- Beginners - Haskell.org

is Haskell practical?
by Dennis Raddle 27 Nov '15

27 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

8 11

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

2 4

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

1 0

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>

6 10

Comparison between fields of each record
by Jeon-Young Kang 24 Nov '15

24 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

4 4

Re: [Haskell-beginners] Beginners Digest, Vol 89, Issue 38 How to define a simple function
by Francisco Gutierrez 24 Nov '15

24 Nov '15

Dear sirs:I am defining a function halve, that breaks a list in two halfs. But I always get an error. Below is my code. What am I doing wrong?Thanks!halve xs = [take (floor (longitud/2)), drop (floor (longitud/2))] where longitud = length xs From: "beginners-request(a)haskell.org" <beginners-request(a)haskell.org> To: beginners(a)haskell.org Sent: Saturday, November 21, 2015 8:21 PM Subject: Beginners Digest, Vol 89, Issue 38 Send Beginners mailing list submissions to beginners(a)haskell.org To subscribe or unsubscribe via the World Wide Web, visit http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners or, via email, send a message with subject or body 'help' to beginners-request(a)haskell.org You can reach the person managing the list at beginners-owner(a)haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Beginners digest..." Today's Topics: 1. Re: Warning: Tab Character (Dan Stromberg) 2. Re: Warning: Tab Character (Jeon-Young Kang) 3. Re: Warning: Tab Character (Thomas Jakway) ---------------------------------------------------------------------- Message: 1 Date: Sat, 21 Nov 2015 16:44:21 -0800 From: Dan Stromberg <strombrg(a)gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> Subject: Re: [Haskell-beginners] Warning: Tab Character Message-ID: <CAOvKW55oxCcEhtqppC0bXQZjOEEv5vcYnBTWHaj9C5vhhm7g+w(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" I believe the first version of make required tabs, but some subsequent versions have relaxed that requirement to make things easier. Here's an example on a Linux Mint system (the 20's are of course spaces) : $ od -cx < Makefile cmd output started 2015 Sat Nov 21 04:39:47 PM PST 0000000 \n g o : \n . / t \n \n 670a 3a6f 200a 2020 2e20 742f 0a0a 0000016 above cmd output done 2015 Sat Nov 21 04:39:47 PM PST dstromberg@dell-inspiron:/var/tmp/make-tabs i686-pc-linux-gnu 25234 $ make cmd output started 2015 Sat Nov 21 04:39:48 PM PST Makefile:3: *** missing separator. Stop. above cmd output done 2015 Sat Nov 21 04:39:48 PM PST dstromberg@dell-inspiron:/var/tmp/make-tabs i686-pc-linux-gnu 25234 $ make --version cmd output started 2015 Sat Nov 21 04:39:53 PM PST GNU Make 3.81 Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. This program built for i686-pc-linux-gnu above cmd output done 2015 Sat Nov 21 04:39:53 PM PST dstromberg@dell-inspiron:/var/tmp/make-tabs i686-pc-linux-gnu 25234 I'll add that vim's Makefile syntax highlighting puts the space-indented line in red, suggesting an error. Changing the spaces to a single tab makes the red go away. HTH On Sat, Nov 21, 2015 at 3:34 PM, Thomas Jakway <tjakway(a)nyu.edu> wrote: > Why would make require tabs...? > Admittedly it's been a long time since I've written any makefiles by hand > but I always use spaces instead of tabs and have never had any problems. > > On Nov 21, 2015 6:01 PM, David Moberg <kaddkaka(a)gmail.com> wrote: > > > > Have you found "soft tabs"? That option will make TextMate use spaces > instead of tab characters. Read 4.11 "Using Spaces Instead of Tabs" in Dan > Strombergs link for instrucitons. > > > > 2015-11-21 21:36 GMT+01:00 Jeon-Young Kang <jykang22(a)gmail.com>: > >> > >> I've tried to choose "instead of spaces". But there is no option of > mine (Textmate 2 beta). > >> > >> On Sat, Nov 21, 2015 at 3:14 PM, Dan Stromberg <strombrg(a)gmail.com> > wrote: > >>> > >>> > >>> Perhaps enable soft tabs? > >>> https://manual.macromates.com/en/working_with_text > >>> > >>> This is an issue in Python too (at least from PEP8's perspective) - > but note that many "make" programs require tabs instead of spaces. > >>> > >>> On Sat, Nov 21, 2015 at 11:13 AM, Jeon-Young Kang <jykang22(a)gmail.com> > wrote: > >>>> > >>>> Hello everyone. > >>>> > >>>> > >>>> I am a newbie of Haskell, using Textmate on mac. > >>>> > >>>> > >>>> I got the following warning. How can I fix it? > >>>> > >>>> > >>>> Warning: > >>>> > >>>> Tab character > >>>> > >>>> Ok, modules loaded: Main. > >>>> > >>>> > >>>> > >>>> Sincerely, > >>>> Young > >>>> > >>>> _______________________________________________ > >>>> Beginners mailing list > >>>> Beginners(a)haskell.org > >>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > >>>> > >>> > >>> > >>> > >>> -- > >>> Dan Stromberg > >>> > >>> _______________________________________________ > >>> Beginners mailing list > >>> Beginners(a)haskell.org > >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > >>> > >> > >> > >> > >> -- > >> Department of Geography > >> State University of New York at Buffalo > >> > >> jykang22(a)gmail.com > >> > >> Jeon-Young Kang > >> > >> _______________________________________________ > >> Beginners mailing list > >> Beginners(a)haskell.org > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > >> > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -- Dan Stromberg

