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Re: [Haskell-cafe] Printing list of enum type
by michael rice 14 Apr '09

14 Apr '09

I didn't know that, but there's a lot I don't know about Haskell. It works great. Thanks! Michael --- On Tue, 4/14/09, John Dorsey <haskell(a)colquitt.org> wrote: From: John Dorsey <haskell(a)colquitt.org> Subject: Re: [Haskell-cafe] Printing list of enum type To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Tuesday, April 14, 2009, 3:33 PM Michael, > What do I need to add to this Color enum type to print a list of them? You can also easily print a list of /all/ of them. Regards, John scratch$ cat color.hs data Color = Red | Blue | Green | Yellow | Orange | Brown | White | Black deriving (Show,Enum,Bounded) scratch$ ghci color.hs *Main> [minBound..maxBound] :: [Color] [Red,Blue,Green,Yellow,Orange,Brown,White,Black]

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GHC needs a 64 bit machine to be fast?
by FFT 14 Apr '09

14 Apr '09

Not that this is a very good benchmark, but I compiled the nearly equivalent C and Haskell (1st, recursive version) programs from this blog post: http://donsbot.wordpress.com/2008/06/04/haskell-as-fast-as-c-working-at-a-h… There, in both versions, 1e9 iterations take 1.8s. However, in my timing on 32-bit Linux machines, the C version is 5-10 times faster. Does the bitness of the OS make a big difference? Here are my timings on a 3.4GHz Pentium D: C : 4.072s (GCC 4.2.4) Haskell : 22.481s (GHC 6.10.2 and 6.8.2 are about the same here) I used the exact same compiler options as in the blog post. I only added import System import Text.Printf to the Haskell program, which seems to be missing. Suspiciously, "ghc -O2 --make" is almost as fast at 24.438s (6.10.2)

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Printing list of enum type
by michael rice 14 Apr '09

14 Apr '09

What do I need to add to this Color enum type to print a list of them? Michael =============== data Color = Red | Blue | Green | Yellow | Orange | Brown | White | Black instance Show Color where show Red = "Red" show Blue = "Blue" show Green = "Green" show Yellow = "Yellow" show Orange = "Orange" show Brown = "Brown" show White = "White" show Black = "Black" ====================== Ok, modules loaded: Main. *Main> Red Red *Main> Black Black *Main> [Red Black White] <interactive>:1:1: Couldn't match expected type `Color -> Color -> t' against inferred type `Color' In the expression: Red Black White In the expression: [Red Black White] In the definition of `it': it = [Red Black White]

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Re: [Haskell-cafe] Printing list of enum type
by michael rice 14 Apr '09

14 Apr '09

Dumb, dumb, dumb, dumb, DUMB!, he Lisped. Thanks. Michael --- On Tue, 4/14/09, Andrew Wagner <wagner.andrew(a)gmail.com> wrote: From: Andrew Wagner <wagner.andrew(a)gmail.com> Subject: Re: [Haskell-cafe] Printing list of enum type To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Tuesday, April 14, 2009, 9:19 AM You don't need to add anything, you just need to use list notation correctly. Try typing [Red, Black, White] in at the prompt instead. The commas are part of the list syntax. On Tue, Apr 14, 2009 at 9:15 AM, michael rice <nowgate(a)yahoo.com> wrote: What do I need to add to this Color enum type to print a list of them? Michael =============== data Color = Red | Blue | Green | Yellow | Orange | Brown | White | Black instance Show Color where show Red = "Red" show Blue = "Blue" show Green = "Green" show Yellow = "Yellow" show Orange = "Orange" show Brown = "Brown" show White = "White" show Black = "Black" ====================== Ok, modules loaded: Main. *Main> Red Red *Main> Black Black *Main> [Red Black White] <interactive>:1:1: Couldn't match expected type `Color -> Color -> t' against inferred type `Color' In the expression: Red Black White In the expression: [Red Black White] In the definition of `it': it = [Red Black White] _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANN: hmatrix-static: statically-sized linear algebra
by Reiner Pope 14 Apr '09

14 Apr '09

Hi everyone, I am pleased to announce hmatrix-static[1], a thin wrapper over Alberto Ruiz's excellent hmatrix library[2] for linear algebra. The main additions of hmatrix-static over hmatrix are: - vectors and matrices have their length encoded in their types - vectors and matrices may be constructed and destructed using view patterns, affording a clean, safe syntax. hmatrix-static includes statically-sized versions of hmatrix's linear algebra routines, including: - simple arithmetic (addition, multiplication, of matrices and vectors) - matrix inversion and least squares solutions - determinants / rank / condition number - computing eigensystems - factorisations: svd, qr, cholesky, hessenberg, schur, lu - exponents and sqrts of matrices - norms See http://code.haskell.org/hmatrix-static/examples/ for example code. Installation and requirements: Because of the use of type families and view patterns, hmatrix-static requires GHC 6.10 to build. The linear algebra routines are ultimately implemented by BLAS, GSL and LAPACK; see the hmatrix manual [3] for instructions on installing these, prior to installing hmatrix. Once this has been done, hmatrix-static can be installed by $ cabal install hmatrix-static I would be very interested in your feedback, so please try it out. Kind regards, Reiner Pope [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmatrix-static [2] http://www.hmatrix.googlepages.com/ [3] http://www.hmatrix.googlepages.com/installation

