kang

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