- Beginners - Haskell.org

deducing type of multi-parameter type class
by Alex 15 Jul '10

15 Jul '10

Hi, I'm trying to get the following code to compile: ======= Main.hs ========== import IO data MyInt = MyInt Int data MyString = MyString String deriving Show class Show b => MyClass a b where fn :: a -> b instance MyClass MyInt MyString where fn (MyInt i) = MyString (show i) myprint :: (MyClass a b) => a -> IO () myprint a = putStrLn $ show (fn a) main = myprint 3 ======= Main.hs ========== with |ghc Main.hs -XMultiParamTypeClasses|. However, the compiler cannot deduce the type of the |b| type variable (which in this case is |MyString|). How can I explicitly tell this information to the compiler? Many thanks, Alex.

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Creating databases with sqlite3 using HDBC
by George Roberts 15 Jul '10

15 Jul '10

I'm trying to have a program attempt opening an sqlite3 database, and if it isn't created then I need some way of creating it. However, after looking through the HDBC api and the sqlite3 specific portion, I cannot figure out how to create a new sqlite3 database.

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newtype record syntax
by Tom Doris 15 Jul '10

15 Jul '10

Can someone give an explanation of the use of record syntax in newtype declarations that helps one understand how it's meant to be interpreted? I understand the "facts" according to the language definition, but I just don't see why this notation was used; does it have any relationship with records? The only docs I can find that cover this are in the language report itself: >|A newtype declaration may use field-naming syntax, though of course there may only be one field. Thus: >| newtype Age = Age { unAge :: Int } >|brings into scope both a constructor and a de-constructor: >| Age :: Int -> Age >| unAge :: Age -> Int I understand what this says, I just don't understand why Haskell does it this way - unAge ends up defining a function, using the notation used to declare a field in a record, and the type of the function is inferred from the type of the field and the constructor - it just seems a bit barmy. I'd appreciate anyone who can help, or a reference to some docs that make sense of this. Thanks t

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A difficult package
by Ben Wise 14 Jul '10

14 Jul '10

Folks, I'm trying to work through Conrad Barski's Haskell tutorial, http://www.lisperati.com/haskell/, and I've run into a difficult package problem. I've googled around for two days, but nothing seems to work. Basically, I can not get it recognize matchRegex, mkRegex, and similar. They are all in the Text.Regex.Posix package, and version 0.94.1 is installed according to 'cabal list regex'. The 'import Text.Regex.Posix' line compiles just fine, so it is finding the package: it just doesn't find the symbols in it ?! But (under all four combinations of command line ghc or Leksah on Windows 7 or Ubuntu), it keeps trying to use the 'base' version, not posix. At least, that's what 'ghc -v main.hs' seems to be saying. In Leksah, adding some of the suggested build dependencies fixed earlier problems with importing the Text and Random things, but adding 'regex-compat' as suggested did not help. Has anyone else gotten CB's tutorial to work? Any suggestions as to how to get Text.Regex.Posix imported? -- Ben Wise, PhD 0xCAF514E1

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regular expressions
by prad 14 Jul '10

14 Jul '10

there seem to be a lot of regular expression modules: Text.Regex.TDFA Text.Regex.PCRE Text.Regex.PCRE.Light there is a bit info on how to use it in real world haskell but only for the POSIX stuff (eg TDFA) any suggestions or preferences? -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

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Re: [Haskell-cafe] lists of arbitrary depth
by Christopher Done 13 Jul '10

13 Jul '10

On 13 July 2010 10:58, vadali <shlomivaknin(a)gmail.com> wrote: > > hello, > iam really new to haskell, > > i want to define a function which takes as a parameter a list which can > contain other lists, eg. [1,[2,3],[4,[5,6]]] > > how would i define a function that can iterate through the items so (in this > example) > iter1 = 1 > iter2 = [2,3] > iter3 = [4,[5,6]] > > ? > > ( can i do that without using the Tree data type? ) Maybe this should be moved to Haskell-Beginnners. It's impossible to have a function that works on a list of arbitrary nesting. But you could use a tree data type, yeah.

