January 2006 - Haskell-Cafe

using cabal? add your package to the list!
by Isaac Jones 15 Jan '06

15 Jan '06

I started a list of packages using cabal (including those uplaoded to Hackage). Please either upload your packages to hackage, or add it here. I want this list so that I can try to figure out things like how much a given modification to Cabal will break stuff: http://hackage.haskell.org/trac/hackage/wiki/CabalPackages To add your package to the list, hit "Edit Page" at the bottom of the page. Feel free to make the page into a table, add links for the packages already there, reorganize, etc. peace, isaac

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Inheritance without OOHaskell
by John Goerzen 15 Jan '06

15 Jan '06

Hello, I'd like to put forth two situations in which I miss the ability to use inheritance in Haskell, and then see if maybe somebody has some insight that I'm missing out on. Situation #1: In HDBC, there is a Connection type that is more or less equivolent to a class on a OOP language. In Haskell, we use a record with named fields that represent functions. Closures are used to permit those functions to access internal state. That works well enough. But there is no way at all to extend this. Say a database such as PostgreSQL has some extra features -- it would be nice for the PostgreSQL objects to support additional functions, whereas Connection objects from other databases might not support those functions. Any Haskell function could expect a Connection object (in which case it could access only the standard functions) or a PostgreSQL object (in which case it could access the standard plus the enhanced functions). The internal state of a Connection object is DB-specific, so there can be no general function to expose it. Situation #2: In Python, every exception is an object, and every object can be extended. Therefore, I could write an exception handler for, say, an IO error, and have it work for anything that's a subclass of the generic IO error -- even if these subclasses weren't known at the time the program was written. Other handlers could be as specific or as general as desired. In Haskell, it seems that all of this has to be anticipated in advance; it's not very easy to extend things. So my questions are: 1. I have sometimes used typeclasses instead of data records. This provides some of what I'm searching for, but has the unfortunate side-effect that one can't very easily build a list of objects that may not come from the same place but are nonetheless part of the class. For instance, had I used typeclasses for HDBC, I couldn't have a list of Connection objects where some are from MySQL, some from PostgreSQL, etc. 2. As a library designer, what is the most friendly way around these problems that adheres to the principle of least surprise for Haskell programmers? 3. How does one choose between a type class and a data record of functions when both would meet the general needs? 4. Is there a way to solve these inheritance problems without resorting to a library such as OOHaskell, which many Haskell programmers are not familiar with? Thanks, -- John -- John Goerzen <jgoerzen(a)complete.org> GPG: 0x8A1D9A1F www.complete.org "Value your freedom, or you will lose it, teaches history. `Don't bother us with politics,' respond those who don't want to learn."

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powerful type checking & false expectations that a program is correct
by Isaac Gouy 15 Jan '06

15 Jan '06

Programmers who use languages without static type checking sometimes claim that static type checking gives folk the false impression that once the program passes type checking the program is correct. That always seemed silly to me, but I'm starting to wonder ;-) Of course, the shootout programs are a silly entertainment; but after taking the time to optimise and write the programs, I'm a little surprised that there have been Haskell contributions which just don't give 'the correct answer' in a way that visual inspection with the expected answer would detect. For example ackermann Haskell GHC program regex-dna (new) Haskell GHC #2 program Oh well, they are just easy to fix formating bugs :-) __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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Formated output with ghci
by Frank 15 Jan '06

15 Jan '06

A 'stupid' question (sort of an embarassing one): I want to do formated output by converting data types to strings and the bring them in a formated way together. I use ghci 6.4 and when I want to see this text then it is shown as a string (i.e. with "... " added around) and the formating characters in the text (e.g. \n) are quoted and not executed. (if I apply show to the text string, then the above is done twice!) This does occur if I use a defined function (something like 'toString' or 'text') and does not occur if I use show (but I have other reasons not to use show). Is there a cure to have output formated with ghci without using show to individually convert data types into strings? Here an example output: text s2 "last id used #2\n\n(#2,EntType {unEntType = \"Point\"})\n(#1,EntType {unEntType = \"Point \"})\n\n\n deleted\nRelX empty\n\nHCons (Region2start \"start of region\",RelX empty\n) ( HCons (Region2end \"end of region\",RelX empty\n) (HCons (Point2coord \"Coordinates of Poi nt\",\n(#2,\n[NV 0.0 100.0])\n(#1,\n[NV 0.0 0.0])\n\n) HNil))" Wrapping the text string into a data type and then apply show to this data type gives also the desired result. Is there an easier way? Help is highly appreciated module OutputTest where data X = X Int Int Float deriving (Show) x1 = X 3 4 4.5 text :: X -> String text (X a b c) = show a ++ show b ++ " \n" ++ show c -- example output -- >text x1 --"34 \n4.5" data T a = T a instance Show a => Show (T a) where show (T a) = "Tx " ++ show a tx1 = T x1 instance Show (T X) where show (T a) = text a -------- works: -- >tx1 --34 --4.5

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What does the Haskell type system do with "show (1+2)"?
by Jeff.Harper＠handheld.com 14 Jan '06

