- Haskell-Cafe - Haskell.org

"show" for functional types
by Greg Buchholz 05 Apr '06

05 Apr '06

In section 5 of _Fun with Phantom Types_, Hinze states... "Let us move on to one of the miracles of theoretical computer science. In Haskell, one cannot show values of functional types. For reasons of computability, there is no systematic way of showing functions and any ad-hoc approach would destroy referential transparency (except if show were a constant function). For instance, if show yielded the text of a function definition, we could distinguish, say, quick sort from merge sort. Substituting one for the other could then possibly change the meaning of a program." ...I guess I'm not understanding what that means. Would there be some sort of contradiction that arises when trying to evaluate "show (show)" or somesuch? Can anyone point me in the direction of information that tries to explain why this is? I'm at a loss as to what search terms to use. Thanks, Greg Buchholz

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Re: [Haskell] reading binary files
by Benjamin Franksen 05 Apr '06

05 Apr '06

[questions such as this one should go to cafe] On Wednesday 05 April 2006 20:41, minh thu wrote: > 1/ i want to read some binary file (e.g. targa file format : *.tga). > > -- first way : via IOUArray > showInfoHeader1 handle = do > a <- newArray_ (1,8) :: IO (IOUArray Int Word8) > hGetArray handle a 8 > idLength <- readArray a 1 -- or getElems... > putStrLn ("id length : " ++ show idLength) > return () > > -- second way : via c-like array > showInfoHeader2 handle = do > b <- mallocArray 8 :: IO (Ptr Word8) > hGetBuf handle b 8 > [idLength] <- peekArray 1 b -- or peakArray 8 b The index should be 0 if you want to read the first byte. Also, if you are only interested in the first byte, you could simply idLength <- peek b or if it is not the first byte, then idLength <- peekByteOff b i However, it is better to use arrays, than pointers. > putStrLn ("id length : " ++ show idLength) > free b > return () > > so, briefly, i have to read some content into some kind of buffer > (IOUArray Int Word8 or Ptr Word8), then get one (or more) elements > from the buffor into a standard haskell variable (is it the correct > word ?) (or list). > > in the second case, i also have to free the buffer. Or use alloca or allocaBytes, which are both a lot faster than malloc and free. > in some case, when the data is more than one Word8 long, i have to > 'reconstruct' it, i.e.: > > [x1,x2] <- getElems a This will give you a run-time error, because you array is 8 elements long, not 2. You can do x1:x2:_ <- ... or better still x1 <- readArray b 1 x2 <- readArray b 2 Still better: Use one of the available binary serialisation libraries. They are already tuned for efficiency and give you a much nicer high-level API. > let x = fromIntegral x1 + fromIntegral x2 * 256 :: Int > > is it the correct way to read binary files ? Depends on the byte order that is used in your file format. If it is big-endian then correct, else not correct. (I hope I did get this right; I always tend to confuse big- and little-endian.) > 2/ haskell is (i heard that once ... :-) a high level language, so it > has (must have) good support for abstraction... Sure. See abve mentioned libraries. > but in 1/, i have to choose between different kind of array > representation (and i dont know which one is better) and it seems to > me that the resulting code (compiled) would have to be the same. I strongly recommend using some Array type (IOU or whatever). Ptr is really just a raw pointer into memory: no protection from out-of-bounds access, etc. much like in C. Ptr has been invented for interfacing with C routines, not for regular Haskell programming. HTH, Ben

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Re: [Haskell] What's up with this Haskell runtime error message:
by Jon Fairbairn 05 Apr '06

05 Apr '06

On 2006-04-05 at 14:03EDT "Michael Goodrich" wrote: > BTW, I can't seem to locate 'haskell-cafe'. http://www.haskell.org/mailman/listinfo/haskell-cafe > The message responding to my sign-up said nothing about > haskell-cafe. Perhaps it should. It's so long since I signed up to haskell that I've forgotten what the sign-up message says. > Also, in my infinte-looping application, I am wrapping the > calculation in a 'take 1000' function - shouldn't this > guarantee termination? Not on its own loop = loop:: Char l = [1,2,3,loop,5,6] putStr $ show $ take 4 l will print 1 2 and 3 and then loop when trying to find the value for the fourth. -- Jón Fairbairn Jon.Fairbairn at cl.cam.ac.uk

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Re: [Haskell] What's up with this Haskell runtime error message:
by Jon Fairbairn 05 Apr '06