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My first functioning haskell project - a steganography utility
by Tim Cowlishaw 13 Jul '10

13 Jul '10

Hey there all, I've just completed my first functional haskell project - a simple utility for steganography - hiding messages within the least significant bit of another sort of data file. Therefore, I was wondering if any of you had any pointers about how I could refactor or otherwise improve my code? Any input would be greatly appreciated - whether howling great errors or smaller points of "good haskell style". In particular, I'd be really interested in whether my type declarations are correct - for instance, whether I have been to specific or not specific enough in specifying the types of my functions (Integral vs Int, etc). In addition, I keep feeling that my Steganograph 'smells like' a monad and/or functor , as it 'wraps around' a message - however, I'm having trouble defining quite how I could achieve construct a monadic type representing a steganograph. Is my hunch incorrect, or is there a way of doing this that I haven't yet discovered? The sources is here: http://gist.github.com/473862 Cheers, Tim

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Multidimensional Matrices in Haskell
by Mihai Maruseac 12 Jul '10

12 Jul '10

Hi, A friend of mine wanted to do some Cellular Automata experiments in Haskell and was asking me what packages/libraries are there for multidimensional matrices. I'm interested in both immutable and mutable ones but I don't want them to be trapped inside a monad of any kind. Any hints? -- MM

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Some Code from XMonad
by matthew coolbeth 10 Jul '10

10 Jul '10

Hi. This is not standard haskell98, right? It appears as though a container module can very succinctly expose ALL items exposed by "submodules" that it has imported within itself. Does anyone know where I can find a document specifying the language behaviour for code that does this? ------------------------------------------------------------------------------------------------------------ module XMonad ( module XMonad.Main, module XMonad.Core, module XMonad.Config, module XMonad.Layout, module XMonad.ManageHook, module XMonad.Operations, module Graphics.X11, module Graphics.X11.Xlib.Extras, (.|.), MonadState(..), gets, modify, MonadReader(..), asks, MonadIO(..) ) where -- core modules import XMonad.Main import XMonad.Core import XMonad.Config import XMonad.Layout import XMonad.ManageHook import XMonad.Operations -- import XMonad.StackSet -- conflicts with 'workspaces' defined in XMonad.hs -- modules needed to get basic configuration working import Data.Bits import Graphics.X11 hiding (refreshKeyboardMapping) import Graphics.X11.Xlib.Extras import Control.Monad.State import Control.Monad.Reader -- mac

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Parsec Monad Type Confusion
by Aleksandar Dimitrov 10 Jul '10

10 Jul '10

Hello list, I have a rather trivial question, but one that I can't seem to get my head around. In the following example function (using Text.ParserCombinators.Parsec) I'm getting an error without the type declaration, but it compiles just fine with it: > parseNumber' :: Parser LispVal > parseNumber' = do x <- many1 digit > return $ Number . read $ x The error message when compiling without the type declaration is the following: No instance for (Text.Parsec.Prim.Stream s m Char) arising from a use of `digit' at Scheme.hs:33:30-34 Possible fix: add an instance declaration for (Text.Parsec.Prim.Stream s m Char) In the first argument of `many1', namely `digit' In a stmt of a 'do' expression: x <- many1 digit In the expression: do { x <- many1 digit; return $ Number . read $ x } The function is an example in "Write yourself a Scheme in 48 hours." I'd consider myself fairly accustomed to programming with monads; and still, I can't seem to find out *why* exactly GHC ( 6.12.1 ) cannot compile this without the type signature. What does it try to build here? Why does the inferencing fail? My best guess is that it somehow doesn't get that 'x' is supposed to be a String value ([Char], not Char.) What really confuses me, though, is that adding the instance declaration makes the whole thing work! The type declaration for LispVal looks like this: > data LispVal = Number Integer So it should be pretty clear to GHC that, due to `Number . read` demanding a [Char], I actually want x to be a [Char]. I'm assuming here that I'm just in the wrong Monad, starting with `do`. Somehow, GHC assumes it's not Parser LispVal, but something more complex, possibly Text.Parsec.Prim.Stream s m LispVal or so? Thank you everybody for you help :-) This list is a truly amazing place with a lot of helpful people! Aleks

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