3 2

help with type error in a wxHaskell program
by Jason J. Corso 22 Nov '15

22 Nov '15

Hi: I'm new to Haskell and to wxHaskell (but found playing with wxHaskell to be a nice mid-ground between my years of imperative experience and the pure functional). Thanks for the help. I wrote the following code to create a set of staticText widgets by mapping the layout over a list. Easy enough... module Main where import Graphics.UI.WX main :: IO () main = start gui gui :: IO () gui = do f <- frame let s = map (\t -> staticText f [text := show t]) [1..10] set f [layout := margin 10 ( row 2 $ map (\x -> (widget x)) s)] But, I get a type error that I do not understand Couldn't match expected type ‘IO (Window a0)’ with actual type ‘[Prop (Frame ())] -> IO (Frame ())’ Probable cause: ‘frame’ is applied to too few arguments Thank you in advance, Jason

3 3

Warning: Tab Character
by Jeon-Young Kang 21 Nov '15

21 Nov '15

Hello everyone. I am a newbie of Haskell, using Textmate on mac. I got the following warning. How can I fix it? Warning: Tab character Ok, modules loaded: Main. Sincerely, Young

4 8

Ambiguous type of WriterT result I am not using
by Martin Vlk 21 Nov '15

21 Nov '15

Hi I have two functions, foldrEntries and foldrEntriesW, where the latter is a WriterT version of the former. Here are the type signatures: foldrEntries :: (Entry -> a -> a) -> a -> (String -> a) -> Entries -> a foldrEntriesW :: (Monoid w, Monad m) => (Entry -> a -> WriterT w m a) -> WriterT w m a -> (String -> WriterT w m a) -> Entries -> WriterT w m a I want to implement foldrEntries in terms of foldrEntriesW using the Identity monad and ignore/not use the writer result. I am doing this in order to reuse the foldrEntriesW implementation and avoid code duplication. This is what I have so far: http://lpaste.net/145641 But the compiler complains about ambiguous type for the writer reult I am ignoring (message in the above lpaste). Normally I think the way around this is to provide explicit type annotation for "foldIt", but in this case the result type depends on the type of "a" in foldrEntries type and I don't know how to express this and make the compiler happy. (I was able to make it work by calling "tell ()", basically writing a dummy value, which lets compiler know what the type is, but this is not so good - I don't want to make artificial function calls like this.) Can anybody help me with that? Many Thanks Martin

3 3

Re: [Haskell-beginners] could not install "process-leksah" due to dependencies errors....
by André Verwijs 21 Nov '15

21 Nov '15

OS: OpenSUSE tumbleweed 2015-11-18 Plasma 5.4.2 QT version: 5.5.1 Kernel: 4.3.0-2 (needed for building from source) gtksourceview-devel 3.18.1-1.1 libwebkitgtk-devel 2.4.9-3.2

1 0