- Beginners - Haskell.org

Large binary overhead?
by Christopher Howard 26 May '11

26 May '11

I'm still going through the GHC tutorial, so be patient with me: But why is it that my GHC-produced executables are always so large? Even my one line Hello World program ends up being over 500KB. I thought maybe it was because all of the functionality was statically compiled in, but I used ldd and saw that my hello world binary links dynamically to 10 different C libraries. (I compile on Gentoo Linux x86, if that makes a difference.) Anyway, is that just the way it is, or does this become a non-issue once I learn proper optimization/linking techniques? -- frigidcode.com theologia.indicium.us

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Quickcheck Preconditions
by Guy 25 May '11

25 May '11

I have some functions with preconditions. For example, foo::[a]->IO Bool requires a list of at least three elements, and bar::[a]->Int->IO () requires that its second argument is some position within the list. How can I test these with quickcheck? If I use ==>, quickcheck gives up with very few test cases.

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Handling Maybes
by Christopher Howard 25 May '11

25 May '11

Since I started into Haskell, I frequently run into a scenario like this: some function returns a Maybe value. If it is a Nothing, I want to do one thing, but if it isn't, I want to do something with the value encapsulated by the Maybe. But before I can use that value (say, to print it) I've got to pull it out of the Maybe in a way that satisfies the compiler. So one option is to use a separate function with pattern matching, for example: handleMaybeVal Nothing = do someAction someOtherActionEtc handleMaybeVal (Just a) = putStrLn ("The string in reverse is " ++ (reverse a)) Or I can use the maybe function with a conditional main = do val <- someIOFunc -- val is a Maybe String if val == Nothing then do someAction someOtherActionEtc else putStrLn ("The string in reverse is " ++ ((reverse . unpack) val)) where unpack a = maybe (error "unreachable error") (\b -> b) a The second option I like better, for some reason, but it is a bit dangerous, as I have to double check my code to make sure the default value passed to "maybe" cannot possibly be evaluated. (Alternatively, I could pass in a regular default value, like a blank string, though that is actually more dangerous in my mind because a bad value might be used silently.) Anyway: how do you pros go about handling situations like these? -- frigidcode.com theologia.indicium.us

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question for advice on packages for a retro-style pocket money ledger program
by DekuDekuplex＠Yahoo.com 25 May '11

25 May '11

For some time, I have been considering rewriting my original pocket money ledger program in Haskell, but am not sure as to which packages would be most useful. My original pocket money ledger program was first written in circa 1983 in N80-BASIC (a JIS-compatible ROM-based line BASIC) on an NEC PC-8001 mkII [1]. The main difficulty is that I would like to preserve the original look and feel of my original program while still rewriting it in Haskell. However, my original program ran in an age before any kind of Macintosh-style or Windows-style GUI had become common, and used basic built-in ROM-based graphics and sound commands to draw a double-line colored graphical border on a centered-colored-text screen, and saved files to a floppy disk drive (floppy disks were actually considered advanced for that age, since most personal computers then used a cassette tape drive for external file storage). As a result, I am uncertain as to which packages would be useful for preserving the original look and feel of my program. At minimum, I would like my program to exhibit the following behavior: 1. Start out by drawing a lime-green-colored double two-pixel wide box surrounding the screen, with the title and author name displayed as separately-colored text centered in the screen. 2. If possible, play a suitable music file while on this screen. Also, display an option to mute this music, and if this music is once muted, save this option for future runs of this program, so that the program will start out with the music muted as the default option. Also, display an option to control the volume of this music, and if possible, also display a separate option to choose a different tune to play. The tunes should be selectable from a selectable, expandable menu. 3. If possible, include a suitable screen saver to be displayed automatically, separate from the GUI screen saver, upon a user-defined period in which there has been no user input. The screen saver should be configurable in a separate configuration menu. 4. Upon the user hitting the Return key, stop playing the music (if playing), and move to the main menu screen. 5. The options displayed in the menu screen should include, at minimum, the following: a. Create a new ledger book. b. Load a ledger book (preferably from floppy disk). c. Add an entry to the ledger book. d. Delete an entry from the ledger book. e. Erase the ledger book. f. Save the current ledger book (preferably to floppy disk). g. Tabulate and display income and expenditures (leads to a separate screen requesting a time period for tabulation). (The income and expenditures should be displayed in detail both as numerical charts and as colored graphs in a preferred form (pie chart, bar graph, etc.).) h. Set configuration options (leads to a separate screen for configuring default background music, default volume, default application screen saver, default theme, default fonts, default menu behavior, default keyboard remappings, default screen height/width ratio, etc.). i. Exit the application. 5. For each option (except for option i), move to a separate sub-menu screen. When input/output and processing have been completed on the separate sub-menu screen, return to the main menu screen. Most of the functionality of this program is related to some form of input and output: colored formatted combined graphical and textual title screen with background music, manual data entry by a user, saving files to a floppy disk, reading files off a floppy disk, deleting files from a floppy disk, configuring various default music/volume/screen saver/theme/font/menu behavior/keyboard remapping/screen height and width ratio options, and combined numerical and graphical representation of statistical ledger information. Most of the computation involves only simple arithmetic (mainly addition and subtraction, with perhaps some multiplication and/or division, and no matrix manipulation). The main point of this program is that program usage should be interactive, and not require a separately entered file. The idea is that the program will play the role of an interactive personal ledger assistant. However, I am not sure as to how to implement this program in Haskell easily. Because most of the program is mainly concerned with side effects, it is difficult to write it easily while preserving referential transparency. This type of program seems relatively straightforward to write in N80-BASIC (which is no longer available in the original version), but is relatively less trivial in a purely functional programming language such as Haskell. However, I am tired of spaghetti code, and although writing each line of code in N80-BASIC is trivial, managing control flow is not. It is very difficult to manage control flow without writing spaghetti code in a dialect of line BASIC. However, most of the associated graphics and sound commands are proprietary and implementation-dependent, and I am not sure how to rewrite that part of the functionality in an implementation-independent language without spending lots of time on API-related issues. -- Benjamin L. Russell [1] _OLD-COMPUTERS.COM Museum ~ NEC PC 8001 MK 2_. NYI (New York Internet). n.d. Web. May 25, 2011. <http://www.old-computers.com/museum/computer.asp?st=1&c=396>. -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 90-6526-1406 "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Type-class instance for any type that has an instance of another type
by Arlen Cuss 24 May '11

