June 2017 - Beginners

a way to check whether a file is actually being written on
by Silent Leaf 28 Jun '17

28 Jun '17

Hi, i created a small clone of dd in haskell. I made it so it only copies block by block and only if there's any difference between each pair of blocks from each file. the idea is to use this dd clone as backup system, especially since my partitions are nearly full, so no real loss in copying the whole things. I'm wondering if there's any way to check if my program never ever writes onto the target unless actually needed. obviously by reading the code i'd say it does what i want, but we do make test cases rather than rely on what we think the code does. i can't run it with a target file that would be made read-only in the filesys (and hope for an error for trying to write on it) since obviously i need to open it in read-write right from the beginning, in case of actual need of writing (as apparently i can't have two handles on the same file... although maybe there's a way to change the mode of opening on the run? did not find it in System.IO nor in Hoogle or Hayoo) so if anyone has an idea, in or outside of haskell, that would be great!

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Re: [Haskell-beginners] Beginners Digest, Vol 108, Issue 12
by Ning Yin 28 Jun '17

28 Jun '17

发自我的 iPad > 在 2017年6月24日，20:00，beginners-request@haskell.org 写道： > > 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: help designing types for a gsl fit (David McBride) > 2. What to use when you need random values? (Silent Leaf) > 3. Re: What to use when you need random values? (David McBride) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 23 Jun 2017 09:58:35 -0400 > From: David McBride <toad3k(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] help designing types for a gsl fit > Message-ID: > <CAN+Tr42LkXcggzKVfzcsmXuK79t39F36_eNa4CWDmP1PX_gqHw(a)mail.gmail.com> > Content-Type: text/plain; charset="UTF-8" > > If you look at the type of labelNew > > GlibString string => Maybe string -> IO Label > > If you look at the instances for GlibString, they could be Text or > [Char]. You have to decide which. The fact that you are using > Nothing does not tell you the entire final type. It could be Maybe > [Char], or Maybe Text. Even though the choice seems arbitrary in this > instance, you have to decide which it is. So try this. > > lprog <- G.labelNew (Nothing :: Maybe [Char]) > > On Fri, Jun 23, 2017 at 3:22 AM, Agustin Larreinegabe > <alarreine(a)gmail.com> wrote: >> Hello, >> I'm trying to install an application Termite - Debug but I get this error in >> line 929:14 >> >> when it try to do this >> >> >> >> lprog <- G.labelNew Nothing >> >> >> The error says: >> >> Could not deduce (glib-0.13.4.1:System.Glib.UTFString.GlibString string0) >> arising from a use of ‘G.labelNew’ from the context (D.Rel c v a s) bound by >> the type signature for sourceWindowCreate :: D.Rel c v a s => RSourceView c >> a u -> IO G.Widget at Debug/SourceView.hs:913:23-73 instance >> glib-0.13.4.1:System.Glib.UTFString.GlibString [Char] -- Defined in >> ‘glib-0.13.4.1:System.Glib.UTFString’ In a stmt of a 'do' block: lprog <- >> G.labelNew Nothing In the expression: do { vbox <- G.vBoxNew False 0; >> G.widgetShow vbox; spec <- getIORef svInputSpec ref; code <- codeWinNew >> spec; .... } In an equation for ‘sourceWindowCreate’: sourceWindowCreate ref >> = do { vbox <- G.vBoxNew False 0; G.widgetShow vbox; spec <- getIORef >> svInputSpec ref; cabal: Error: some packages failed to install: >> >> I really don't know how to proceed, I'm new with Haskell >> >> >> Thanks in advance. >> >> >> ----------------- >> Agustin Larreinegabe >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > ------------------------------ > > Message: 2 > Date: Fri, 23 Jun 2017 16:24:07 +0200 > From: Silent Leaf <silent.leaf0(a)gmail.com> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: [Haskell-beginners] What to use when you need random values? > Message-ID: > <CAGFccjP6ai7x_n9+AGQGGgjGAMWtqOU-+YNSUbDHAEN4Sp+wNA(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > Hi, > I've had trouble finding the best way(s) to use random values in haskell, > as it seems like there are several modules that either do the same thing or > reuse one another i'm not sure. > > There is System.Random > - is it better to use the streams random(R)s or a more imperative randomRIO? > - is it better to use mkStdGen or newStdGen or getStdGen? > There is Test.QuickCheck and its type(class?) Gen > There is a module in Control.Monad (i think) which exports the type Rnd > > What about performances, and all those options? What do you like to use > with random numbers? > > I know that's a lot of questions. feel free to only answer to a few of > them. :) >

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R/W from/to partitions (on linux)
by Silent Leaf 26 Jun '17

26 Jun '17

Hi, I'd like to be able to read and write from/to partitions directly. I've had trouble with the documentation (honestly i can't find anything, and any mention of partitions leads to mathematical partitioning of lists or whatever). I obviously would need to be able to write or read from a specific position in the partition. Mind you that would be good too for files (that is, being able to read/write from a specific position in it) since i plan on making disk images. Thanks in advance!

