- Beginners - Haskell.org

Re: [Haskell-beginners] Improve my lambda expressions
by PATRICK BROWNE 28 Jun '17

28 Jun '17

Thanks for all your help. I was unaware that there was a relation between let/where and lambdas. Here is my effort to use a single lamda md3 n = (\n -> (dist (Point (4.0 + 0.5 * n) (4.0 - 0.5 * n)) (Point (n * 1.0) ( n * (-1.0))))) n I imagine that this function could be written without lambdas, let, or where. Is it generally true the all/most functions could be written without lambdas, let, or where? Thanks, Pat On 26 June 2017 at 12:04, Frerich Raabe <raabe(a)froglogic.com> wrote: > On 2017-06-26 11:38, PATRICK BROWNE wrote: > >> 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. >> > > [..] > > -- 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 >> > > Maybe you could reduce the number of lambda expressions by extracting > common logic. It seems to me that 'mp1' and 'mp2' are moved versions of the > same point (2,2) except that they apply different functions to the > coordinates. These functions follow a pattern (a factor is applied to the > component and then an 'offset' is added). > > For instance, it might be worthwhile to define > > movePoint :: Float -> Float -> Point -> Point > movePoint offset factor (Point x y) = Point (offset + factor * x) > (offset - factor * y) > > ...such that you could then define > > md = let p = Point 2 2 in dist (movePoint 4 0.5 p) (movePoint 0 1 p) > > -- > Frerich Raabe - raabe(a)froglogic.com > www.froglogic.com - Multi-Platform GUI Testing > -- 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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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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