July 2009 - Haskell-Cafe

Importing Control.Arrow changes inferred type of (m >>= f) x in ghci
by Dan Weston 28 Jul '09

28 Jul '09

The following inferred type has a constraint that can be trivially satisfied, but isn't: Control.Monad> :t \ (m,f,x) -> (m >>= f) x \ (m,f,x) -> (m >>= f) x :: forall t a b. (Monad ((->) t)) => (t -> a, a -> t -> b, t) -> b -- In Control.Monad there is forall t. instance Monad ((->) t), -- so why is the vacuous Monad constraint still there? -- Nor can I remove it with a type annotation: Control.Monad> :t \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b <interactive>:1:13: Inferred type is less polymorphic than expected [snip] -- Then if I just import a module: Control.Monad> :m + Control.Arrow -- Now, the Monad ((-> t) constraint disappears: Control.Monad Control.Arrow> :t \ (m,f,x) -> (m >>= f) x \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b -- Although it still will not accept an explicit type annotation: Control.Monad Control.Arrow> :t \ (m,f,x) -> (m >>= f) x :: forall t a b. (t -> a, a -> t -> b, t) -> b <interactive>:1:13: Inferred type is less polymorphic than expected [snip] Is there some explicit kind annotation I can/should give to get the most general type? Dan

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Re: [Haskell] Re: 20 years ago
by Ketil Malde 27 Jul '09

27 Jul '09

[redirected from haskell@] Benjamin L.Russell <DekuDekuplex(a)Yahoo.com> writes: >> One often amusing outgrowth of this is that FP (OOP) fanatics anthropomorphize >> their functions (objects). Well, I don't think we do. Functions are just mappings of values to values, they may be opaque, but they're predictable, unchanging, and just...too boring to be antropomorphized. Objects contain all kinds of hidden state and dependencies, and the sheer unpredicatability of it all is the reason for the anthropomorphics - it a symptom of a disease, not a desirable quality. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Testing nested implication properties with QuickCheck?
by Ahn, Ki Yung 27 Jul '09

27 Jul '09

How should I you use QuickCheck for testing a property that is a nested implecation such as (A ==> B) ==> C ? The problem is (==>) in QuickCheck is that its type is Testable prop => Bool -> prop -> Property rather than Testable prop => prop -> prop -> Property. So, A ==> (B ==> C) would work but (A ==> B) ==> C won't work when A, B, C are boolean expressions. Is there some trick to handle the latter? Thanks, -- Ahn, Ki Yung

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Testing nested implication properties with QuickCheck?
by Ahn, Ki Yung 27 Jul '09

27 Jul '09

How should I use QuickCheck for testing a property that is a nested implecation such as (A ==> B) ==> C ? The problem is (==>) in QuickCheck is that its type is Testable prop => Bool -> prop -> Property rather than Testable prop => prop -> prop -> Property. So, A ==> (B ==> C) would work but (A ==> B) ==> C won't work when A, B, C are boolean expressions. Is there some trick to handle the latter? Thanks, -- Ahn, Ki Yung

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'cabal install lhs2tex' fails for me on os x -- help please
by Conal Elliott 27 Jul '09

27 Jul '09

Fails during configuration: bash-3.2$ cabal install lhs2tex Resolving dependencies... [1 of 1] Compiling Main ( /tmp/lhs2tex-1.1423397/lhs2tex-1.14/Setup.hs, /tmp/lhs2tex-1.1423397/lhs2tex-1.14/dist/setup/Main.o ) Linking /tmp/lhs2tex-1.1423397/lhs2tex-1.14/dist/setup/setup ... Configuring lhs2tex-1.14... cabal: Error: some packages failed to install: lhs2tex-1.14 failed during the configure step. The exception was: exit: ExitFailure 11 Note that the the first module to be compiled is Main. On my linux machine, Main is the *last* of several modules to be compiled. Here's my failure log: http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=3096#a3096 . If instead I use ./configure and make, i get errors due to use of the old Exception API: ... [17 of 19] Compiling FileNameUtils ( FileNameUtils.lhs, FileNameUtils.o ) FileNameUtils.lhs:81:35: Ambiguous type variable `b' in the constraint: `GHC.Exception.Exception b' arising from a use of `catch' at FileNameUtils.lhs:(81,35)-(89,60) Probable fix: add a type signature that fixes these type variable(s) ... Suggestions greatly appreciated! - Conal

