November 2005 - Haskell-Cafe

Existential quantification of environments.
by Keean Schupke 22 Nov '05

22 Nov '05

If I have a function: f x y = add x y and I want to type the function in isolation, then the type of 'add' is essentially carried in the environment... Lets say I want to make this type explicit in the type signature (where f is valid for any a where there is an add function on a - ignoring the class that Haskell would require for the overloading): add :: Int -> Int -> Int add :: Float -> Float -> Float f :: forall a . exists (add :: a -> a -> a) => a -> a -> a or a step further: class Add a where add :: a -> a -> a instance Add Int where ... instance Add Float where ... f :: forall a . Add a => a -> a -> a This seems to suggest: Add a == exists (add :: a -> a -> a) Does this seem in any way right? It seems that the definition of 'f' does require the existance of 'add'. That is the definition is valid iff there exists a function called 'add' of the correct type. Also doesn't the existential quantifier stop you looking inside 'add' - obviously you cannot inspect the definition as it may not be defined (yet?), but presumably you can still apply 'add'. Regards, Keean.

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GHCi always optimizes?
by Dusan Kolar 22 Nov '05

22 Nov '05

Hello all, My question is rather strange, maybe, even if simple: Does ghci always translates with "-O" option set on? I've done some measurements on an application using ghc and ghci. ghc compiled with no opmitization => program running real 988m59.260s user 989m1.325s sys 0m0.704s ghc compiled with optimization set on => program running real 15m54.343s user 15m54.168s sys 0m0.172s ghci alaways: 951.97 secs, 7445117252 bytes (which is 15m51.97sec) To be honest, I don't mind ghci optimizes always but saying it uses the same options as ghc is not entirely true and, for optimization, it's quite painful because one searches error where it is not. (/Understand, I was wondering why program compiled by ghc crashes and/or evaluates so long while running correctly and fast in ghci./) Dusan

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Pickling HList
by Joel Reymont 22 Nov '05

22 Nov '05

Folks, I'm having trouble creating a pickler for HLists and would appreciate a solution. The code for (HCons e HNil) works fine but I get an error trying to implement puHList for (HCons e l) where l is supposed to be (HCons e ...), i.e. another HList. Bar.hs:21:37: Couldn't match the rigid variable e' against PU e' `e' is bound by the instance declaration at Bar.hs:17:0 Expected type: HCons (PU e) l Inferred type: HCons e l In the first argument of puHList', namely l' In the second argument of pair', namely (puHList l)' Failed, modules loaded: none. ---- module Bar where import Data.Word import OOHaskell main = print "We are here!" class HList l => HLPU l where puHList :: HCons (PU e) l -> PU (HCons e l) instance HLPU HNil where puHList (HCons pe HNil) = wrap (\e -> HCons e HNil, \(HCons e HNil) -> e) pe instance HList l => HLPU (HCons e l) where puHList (HCons pe l) = wrap (\(a, b) -> HCons a b, \(HCons a b) -> (a, b)) (pair pe (puHList l)) newtype TourType = TourType TourType_ deriving (Show{-, Typeable-}) newtype AvgPot = AvgPot Word64 deriving (Show{-, Typeable-}) data TourType_ = TourNone | TourSingle | TourMulti | TourHeadsUpMulti deriving (Enum, Show{-, Typeable-}) --- Pickling data PU a = PU { appP :: (a, [Word8]) -> [Word8], appU :: [Word8] -> (a, [Word8]) } pickle :: PU a -> a -> [Word8] pickle p value = appP p (value, []) unpickle :: PU a -> [Word8] -> a unpickle p stream = fst (appU p stream) lift :: a -> PU a lift x = PU snd (\s -> (x, s)) sequ :: (b -> a) -> PU a -> (a -> PU b) -> PU b sequ f pa k = PU (\ (b, s) -> let a = f b pb = k a in appP pb (b, appP pa (a, s))) (\ s -> let (a, s') = appU pa s pb = k a in appU pb s') pair :: PU a -> PU b -> PU (a,b) pair pa pb = sequ fst pa (\ a -> sequ snd pb (\ b -> lift (a,b))) wrap :: (a -> b, b -> a) -> PU a -> PU b wrap (i, j) pa = sequ j pa (lift . i) Thanks, Joel -- http://wagerlabs.com/

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using of data types as kinds
by Bulat Ziganshin 22 Nov '05

