May 2005 - Haskell-Cafe

Question about classes from a Haskell newbie.
by Jeff.Harper＠handheld.com 23 May '05

23 May '05

Hi, I'm new to Haskell and even newer to this list. I have over 20 years of experience in C or C++. For fun, I decided to write a Fourier transform in Haskell. It would be convenient if I had a function that converts any real or complex number into a complex number. My attempt at doing this is below. Hugs produces errors when I try to load this code. Essentially, I'm trying to define a class called ConvertibleToComplex. There is a function in this class called toComplex. toComplex has the type: class ConvertibleToComplex a where toComplex :: RealFloat b => a -> Complex b I'd like some instances of toComplex to return a Complex Double while other instances return a Complex Float. Doing this sort of thing in C++ is fairly easy. However, I get the feeling that I'm pushing the Haskell type system to do something it isn't designed to do. Is there a way to define ConvertibleToComplex so that toComplex's return type is generic and a particular instance of ConvertibleToComplex decides what the return type is? Here is my attempt at doing this. If someone can give me some pointers on how to make this work, I'd appreciate it. module Main where import Complex -------------------------------------------------------------------------------- -- toComplex should convert a real or complex number to a complex number. -- -- Here's my goal for the type of toComplex. Given: -- f :: Float -- d :: Double -- cf :: Complex Float -- cd :: Complex Double -- -- When toComplex is applied to these, I'd like it to evaluate to the -- following types: -- -- toComplex f :: Complex Float -- toComplex d :: Complex Double -- toComplex cf :: Complex Float -- toComplex cd :: Complex Double class ConvertibleToComplex a where toComplex :: RealFloat b => a -> Complex b -- If I uncomment the following code, Hugs produces errors: -- ERROR "c:\tmp\test2.hs":29 - Inferred type is not general enough -- *** Expression : toComplex -- *** Expected type : (ConvertibleToComplex Float, RealFloat a) => Float -> Complex a -- *** Inferred type : (ConvertibleToComplex Float, RealFloat Float) => Float -> Complex Float {- instance ConvertibleToComplex Float where toComplex f = f :+ 0 instance ConvertibleToComplex Double where toComplex d = d :+ 0 instance ConvertibleToComplex (Complex a) where toComplex c = c -}

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Open mutable records
by Einar Karttunen 23 May '05

23 May '05

Hello I recently ended up hacking a quite concise implementation of mutable open (extensible) records in Haskell. Most of the ideas came from the HList-paper, but this seems like a very simple way of doing things. Run with ghci -fglasgow-exts -fallow-overlapping-instances. Import some stuff we are going to need later: > import Control.Monad.Reader > import Data.IORef > import System Monad for mutable record calculations - to get implisit this/self in the OO sense. > newtype OO t r = OO (ReaderT t IO r) deriving(Monad, MonadReader t, MonadIO) > > with :: s -> OO s a -> OO b a > with this (OO c) = liftIO (runReaderT c this) > > ooToIO :: OO s a -> IO a > ooToIO (OO c) = runReaderT c undefined Records First the record constructor - followed by the terminator. > data a :.: r = RC !a !r > infixr :.: > data END = END Next we define a field access method. > class Select r f t | r f -> t where (!) :: r -> f -> Ref t > instance Select (Field f t :.: r) f t where (!) (RC (F x) _) _ = x > instance Select r f t => Select (a :.: r) f t where (!) (RC _ t) = (!) t And finally the type of mutable fields. > type Ref a = IORef a > newtype Field name rtype = F (Ref rtype) Next we define a way to construct record values. > infixr ## > (##) :: v -> OO s r -> OO s ((Field f v) :.: r) > (##) v r = do { h <- liftIO (newIORef v); t <- r; return (RC (F h) t) } > end = return END :: OO s END Get the value of a field. > value :: Select s f t => f -> OO s t > value a = do x <- asks (\s -> s!a) > liftIO (readIORef x) Or set the value of a field. > (<-:) :: Select s f t => f -> t -> OO s () > a <-: b = do x <- asks (\s -> s!a) > liftIO (writeIORef x b) And as a convenience add value to an int field. > (+=) :: Select s f Int => f -> Int -> OO s Int > a += b = do x <- asks (\s -> s!a) > val <- liftIO (readIORef x) > let z = val+b > z `seq` liftIO (writeIORef x z) > return z Now implement the classic ocaml OO tutorial: > data X = X > type Point = Field X Int :.: END > > newPoint :: OO s Point > newPoint = 0 ## end > > getX :: Select s X t => OO s t > getX = value X > > move d = X += d > data Color = Color > type ColoredPoint = Field Color String :.: Point > > newColoredPoint :: String -> OO s ColoredPoint > newColoredPoint c = c ## 0 ## end > > color :: Select s Color t => OO s t > color = value Color The code looks in more complex examples like this: ((~=) is prepending into list fields.) newArrival :: Patient -> OO Hospital () newArrival patient = do with patient (HospitalVisits += 1) staff <- value FreeStaff if staff > 0 then do FreeStaff += (-1) Examination ~= patient with patient (do HospitalTime += 3 RemainingTime <-: 3) else do Triage ~= patient > main = ooToIO (do c1 <- newPoint > c2 <- newColoredPoint "green" > with c1 $ move 7 > with c2 $ move 4 > let p x = liftIO (print x) > p =<< with c1 getX > p =<< with c2 getX) - Einar Karttunen

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terrible Haskell pun
by Michael Vanier 22 May '05

22 May '05

I came up with a terrible Haskell pun that I had to share with this list: Haskell provides special syntactic support for monads in terms of the "do notation". There is a straightforward translation between this notation and the core language, which constitutes its "do-notational semantics", as it were. Sorry, Mike

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basic haskell question
by wenduan 22 May '05

22 May '05

hi, Anybody could please tell me that in the following two expressions what value does the [] take? foldl (/) 3 [] foldr (/) 3 [] when both of them are evaluated I got 3.0,but I thought I could get nothing out of there,cause its an empty list,does Haskell assume any default value for a empty list? -- ------------------------------------------------------------------------------------------------------- If you didn't succeed at the first attempt, you are just about average.