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Improve my lambda expressions
by PATRICK BROWNE 26 Jun '17

26 Jun '17

The code below provides a distance function that works for points and moving point. I am happy with the result, but I have a problem with the lambda expressions. Instead of supplying an argument to each 'mpXX', is it possible to have a single lambda argument on the entire final 'md' function? (e.g. 'md 2') Thanks in advance, Pat data Point a = Point { abscissa :: a, ordinate :: a } deriving Show dist a b = sqrt (((abscissa a) - (abscissa b))^2 + ((ordinate a) - (ordinate b))^2) -- Sttic points p1, p2 :: Point Float p1 = Point 0.0 0.0 p2 = Point 4.0 4.0 d = dist p1 p2 -- Moving points mp1, mp2 :: Point Float mp1x = (\t -> 4.0 + 0.5 * t) mp1y = (\t -> 4.0 - 0.5 * t) mp1 = Point (mp1x 2) (mp1y 2) mp2x = (\t -> 0.0 + 1.0 * t) mp2y = (\t -> 0.0 - 1.0 * t) mp2 = Point (mp2x 2) (mp2y 2) md = dist mp1 mp2 -- 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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What to use when you need random values?
by Silent Leaf 26 Jun '17

26 Jun '17

Hi, I've had trouble finding the best way(s) to use random values in haskell, as it seems like there are several modules that either do the same thing or reuse one another i'm not sure. There is System.Random - is it better to use the streams random(R)s or a more imperative randomRIO? - is it better to use mkStdGen or newStdGen or getStdGen? There is Test.QuickCheck and its type(class?) Gen There is a module in Control.Monad (i think) which exports the type Rnd What about performances, and all those options? What do you like to use with random numbers? I know that's a lot of questions. feel free to only answer to a few of them. :)

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help designing types for a gsl fit
by Agustin Larreinegabe 23 Jun '17

23 Jun '17

Hello, I'm trying to install an application Termite - Debug <https://raw.githubusercontent.com/termite2/debug/master/Debug/SourceView.hs> but I get this error in line 929:14 when it try to do this lprog <- G.labelNew Nothing The error says: Could not deduce (glib-0.13.4.1:System.Glib.UTFString.GlibString string0) arising from a use of ‘G.labelNew’ from the context (D.Rel c v a s) bound by the type signature for sourceWindowCreate :: D.Rel c v a s => RSourceView c a u -> IO G.Widget at Debug/SourceView.hs:913:23-73 instance glib-0.13.4.1:System.Glib.UTFString.GlibString [Char] -- Defined in ‘glib-0.13.4.1:System.Glib.UTFString’ In a stmt of a 'do' block: lprog <- G.labelNew Nothing In the expression: do { vbox <- G.vBoxNew False 0; G.widgetShow vbox; spec <- getIORef svInputSpec ref; code <- codeWinNew spec; .... } In an equation for ‘sourceWindowCreate’: sourceWindowCreate ref = do { vbox <- G.vBoxNew False 0; G.widgetShow vbox; spec <- getIORef svInputSpec ref; cabal: Error: some packages failed to install: I really don't know how to proceed, I'm new with Haskell Thanks in advance. ----------------- Agustin Larreinegabe

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Can i post code here for review and getting better?
by Silent Leaf 23 Jun '17

23 Jun '17

Hi, I'd like to know if this mailing list would be an appropriate place to post code and ask for review. The purpose would not be to help me do it, but to tell me how to do it better, more idiomatically, using more appropriate or powerful tools. If not, and even if yes, are there any other places where this might be appropriate? Are there haskell forums somewhere? mind you i just got the idea, so i'm gonna check, but maybe you all know which ones are better than others, or more active. Also, i've searched in vain for places where one could maybe get together into projects that would not require being fully fluent yet in haskell (with the will to learn and still some solid, yet relatively untested, bases), nor that would require getting updated on a projects of perhaps several thousands of more or less obscure code (for those that come from the outside *cough* operator maniacs *cough*). Otherwise, i'd also welcome ideas of very simple ideas of projects just to learn by doing. The idea would be to know how to do projects that actually do something, so usually there's some sort of contact with the exterior. Thanks verymuch in advance!