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Retiring forkIO
by Günther Schmidt 27 Jul '09

27 Jul '09

Hi, I'd like to retire forkIO and friends by using Delimited Continuations instead. Am I dead wrong here in my understanding of Delimited Continuations or can I pursue in this direction? The most immediate use for this is actully GUI problems where I'd use del-conts in place of forkIO for long-running computations because once I forkIO the computation so as not to block the UI thread I cannot call GUI code from the end of that thread. Well I can but then the app crashes because of single threading issues. Günther

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Is there a sample code for hdbus?
by Magicloud Magiclouds 27 Jul '09

27 Jul '09

Hi, Although I have used dbus with ruby a little, but still, I do not understand hdbus's APIs.... Is there any sample that I could learn? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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Data.Binary.encode slower than show
by Grzegorz Chrupała 27 Jul '09

27 Jul '09

Hi all, I have a piece of code where I'm serializing a datastructure with the following type [(Int, (Map DType (IntMap Int)))], using Binary.encode The thing is it is very slow: actually quite a bit slower than just using show. This seems rather suspicious. Any idea what could be going on? import qualified Data.Map as Map import qualified Data.IntMap as IntMap import Data.IntMap ((!)) import Vector import Atom import Control.Monad.State import Indexing (indexBy) import Data.List (foldl') import System import System.IO import Debug.Trace import qualified Data.Binary as Binary import qualified Data.ByteString.Lazy as BS data DType = Fol | For | Sol | Sor deriving (Eq,Enum,Ord,Show,Read) instance Binary.Binary DType where get = do x <- Binary.get return (toEnum x) put x = Binary.put (fromEnum x) type Word = Int type Count = Int type WordMap = IntMap.IntMap find = IntMap.findWithDefault IntMap.empty distReps :: Map.Map (Word,Word) Count -> [(Int, (Map.Map DType (WordMap Count)))] distReps bidict = let bigrams = Map.toList bidict for = indexBy id bigrams fol = indexBy swap bigrams ws = map (fst . fst) bigrams r = flip map ws $ \w -> (w,Map.fromList [ (Fol , find w fol) , (For , find w for) , (Sol , sox for fol w ) , (Sor , sox fol for w ) ]) in trace (show $ r == r) r sox :: WordMap (WordMap Count) -> WordMap (WordMap Count) -> Word -> WordMap Count sox fox foy w = let xs = IntMap.keys (find w fox) f !z x = let xv = find x foy in xv == xv `seq` z `plus` xv in foldl' f IntMap.empty xs swap (!a,!b) = (b,a) readBigrams = fmap (map (\ [w,w',c] -> ((w,w'),read c)) . map words . lines ) getContents main = do [f] <- getArgs bigrams <- readBigrams let (bigrams',as) = flip runState empty (atomize bigrams) dr = distReps (Map.fromList bigrams') write f dr hPutStrLn stderr "Done this" write (f++".atom") as --write f d = writeFile f (show d) write f d = BS.writeFile f (Binary.encode d) atomize xs = mapM f xs where f ((w,w'),!i) = do !i_w <- toAtom w' !i_w' <- toAtom w let r = ((i_w,i_w'),i) r == r `seq` return r -- Grzegorz

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TBC: Testing By Convention
by Peter Gammie 27 Jul '09

27 Jul '09

Hello, Mark and I would like to announce our test harness, which has features complementary to existing harnesses. TBC provides two main features: - It attempts to compile and run all tests, even if some do not compile or run. - Aspiring to the write-it-once principle, tests following conventions require a lot less boilerplate. It is at an embryonic stage of development, and would greatly benefit from feedback and/or patches. :-) Hackage: http://hackage.haskell.org/package/TBC git: http://github.com/peteg/TBC/ Enjoy! cheers peter -- http://peteg.org/

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Is a bug?
by Linker 26 Jul '09

26 Jul '09

Hi,All! I find out that diff between GHC and Hugs: GHC: *Prelude> sqrt 3+4+9 14.732050807568877 Prelude> sqrt 16 4.0 Prelude> sqrt $3+4+9 14.732050807568877* Hugs:*Hugs> sqrt $ 3+4+9* *4.0* *Hugs> sqrt 3+4+9* *14.7320508075689* Which one is right? Thanks. -- Regards, Linker Lin linker.m.lin(a)gmail.com

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