July 2010 - Beginners

Thompson Exercise 9.13
by dan portin 15 Jul '10

15 Jul '10

Hi, I am new to Haskell (and programming). Thompson's exercise 9.13 in *Craft of Functional Programming *gave me trouble. Searching the list archives, I saw people define init (xs), last (xs), and so on, in a variety of complex ways (using the Maybe monad, using fairly complex post-processing). This seems to be a hard problem for beginners; at least, it was rather hard for me. The problem is to define the Prelude functions *init* and *last* using * foldr*. After a while, I came up with: -- *Exercise 9.13*: Use foldr (f, s, xs) to give definitions of the prelude functions -- unzip, last, and init. -- Clearly, -- [(x, y), (x1, y1)] = (x, y) : (x1, y1) : ([], []) -- foldr f ([], []) ((x, y):(x1, y1):[]) = f (x, y) (f (x1, y1) ([], [])) -- Hence, f (x, y) (xs, ys) must equal (x:xs, y:ys) for any xs, ys. unzip :: [(a, b)] -> ([a], [b]) unzip xys = foldr f ([], []) xys where f :: (a, b) -> ([a], [b]) -> ([a], [b]) f (x, y) (xs, ys) = (x:xs, y:ys) *last* :: [a] -> a last xs = head $ foldr f [] xs where f :: a -> [a] -> [a] f x [] = [x] f x ys = ys ++ [x] *init* :: [a] -> [a] init xs = tail $ foldr f [] xs where f :: a -> [a] -> [a] f x [] = [x] f x (y:xs) = y : x : xs Now, these seemed to be hard questions. So, I have three questions: (1) are these correct? They work on test cases, and I *did* do some quick proofs. They seem okay. (2) Is there a way to eliminate the post-processing of the lists (i.e., *head* in *last* and *tail* in *init*)? (3) Why the complex answers in the list archives? Am I missing something?

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why is something different within a function when it comes out?
by prad 15 Jul '10

15 Jul '10

i have this: main = do c <- readFile "test.txt" let tleP = "<title>\n(.*)\n</title>" let tle = c=~tleP::[[String]] putStrLn $ tle!!0!!1 let g = xtract tleP c putStrLn $ show g xtract p c = do let r = c=~p::[[String]] return r!!0!!0!!1 for the first putStrLn i get: League of Humane Voters Home Page for the second i get "League of Humane Voters Home Page" but only after i have done r !!0 !!0 !!1 whereas i only needed tle !!0 !!1 it seems to me that both tle and r serve the same purpose though they come out as different types r being a string and tle being i don't know what because i get a "Not in scope: 'tle' so even though both r and tle are set as [[String]], they don't seem to be the same creature. -- 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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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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