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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 13 Jan '06

13 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 13 Jan '06

13 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 12 Jan '06

12 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 12 Jan '06

12 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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I/O and utf8
by Andreas Kägi 12 Jan '06

12 Jan '06

hello i want to read a file encoded in utf8 and at a later time output portions of it on the console. Is there an easy way to do this in haskell? using the standard i/o functions i can read the file but the output gives me \1071 ... instead of the unicode characters.

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Haskell DB bindings
by oleg＠pobox.com 12 Jan '06

12 Jan '06

Krasimir Angelov wrote: > There are three active database libraries: HDBC, HSQL and Takusen. > It is quite disappointing from my point of view. Recently there was > the same situation with the GUI libraires. I think the dichotomy between lower-level Haskell libraries (whose API is closer/faithful to the underlying library) and higher-level, more idiomatic libraries will always exist, because there is need for both. Regarding GUI libraries, the bulk of wxHaskell is close to wxWindows. However, there are also more higher-level, more idiomatic interfaces built on the top of it. Takusen has its own lower-level layer -- which can be replaced with other suitable lower-level layer, for example, HSQL. By suitable I mean the layer should not improve the underlying database API too much, e.g., add finalizers to database and session handlers, because avoiding such finalizers was one of the main reason for the design of Takusen. It may be an interesting experiment to see if Takusen proper can be moved to use HSQL, for example, as the lower-level layer. Also, perhaps HSQL may find something useful to borrow from Takusen's lower-level layer.

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Re: [Haskell-cafe] multiline strings in haskell?
by Michael Vanier 12 Jan '06

12 Jan '06

Yes, just like that ;-) Thanks! Now if somebody has a string interpolation library, I'd be a pretty happy camper ;-) Mike > mvanier: >> 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. > > Do you mean like this: > > string = " line one\n\ > \ line two is here\n\ > \ line three is this line\n" > > > $ echo 'putStr string' | ghci A.hs > line one > line two is here > line three is this line > > -- Don

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Help compiling School of Expression graphics library?
by Rakesh Malik 11 Jan '06

11 Jan '06

I'm trying to build the GraphicsLib code in Linux, Fedora. I have the March '05 version of Hugs installed (and ghc, but that doesn't seem to be relevant to this). The first bit of puzzlement comes from the error messages I'm getting: ffihugs +G +L"X_stub_ffi.c" X.hs Warning: unknown toggle `G'; ignoring. Warning: unknown toggle `L'; ignoring. runhugs: Unable to initialise Hugs (Unrecognised option) Hugs is on my path ($HOME/hugs/bin), and I edited the Makefile to point the hugs_install home directory to $HOME/hugs. X_dir is correct, it has lib/libX11.a and include/X11/X.h. My gc version is: gcc (GCC) 4.0.0 20050519 (Red Hat 4.0.0-8) (I don't know whether that's relevant, but just in case.) Thanks for any help you can provide!

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