05 Apr '06

On 2006-04-05 at 12:35EDT "Michael Goodrich" wrote: > Greetings All: > > GHC gives: Fail: <<loop>> > > Hugs gives: [(ERROR - C stack overflow Nowt's up wi' ' runtime error message. GHC's perfectly lucid. It says your programme went into an infinite loop. This sort of thing belongs on haskell-cafe, by the way. I've moved it there. Jón -- Jón Fairbairn Jon.Fairbairn at cl.cam.ac.uk

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PPM image manipulation
by newb 05 Apr '06

05 Apr '06

having some trouble trying to be able to manipulate PPM images. I want to be able to desaturate and also double the scale of them. im a first time user of haskell so am not very familiar with it. any info would be great! thanks! -- View this message in context: http://www.nabble.com/PPM-image-manipulation-t1382306.html#a3711988 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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RE: generics question 2
by Ralf Lammel 05 Apr '06

05 Apr '06

Hi Frederik, [resending; as it bounced because of size.] That’s a tricky one. Let’s first recall that this one is still fine: *Main> :t cast True :: (Typeable a, Typeable b) => Maybe (a,b) cast True :: (Typeable a, Typeable b) => Maybe (a,b) :: (Typeable b, Typeable a) => Maybe (a, b) The type error for your attempt was caused by the attempt to print this polymorphic maybe. The overloading resolution for the show instance cannot possibly deal with this polymorphism. More generally, any dependent SYB functionality or class-based code would run into this problem. However, the above polymorphic cast, as such does not work as you would expect. Here is a clear example: sigh = maybe 0 (\(a,b) -> gsize a + gsize b) (cast True) The component types of this product are *ambiguous*. Cast is monomorphic. No way to use it for polymorphic problems. So we need polymorphic type extension, here: dataCast2 provided by the Data class. So here is how it works with polymorphic type extension. See below and check out the 2nd paper for some deeper discussion. Or if you find this all too complicated, use class-based SYB style (3rd SYB paper). Thanks for keeping me off the street. ☺ Regards, Ralf import Data.Generics -- -- A weird gsize function -- g :: Data a => a -> Int g = gsize `extQ` (\(x::Int) -> x) `ext2Q` f where f :: (Data a, Data b) => (a,b) -> Int f (a,b) = g a + g b + 1 -- -- | Flexible type extension -- Should be in Data.Generics.Aliases -- ext2 :: (Data a, Typeable2 t) => c a -> (forall a b. (Data a, Data b) => c (t a b)) -> c a ext2 def ext = maybe def id (dataCast2 ext) -- -- | Type extension of transformations for unary type constructors -- Should be in Data.Generics.Aliases -- ext2T :: (Data d, Typeable2 t) => (forall d. Data d => d -> d) -> (forall d d'. (Data d, Data d') => t d d' -> t d d') -> d -> d ext2T def ext = unT ((T def) `ext2` (T ext)) -- -- | Type extension of queries for type constructors -- Should be in Data.Generics.Aliases -- ext2Q :: (Data d, Typeable2 t) => (d -> q) -> (forall d d'. (Data d, Data d') => t d d' -> q) -> d -> q ext2Q def ext = unQ ((Q def) `ext2` (Q ext)) -- -- | The type constructor for transformations -- Not exported from Data.Generics.Aliases -- newtype T x = T { unT :: x -> x } -- -- | The type constructor for queries -- Not exported from Data.Generics.Aliases -- newtype Q q x = Q { unQ :: x -> q } -- -- Time to test -- data Foo = Foo1 Int | Foo2 (Int,Int) deriving (Typeable, Data, Show) main = do print $ gmapQ g (Foo1 88) print $ gmapQ g (Foo2 (21,20)) > -----Original Message----- > From: Frederik Eaton [mailto:frederik@a5.repetae.net] > > What I want to do is specialise not to a specific type like Bool but > to the class of all pairs (a,b). But this causes the compiler to > complain, even for simpler examples: > > cast True :: (Typeable a, Typeable b) => Maybe (a,b) > > <interactive>:1:0: > Ambiguous type variable `a' in the constraints: > `Typeable a' arising from instantiating a type signature at > <interactive>:1:0-51 > `Show a' arising from use of `print' at Top level > Probable fix: add a type signature that fixes these type variable(s) > > <interactive>:1:0: > Ambiguous type variable `b' in the constraints: > `Typeable b' arising from instantiating a type signature at > <interactive>:1:0-51 > `Show b' arising from use of `print' at Top level > Probable fix: add a type signature that fixes these type variable(s) > > Is there a way to solve this, or do I have to avoid polymorphism? I > can use 'toConstr' to find out dynamically if a particular type is a > pair, and then use unsafeCoerce, but I hear that unsafeCoerce is > unsafe.