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Question regarding error messages
by Choi Waikit 19 Jun '17

19 Jun '17

Hi guys, I have difficulty understanding how GHC determines what kind of error message to return. Could you help explain a bit ? Thanks ! The code can be accessed here <http://lpaste.net/356325> My question is Why does calling `settleDown 9` return different error message from `settleDown "sting"` ? Thanks, David

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Non type-variable argument
by Nikolay Kurtov 14 Jun '17

14 Jun '17

Hello everyone, Please help me compile the program without the -XFlexibleContexts extension. I don't understand the "Non type-variable argument" error message and googling does not shed any light on the problem. Any help is appreciated. % ghc --version The Glorious Glasgow Haskell Compilation System, version 7.10.3 % ghc orfest.hs orfest.hs:6:1: Non type-variable argument in the constraint: AM.MArray a (t, t, t, [i]) m (Use FlexibleContexts to permit this) When checking that ‘go’ has the inferred type go :: forall (m :: * -> *) t t1 t2 t3 (a :: * -> * -> *) i. (Read t, AM.Ix i, AM.MArray a (t, t1, t2, [i]) m) => a i (t, t1, t2, [i]) -> [[Char]] -> t3 -> i -> m (t3, i) This works: % ghc orfest.hs -XFlexibleContexts Regards, Orfest

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2d layout description?
by Mario Lang 12 Jun '17

12 Jun '17

Hi. I am looking for a good method to describe 2-dimensional panels of various technical, mostly music-production related, devices. Motivation: I am blind and am looking for a future-proof solution to write down labels of jacks/knobs/switches on front/back panels of all the devices I use on a daily basis. Complex devices have so many options that you tend to forget the not-so-often used ones, hence, the need to have a reference "card" somewhere. This need has recently surfaced for me rather clearly as I have entered the realm of eurorack modular synthesizers as a hobby. One module is typicall 3U in either and between 2HP and 50HP in vertical size. And there are literally hundreds of these available these days. So while it becomes surprisingly easy to remember the jacks/knobs of regularily used modules after a rather short while, the need for some reference to go back to is unavoidable. Why is this on haskell-beginners? I am a haskell fan, and think this is a perfect job for a functional programming language. What I *want* is some sort of data format/DSL that makes it easy for me to describe the positions, functions and labels of physical interface elements. From that, I'd like to generate easily readable 2d ascii-diagram alike descriptions which would be particularily well fit to be read with a Braille display. Also, knowing the position/function of interface elements (jacks, knobs, switches) it would be neat if I could implement convenience functions on top of the known data, like the ability to calculate cable lengths for a particular patch, or even figuring out a module layout to minimize patch cable lengths given a list of possible patches. While browsing hackage, I found asciidiagrams, which is * pretty cool! * and might be useful to specify the layout However, I would have to fork and rewrite it to fit my actual needs, and it isn't clear if specifying the machine readable description like that wouldn't be a little bit too verbose. As an example, I have written down the layout of http://www.doepfer.de/A114.html (which admittably, is a rather simple module): +-------------+ | | | /-------\ | | | A-114 | | | |RingMod| | | \-------/ | | | | +--------+ | | | /----\ | | | | | X | | | | | |{i} | | | | | \----/ | | | | /----\ | | | | | Y | | | | | |{i} | | | | | \----/ | | | | /----\ | | | | |X*Y | | | | | |{o} | | | | | \----/ | | | +========+ | | | /----\ | | | | | X | | | | | |{i} | | | | | \----/ | | | | /----\ | | | | | Y | | | | | |{i} | | | | | \----/ | | | | /----\ | | | | |X*Y | | | | | |{o} | | | | | \----/ | | | +--------+ | | | +-------------+ This looks cool, but once I add all the tags I probably need (jack(in/out/in|out), rotary knob, n-way rotary, n-way switch, ...) this will probably grow into something quite cumbersome. A DSL might be better there, and somehow recunstruct a simple ascii diagram from that? In essence, I think I am trying to solve a rather general problem here, even though my actual use-case is specific to eurorack modules. Once I have an easily useable DSL, I will likely want to describe everything I own in this format, to be able to remember individual functions at a later day. And once I start to see this as a database (which is the main reason for wanting to move it from text files to a programming language) I can see the need to search for exact descriptions. For instance, I might find something with 7 RJ45 jacks in groups of 4 and 3, and would like to find the description for it without knowing the product name... Or search for a label of a key, without knowing on which remote it was... There are quite some possibilities here. I am basically asking for everything that comes to your mind, if you have read so far without having been bored to hell. Is there any library on hackage that might help? Do you know of similar projects with different goals? Anything else? TL;DR: I am looking for a DSL to describe physical interfaces in a form that makes it easy to make this information available to the blind in a very accessible way. Description files should be easy enough to maintain, to maybe even start a trend of a central repository to avoid people having to re-create them over and over again. The metadata should be detailed enough to allow for search/query operations on the universe of described hardware. -- CYa, ⡍⠁⠗⠊⠕ | Blog: <https://blind.guru/> GitHub: <https://github.com/mlang/> .''`. | Twitter: @blindbird23 FaceBook: disyled : :' : | SoundCloud: <soundcloud.com/mario-lang> `. `' | YouTube: <youtube.com/user/mlang23> `-

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