24 May '11

Hi all, I may be a liiittle in over my head. I'm trying to make a type-class for a type that can be "randomly generated". The class looks like this: > class RandomlyGeneratable a where > rgen :: IO a So far so good. Now, I'd love to be able to define an instance of this on all types that have Enum, because the definition is not hard: > instance Enum a => RandomlyGeneratable a where > rgen = fmap (opts !!) $ randomRIO (0,pred (length opts)) > where opts = enumFrom $ (toEnum 0) :: a Unfortunately I'm wording this incorrectly, as the "instance" line produces: Illegal instance declaration for `RandomlyGeneratable a' (All instance types must be of the form (T a1 ... an) where a1 ... an are *distinct type variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `RandomlyGeneratable a' Could someone point out my error? Am I trying to do something on the edge of possibility? Thanks! Arlen

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Re: [Haskell-beginners] how does the State monad actually work?
by Benjamin Edwards 24 May '11

24 May '11

That should gave been state is a type that takes a state and returns a state tupled with a value. The chapter in yaht is 9.3 On 24 May 2011 08:50, "Benjamin Edwards" <edwards.benj(a)gmail.com> wrote: I highly recommend reading some of the tutorials that show how to derive the state monad. The type of state is really a function that takes a parameter and returns a state tupled with a value. Yet another haskell tutorial has quite a good chapter on this. > > On 24 May 2011 08:40, "Sean Perry" <shaleh(a)speakeasy.net> wrote: > > I am trying to understand ...

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recommendations for a gnuplot tutorial
by Michael Litchard 24 May '11

24 May '11

I've looked for a tutorial for using the Haskell gnuplot wrapper, and can't find anything. If you can think of one off the top of your head, please let me know. I'd like to see how to use it to generate 2D and 3D histograms expressed in a png (or any) file. As far as I can see, the examples that come with gnuplot don't generate files.

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glfw cabal install mac osX
by Britt Anderson 23 May '11

23 May '11

I have just installed the haskell platform on a mac running 10.6 OSX. Using cabal install glfw fails. Who would I write and what data should I include to try and get some help to fix the problem? Thank you.

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learning about exceptions
by Jake Penton 22 May '11

22 May '11

I am working my way through Real World Haskell. I just finished chapter 8, and am moving through chapter 9. I note that RWH uses deprecated "old" exceptions. I am currently importing the old style exceptions as I work through the code, but would like to learn the current (new) exception stuff. Is there a good tutorial on thissomewhere ? Please bear in mind that at my state of knowledge it is still rather difficult to get this knowledge by reading the library documentation alone. I am not all that great at inferring what is going on just by inspecting signatures, etc. Is ch.19 of RWH (which covers exceptions in more depth) still relevant to "new" exceptions? Thanks. - Jake -

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code review?
by Chris Bolton 22 May '11

22 May '11

Hey guys. Pretty new to haskell here. I've started off by writing a small command-line interface to plurk.. I was just wondering if anyone would be willing to give everything a look-over and lemme know what kinds of things I should be looking to improve upon, style-wise. Not sure I'm currently doing things in the 'haskell-way' so to speak :P. Thanks a bunch! https://github.com/saiko-chriskun/hermes (note: the JSON and Plurkell submodules are also mine.)

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