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RE: generics question 2
by Ralf Lammel 04 Apr '06

04 Apr '06

> Hi Ralf, > > I'm looking for a function like extT but with more general type: > > (t a -> s a) -> (t b -> s b) -> (t a -> s a) > > Is there such a thing in the generics library? Hi Frederik, Not sure how you are exactly going to use such an operation ... But here is its implementation anyhow. Thanks for the riddle. Ralf import Data.Generics -- Frederik's weird ext operation :-) ext' :: (Data (t a), Data (s a), Data (t b), Data (s b)) => (t a -> s a) -> (t b -> s b) -> (t a -> s a) ext' f g ta = case cast g of Just g' -> g' ta Nothing -> f ta -- A generic default f (Just x) = [x] f Nothing = [] -- A type-specific case g (Just True) = [True] g (Just False) = [] g Nothing = [] -- A composition using our new type-extension operator test :: Data a => Maybe a -> [a] test = ext' f g -- Let's see whether it works ... main = do print $ test (Just (1::Int)) print $ test (Just False)

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Creating a profiling version of FPS
by Jeremy O'Donoghue 04 Apr '06

04 Apr '06

Hi list, I'm currently working on a pretty simple Haskell program to manipulate memory dumps taken from an embedded environment. The memory dumps are 64MB in length, so I manipulate them using Don Stuart's Fast Packed String library, as it supports mmapped files. I'm a relative Haskell newbie, so unsurprisingly my first attempt has dreadful performance (30% of time spent in GC), and I want to do some profiling to find out why. The difficulty is that I seem to be unable to produce a profiling-enabled version of the FPS library, which means that my application cannot be compiled with profiling enabled. I notice that most of the GHC libraries seem to come with both profiling and non-profiling versions enabled. Does anyone know how I can create both profiling and non-profiling version of FPS? As a matter of interest (feature request), why doesn't Cabal build profiling and non-profiling libraries as a matter of course, given that you can't link non-profiling code with profiling-enabled code? Thanks in advance Jeremy

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Re: Indent for haskell
by dons＠cse.unsw.edu.au 04 Apr '06

04 Apr '06

patc: > > Is there an equivalent of an "indent" program for haskell? I have a > bit of code I want to clean up ... You could run your code through ghc, with -ddump-parsed turned on. Then with a little bit of sed magic, you could recover the original code Before: $ cat B.hs main = do { ;print (f 10) ; print (f 20) ;print (f 0xdeadbeef) } f x = sqrt ( fromIntegral (x * 10133123 `mod` 1231111121 :: Int )) After: $ ghc -fno-code -ddump-parsed B.hs ==================== Parser ==================== main = do print (f 10) print (f 20) print (f 3735928559) f x = sqrt (fromIntegral ((x * 10133123) `mod` 1231111121 :: Int)) Postprocess: $ ghc -fno-code -ddump-parsed B.hs | sed '/^===/d' main = do print (f 10) print (f 20) print (f 3735928559) f x = sqrt (fromIntegral ((x * 10133123) `mod` 1231111121 :: Int)) Yay! Now, I suspect there may be actually be (not well publicised) tools for this. If there are, they should go on the haskell.org "Libraries and tools" page. -- Don

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Strafunski
by Christopher Brown 04 Apr '06

04 Apr '06

Hi, I am trying to use Strafunski with GHC 6.5 and was wondering if someone could help me. I have all the instances for Term and Typeable defined for my data types, but when I try to compile with GHC 6.5 I get lots of "overlapping instance" errors. In particular, it seems the instances I am using (generated by DrIFT) are clashing with the ones in Data.Typeable. Is there a way I can fix this? Also I have heard that it is possible to add "deriving Typeable" to each data type and I don't need to use the instances I have created. However, now it complains that it can't find instances for Term - but I can't derive from Term. Does anyone have any ideas how I can get Strafunski working with GHC 6.5? Thanks. Chris. Christopher Brown PhD Student, University of Kent. http://www.cs.kent.ac.uk/people/rpg/cmb21/ cmb21(a)kent.ac.uk

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