- Haskell-Cafe - Haskell.org

Ambiguous reified dictionaries
by Martijn van Steenbergen 15 Apr '09

15 Apr '09

Good morning, The [1]GHC user's guide, section 8.4.5 says: "The new feature is that pattern-matching on MkSet (as in the definition of insert) makes available an (Eq a) context. In implementation terms, the MkSet constructor has a hidden field that stores the (Eq a) dictionary that is passed to MkSet; so when pattern-matching that dictionary becomes available for the right-hand side of the match." But what happens if there are several dictionaries available? Consider these three modules: ReifyMonoid.hs: > {-# LANGUAGE GADTs #-} > > module ReifyMonoid where > > import Data.Monoid > > data MonoidInst a where > MkMonoidInst :: Monoid a => MonoidInst a ReifySum.hs: > module ReifySum where > > import ReifyMonoid > import Data.Monoid > > instance Monoid Int where > mempty = 0 > mappend = (+) > > intSum :: MonoidInst Int > intSum = MkMonoidInst ReifyProd.hs: > module ReifyProd where > > import ReifyMonoid > import Data.Monoid > > instance Monoid Int where > mempty = 1 > mappend = (*) > > intProd :: MonoidInst Int > intProd = MkMonoidInst Now a function > emp :: MonoidInst a -> a > emp MkMonoidInst = mempty works as you'd expect: *ReifySum ReifyProd> emp intSum 0 *ReifySum ReifyProd> emp intProd 1 But what about this function? > empAmb :: MonoidInst a -> MonoidInst a -> a > empAmb MkMonoidInst MkMonoidInst = mempty Now there are two dictionaries available. GHC consistently picks the one from the second argument: *ReifySum ReifyProd> empAmb intProd intSum 1 *ReifySum ReifyProd> empAmb intSum intProd 0 My questions are: 1) Shouldn't GHC reject this as being ambiguous? 2) Should class constraints only be available on existentially qualified type variables to prevent this from happening at all? 3) Is it possible to implement the following function? > mkMonoidInst :: a -> (a -> a -> a) -> MonoidInst a > mkMonoidInst mempty mappend = ... Thank you, Martijn. [1] http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extension…

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Re: Looking for the fastest Haskell primes algorithm
by Johannes Waldmann 15 Apr '09

15 Apr '09

for the API design, always check others before rolling your own. E.g. the (three) functions with "Prime" in their name from http://java.sun.com/javase/6/docs/api/java/math/BigInteger.html I hope they are there for a reason. While we're at it - do we have modPow? modInverse? And of course check their implementation as well (should be straightforward, but you never know). J.W.

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understanding typeable
by Anatoly Yakovenko 15 Apr '09

15 Apr '09

any idea why this is True data Foo = FooC Int | BarC Int deriving (Data, Typeable, Show) > fromJust $ funResultTy (typeOf FooC) (typeOf (1::Int)) Loading package syb ... linking ... done. ParseG.Foo > typeRepTyCon $ fromJust $ funResultTy (typeOf FooC) (typeOf (1::Int)) ParseG.Foo > let a = typeRepTyCon $ fromJust $ funResultTy (typeOf FooC) (typeOf (1::Int)) > :t a a :: TyCon > typeRepTyCon $ typeOf $ BarC 2 ParseG.Foo > let b = typeRepTyCon $ typeOf $ BarC 2 > a == b True I thought that TyCon can distinguish constructors. it seems no different then a typerep

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error handling (esp. IO/exceptions)
by brian＠lorf.org 14 Apr '09

14 Apr '09

I'm finding it hard to write robust code that isn't really ugly. Suppose I want to write > execute :: FilePath -> [String] -> IO (Either ExecuteError ExitCode) where 'ExecuteError' is a data type representing all the ways 'execute' could fail and that 'execute p args' is supposed to * ensure p exists * get p's permissions * ensure p is readable * ensure p is executable * execute p and return its exit code So 'ExecuteError' would have constructors corresponding to these ways to fail: * could not determine whether p exists * p does not exist * could not get p's permissions * p is not readable * p is not executable * could not execute p for some other reason So if I start to code it, I 'tryJust' 'doesFileExist'. If an exception is thrown, I convert it to Left $AppropriateExecuteError. If not, we know whether p exists. If it doesn't, convert it to Left. Otherwise, 'tryJust' 'getPermissions', etc. It's starting to staircase. I'm needing it to be easier to stop my computation and return specific things when exceptions are thrown or when certain requirements aren't met. Thanks for any help.

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lift in TemplateHaskell?
by Lyle Kopnicky 14 Apr '09

14 Apr '09

Hi folks, I'm having trouble reading the TemplateHaskell docs and trying to do something that seems like it should be simple. In MetaML, there is a 'lift' function which does exactly what I want to do. Here's an example function: let double n = [| 2 * $( litE (integerL n) ) |] This works as intended, but instead of writing "litE (integerL n)" I just want to write "lift n", or something similar. I expect that would involve a type class called something like Liftable that would lift literals using the appropriate functions. Does this exist? Thanks, Lyle

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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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