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The Monad.Reader (14) - Call for copy
by Wouter Swierstra 14 Apr '09

14 Apr '09

Call for Copy The Monad.Reader - Issue 14 Please consider writing something for the next issue of The Monad.Reader. The deadline for Issue 14 is: ** May 15, 2009 ** The Monad.Reader is a electronic magazine about all things Haskell. Check out the website and browse the previous editions to learn more: http://www.haskell.org/haskellwiki/The_Monad.Reader * Submission Details * Get in touch if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (wouter at chalmers.se). Articles should be written according to the guidelines available from: http://www.haskell.org/haskellwiki/The_Monad.Reader Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. Looking forward to your submission, Wouter

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Types and hashes of hashes, trouble for a Java-programmer...
by John Smith 14 Apr '09

14 Apr '09

Hi, a java-programmer running into trouble while trying to learn Haskell. I try to make a hash containing hashes but can not getting a value out of the innermost hash - I keep running into type-trouble where IO and Maybe monad is getting mixed? My attempt: removeMaybeHash x = case x of Just ref -> ref Nothing -> HashTable.new (==) (\key -> key) test = do h <- HashTable.new (==) (\key -> key) h1 <- HashTable.new (==) (\key -> key) HashTable.insert h 3 h1 HashTable.insert h1 1 1000 maybeOuterHash <- HashTable.lookup h 3 res <- removeMaybe (removeMaybeHash maybeOuterHash) 1000 return res Any clues? In java this would be as simple as (pseudocode): h.lookup(3).lookup(1000) Cheers, Pedro

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Monads from Functors
by Sebastian Fischer 14 Apr '09

14 Apr '09

> {-# LANGUAGE Rank2Types #-} Dear Haskellers, I just realized that we get instances of `Monad` from pointed functors and instances of `MonadPlus` from alternative functors. Is this folklore? > import Control.Monad > import Control.Applicative In fact, every unary type constructor gives rise to a monad by the continuation monad transformer. > newtype ContT t a = ContT { unContT :: forall r . (a -> t r) -> t r } > > instance Monad (ContT t) > where > return x = ContT ($x) > m >>= f = ContT (\k -> unContT m (\x -> unContT (f x) k)) Both the `mtl` package and the `transformers` package use the same `Monad` instance for their `ContT` type but require `t` to be an instance of `Monad`. Why? [^1] If `f` is an applicative functor (in fact, a pointed functor is enough), then we can translate monadic actions back to the original type. > runContT :: Applicative f => ContT f a -> f a > runContT m = unContT m pure If `f` is an alternative functor, then `ContT f` is a `MonadPlus`. > instance Alternative f => MonadPlus (ContT f) > where > mzero = ContT (const empty) > a `mplus` b = ContT (\k -> unContT a k <|> unContT b k) That is no surprise because `empty` and `<|>` are just renamings for `mzero` and `mplus` (or the other way round). The missing piece was `>>=` which is provided by `ContT` for free. Are these instances defined somewhere? Cheers, Sebastian [^1] I recognized that Janis Voigtlaender defines the type `ContT` under the name `C` in Section 3 of his paper on "Asymptotic Improvement of Computations over Free Monads" (available at http://wwwtcs.inf.tu-dresden.de/~voigt/mpc08.pdf) and gives a monad instance without constraints on the first parameter.

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Language.Haskell.Parser and literate Haskell source
by Vasyl Pasternak 14 Apr '09

14 Apr '09

Hello, Are there ability to parse literate haskell source with Language.Haskell.Parser, or maybe there is some tool, which converts .lhs files to .hs ? Thanks in advance. -- Best regards, Vasyl Pasternak

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implementing multilingual interface with dynamic language switching
by Vasyl Pasternak 13 Apr '09

13 Apr '09

Hello, Recently, I've tried to use hgettext to implement dynamic language switching and support of several languages simultaneously. All my tries I've described in blog article http://progandprog.blogspot.com/2009/04/multilingual-ui-and-dynamic-languag… . Comments and suggestions are welcome :) -- Best regards, Vasyl Pasternak

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