22 Nov '05

Hello , in the HCAR GHC team wrote "On the type system front, we hope to ... Allow you to use data types as kinds, in a manner similar to Tim Sheard▓s Omega language". can someone point me where i can read about this? -- Best regards, Bulat mailto:bulatz@HotPOP.com

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How to use notFollowedBy function in Parsec
by Sara Kenedy 22 Nov '05

22 Nov '05

Dear all, Using Parsec, I want to represent a string (of anyToken) not ended with symbol semi (;). I use the command notFollowedby as follows: module Parser where import Parsec import qualified ParsecToken as P import ParsecLanguage langDef::LanguageDef () langDef = emptyDef {reservedOpNames = []} lexer::P.TokenParser() lexer = P.makeTokenParser langDef semi = P.semi lexer str1 :: Parser String str1 = do {str <- many anyToken; notFollowedBy semi; return str} However, when I compile, there is an error. ERROR "Test.hs":17 - Type error in application *** Expression : notFollowedBy semi *** Term : semi *** Type : GenParser Char () String *** Does not match : GenParser [Char] () [Char] I do not know how to fix it. Help me. Thanks for your time.

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Re: [Haskell-cafe] records proposals list
by David Menendez 22 Nov '05

22 Nov '05

Keean Schupke writes: > Haskell already has static records (in H98) > > Dynamic records are addressed by the HList library, which uses > extensions already present in GHC and Hugs (namely Multi-parameter > type-classes and function-dependancies). Is this the case? Every implementation of HList that I've seen also uses overlapping and undecidable instances. -- David Menendez <zednenem(a)psualum.com> | "In this house, we obey the laws <http://www.eyrie.org/~zednenem> | of thermodynamics!"

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records proposals list
by Bulat Ziganshin 22 Nov '05

22 Nov '05

Hello Haskell, can anyone write at least the list of record proposals for Haskell? or, even better, comment about pros and contras for each proposal? -- Best regards, Bulat mailto:bulatz@HotPOP.com

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Re: [Haskell-cafe] Spurious program crashes
by Joel Reymont 22 Nov '05

22 Nov '05

On Nov 17, 2005, at 1:44 PM, Sebastian Sylvan wrote: > Are you sure it's safe to kill a thread which has already been killed? It seems so from the docs. > Why do you fork off the killing of the threads? Why not just run them > in sequence? Someone said that they read somewhere that killThread can block. I'm not gonna point any fingers at musasabi ;-). > Also, I'd recommend refactoring the code a bit, write a function > "parIO" which runs IO computations in parallell and then define > timeout in terms of that. I did this by stealing the timeout/either combinators from the Asynchronous Exceptions paper. It did not help a single bit. Joel -- http://wagerlabs.com/

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Function application like a Unix pipe
by Scherrer, Chad 21 Nov '05

21 Nov '05

I'm still trying to settle on a "feel" for good programming style in Haskell. One thing I've been making some use of lately is (\|) = flip ($) infixl 0 \| Then expressions like f4 $ f3 $ f2 $ f1 $ x become x \| f1 \| f2 \| f3 \| f4 I've seen something like this on haWiki using (#), but I prefer this notation because it looks like a Unix pipe, which is exactly how it's used. This style makes the sequential feel of monadic do notation available to non-monadic functions, and would seem to ease the initial confusion most people experience when they first program using lazy evaluation. Many newbies have experience with Unix pipes, and this notation makes that analogy explicit. I have a couple of questions about this... First, it would be convenient if ($) could appear in an fi without extra parentheses, but this would require "infixl (-1) \|". Is there a reason this isn't allowed? The current fixities don't seem to allow much room for expansion. Would it be possible to either (1) allow negative and/or fractional fixities, or (2) shift and spread out the current fixities to allow more flexibility in the introduction of new operators? I'm also trying to determine to what extent this style should really be used. Does anyone see this as "bad style"? Is there an argument why this might not be such a good way to ease someone into the language? Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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getCPUTime ??
by Michael Benfield 21 Nov '05

21 Nov '05

I'm new to Haskell. I'm apparently misunderstanding something here. When I run this program: --------- module Main where import System.Posix import System.CPUTime printTime = getCPUTime >>= putStrLn . show main = printTime >> sleep 5 >> printTime --------- It produces this output: 1430000000000 1430000000000 or similar. In any case, both the numbers are the same. Should the second number not reflect a time 5 seconds later than the first? I've tried this with both GHC and Hugs, and both give me the same thing. Thanks. Mike Benfield

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