14 Jan '06

What does the Haskell type system do with expressions such as these . . . ? show 1 show (1+2) The type of the subexpressions "1" and "1+2" are "ambiguous" since they have type "(Num a) => a". I'm under the assumption before "1+2" is evaluated, the "1" and "2" must be coerced into a "concrete" type such as Int, Integer, Double, etc, and before "show 1" is evaluated, the "1" must be coerced into a "concrete" type. Is my assumption correct? If so, how does Haskell know into which type to coerce the subexpressions? If I try to write a new function, "my_show", which converts an expression into a string representation that includes type information, I run into errors with expressions like "show 1" and "show (1+2)" because of the type ambiguity. class (Show a) => My_show a where my_show :: a -> String instance My_show Int where my_show a = show a ++ " :: Int" instance My_show Integer where my_show a = show a ++ " :: Integer" I can avoid the errors if I change it to "my_show (1::Int)" or "my_show ((1+2)::Int). I'm wondering what the difference is between, my_show and Haskell's built-in show that causes my_show to produce an error message when it is used with ambiguous types, but Haskell's show works okay with ambiguous types.

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Splitting a string into chunks
by Adam Turoff 14 Jan '06

14 Jan '06

Hi, I'm trying to split a string into a list of substrings, where substrings are delimited by blank lines. This feels like it *should* be a primitive operation, but I can't seem to find one that works. It's neither a fold nor a partition, since each chunk is separated by a 2-character sequence. It's also not a grouping operation, since ghc's Data.List.groupBy examines the first element in a sequence with each candidate member of the same sequence, as demonstrated by: Prelude> :module + Data.List Prelude Data.List> let t = "asdfjkl;" Prelude Data.List> groupBy (\a _ -> a == 's') t ["a","sdfjkl;"] As a result, I've wound up with this: -- Convert a file into blocks separated by blank lines (two -- consecutive \n characters.) NB: Requires UNIX linefeeds blocks :: String -> [String] blocks s = f "" s where f "" [] = [] f s [] = [s] f s ('\n':'\n':rest) = (s:f "" rest) f s (a:rest) = f (s ++ [a]) rest Which somehow feels ugly. This feels like it should be a fold, a group or something, where the test is something like: (\a b -> (a /= '\n') && (b /= '\n')) Any thoughts? Thanks, -- Adam

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Re: Haskell-Cafe Digest, Vol 29, Issue 36
by Jeff.Harper＠handheld.com 14 Jan '06

14 Jan '06

Christian Maeder wrote: > Jared Updike wrote: > > http://www.haskell.org/onlinereport/decls.html#default-decls > > http://www.haskell.org/tutorial/numbers.html#sect10.4 > > I still don't see, why it works for show but not for my_show. > > > On 1/12/06, Jeff.Harper(a)handheld.com <Jeff.Harper(a)handheld.com> wrote: > [...] > >> class (Show a) => My_show a where > >> my_show :: a -> String > > If I let this be > > class My_show a where > my_show :: a -> String > > >> instance My_show Int where > >> my_show a = show a ++ " :: Int" > >> > >> instance My_show Integer where > >> my_show a = show a ++ " :: Integer" > > What is the difference to the builtin Show class? I was wondering the same thing myself. Then, I reread the online Haskell report link. >From http://www.haskell.org/onlinereport/decls.html#default-decls: where n>=0, and each ti must be a type for which Num ti holds. In situations where an ambiguous type is discovered, an ambiguous type variable, v, is defaultable if: o v appears only in constraints of the form C v, where C is a class, and o at least one of these classes is a numeric class, (that is, Num or a subclass of Num), and o all of these classes are defined in the Prelude or a standard library (Figures 6.2--6.3, pages -- show the numeric classes, and Figure 6.1, page , shows the classes defined in the Prelude.) Notice the last bullet item. class Show is part of the Prelude. However, class My_show is not part of the prelude. AS far as I can tell, that's the only reason "my_show 1" produces errors while "show 1" is okay.

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Does anybody have a simple example of using continuation Monad?
by Marc Weber 13 Jan '06

13 Jan '06

I'm struggling with this example: http://www.nomaware.com/monads/html/contmonad.html#example After looking at it for the fourth time I got much more.. but still not enough.. because there are so much new things (when beeing translated into some kind of condition ?> thentodo :> elsetodo which is using ThenElse .... It wouldn't be any problem if the next example wasn't using continuation, too.. and that's about combining monads which is important, isn't it? At the tutorial there was mentioned that continuation monads are used for continuation passing style which I've looked up in wikipedia meaning something like splitting a task into different parts beeing executed delayed (for example because of user interaction filling a web form?) I think one simple example like ((+1).(\x->x**2)) in continuation style would make me understand a lot more.. Marc

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Chameneos
by Chris Kuklewicz 13 Jan '06

13 Jan '06

Your "case" tweak was for an older version of Chameneos that used an older Ch channel implementation. But I was inspired by your improvement to use Int# instead of data Color, and I posted a version that seems faster than the winning one that was submitted. http://www.haskell.org/hawiki/ChameneosEntry It still has to box the Int# to put it into the MVar channels. But the fastest complement is now (3# -# a -# b) and "if (other ==# faded)" is faster than the previous "case other of Faded -> ; _ ->". At least on OS X / G4. Also, thanks for cleaning up the SumFile code.

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multiline strings in haskell?
by Michael Vanier 13 Jan '06

13 Jan '06

Is there any support for multi-line string literals in Haskell? I've done a web search and come up empty. I'm thinking of using Haskell to generate web pages and having multi-line strings would be very useful. Mike

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