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Re: [Haskell-cafe] Python?
by karczma＠info.unicaen.fr 21 May '05

21 May '05

Tim Rowe writes: > On 5/11/05, Jerzy Karczmarczuk <karczma(a)info.unicaen.fr> wrote: > >> Give me one single language where [3-d arrays are] natural and immediate. > > I don't know how Matlab does it, but I find the C++ standard library > vector<vector<vector<float> > > > entirely intuitive (apart, perhaps, for the need for those two spaces)! Let's precise what I consider to be important in order to call it natural and immediate. And, in general, useful. 1. The definition of a concrete object, not just its type, but, say, the initialization with constants. Or/and, global initialization with zeros. 2. Easy synthesis of multi-dim matrices out of "planes", of submatrices of lesser dimensions; it can be an 'overlay', like, say, making a colour image out of three R/G/B planes, or making a 3D surfaces, or aking tensors through external products. 3. Easy indexing, and not only A[i][j][k], etc., but slicing, the extraction of sub-dimensional matrices, e.g., one column vector out of a 2D matrix in Matlab: A(3,:). Also, extracting parts (e.g. sub-images). Also, in mathematical context, "intelligent" indexing, e.g. treating a matrix as implicitly anti-symmetric. Here the CAS systems as Maple or Mathematica provide the adequate tools. C++ of course doesn't, unless you overload [] yourself. 4. Readable iterators, perhaps something more compact than insipid do-loops. 5. If those matrices are used as mathematical objects: tensors, etc., I want to have some simple notation for inner multiplications/ contractions, etc. This is not just the syntax problem, but the existence of good libraries as well... 6. Reshaping of those arrays. I thought that Matlab 'reshape' (or something similar in Numeric Python) is a baroque, rarely used construction. Now I use it quite often... So, plenty of things. That's why this is not so trivial... Jerzy Karczmarczuk

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forall in a lambda abstraction
by Scott J. 20 May '05

20 May '05

Hello, what's wrong with the code of test1 below. Ghci does complain with an error message that I don't understand. Thanks Jan id x = x test1 = \f ::(forall a. a -> a) -> (f True,f 'a') test = \f -> (f True,f False) h :: (forall a. a->a) -> (Bool,Char) --h :: (a.a -> a) -> (Bool,Char) h id = (id True, id 'a')

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RE: [Haskell-cafe] Tables in Haskell (data model /business app pr ototyping)
by Bayley, Alistair 20 May '05

20 May '05

> From: Rene de Visser [mailto:rene_de_visser@hotmail.com] > > I would rather use Haskell also for the persistency, global > constraint > maintanence, etc... rather than using an external database. > > The only thing I have found so far is HApps, but I haven't > figure out how > that works. > > Is there anything else? What do you mean by "external" database? If you just mean "on another server, accessed via a separately installed library or API" then you should consider Sqlite as a persistence storage solution. It's a doddle to install on common platforms, and consists of a single library which links to your program i.e. it runs in the same process-space. Both HSQL (and therefore HaskellDB, as it uses HSQL) and Takusen support Sqlite. http://haskelldb.sf.net/ http://cvs.sf.net/viewcvs.py/haskell-libs/libs/takusen/ Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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Re: Why the instance declaration , passing on Hug but failing on GHC 6.2.1?
by Daniel Fischer 19 May '05

19 May '05

Am Donnerstag, 19. Mai 2005 10:39 schrieb Stefan Holdermans: > Wukaichen, > > > data People = A | B | C | D | E | F deriving (Show, Eq, Ord, Enum) > > data Pair a = Pair a a deriving Eq > > > > instance Show (Pair People) where > > show (Pair x y) = "<" ++ show x ++ ", " ++ show y ++ ">" > > GHC is right, actually. Haskell 98 requires instance declarations to > have the form > > instance (...) => C (T a1 ... an) > > where a1 are type variables, not type constructors as in your example. > > However, you can use the Glasgow extensions to get rid of this > restrictions. For example by adding > > {-# OPTIONS -fglasgow-exts#-} > > to the top of your file. > > HTH, > > Stefan > Alternatively, one might define instance Show a => Show (Pair a) where ... By the way, the original code also needs the '-98' option for hugs, so the behaviour isn't really different. Cheers, Daniel

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www.haskell.org
by Colin Paul Adams 19 May '05

19 May '05

I'm trying to download a darcs client, but I get: "The connection was refused when trying to connect to www.haskell.org" from Firefox on Linux. -- Colin Adams Preston Lancashire

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Bit fiddling
by Florian Weimer 19 May '05

19 May '05

I'm toying a bit with Haskell and wondering what's the best way to implement bit fiddling. Most of my applications involve serializing and deserializing small blobs (IP packets, for instance), and after browsing the GHC library documentation, I'm not sure which appraoch I should use. That's why I'd appreciate pointers to sample code. Portability beyond GHC is not required. It's not necessary to update blobs, only to parse them, and serialize new ones. And in the beginning, I'd like to write the boilerplate manually. 8-)

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