August 2010 - Haskell-Cafe

On to applicative
by michael rice 26 Aug '10

26 Aug '10

From: http://en.wikibooks.org/wiki/Haskell/Applicative_Functors ============================= import Control.Applicative f :: (a -> b -> c) fmap :: Functor f => (d -> e) -> f d -> f e fmap f :: Functor f => f a -> f (b -> c) -- Identify d with a, and e with (b -> c) sumsqr :: Int -> Int -> Int -- my f sumsqr i j = i*i+j*j ============================= I'm trying to understand how the above works but... [michael@localhost ~]$ ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :l bozo.hs [1 of 1] Compiling Main ( bozo.hs, interpreted ) bozo.hs:5:0: Invalid type signature Failed, modules loaded: none. Prelude> Michael

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Chez What
by Jeff Rubard 26 Aug '10

26 Aug '10

Haskell CURRY? Curried potatoes? The lambda calculus? Historical actuality? SI!

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Ordering TypeRep from Data.Typeable.
by Serguey Zefirov 26 Aug '10

26 Aug '10

I think, that TypeRep type from Data.Typeable needs Ord class instance. It is unnecessary, but is handy when needed. My use case follows. I try to create graph whose node and arc labels are differently typed. So I can add Int node, Float node and link them by Conversion arc. Right now I am quite good with GADTs that hide values of different types. But when I thinking about querying that structure, I start to want ordered TypeRep. I then can collect different arcs and nodes into different sets and speed up query processing dramatically.

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non-blocking recv from UDP socket
by Vo Minh Thu 26 Aug '10

26 Aug '10

Hi, Is is possible to get Network.Socket.ByteString.recv to be non-blocking (i.e. return directly even if no data is available) ? I have tried ti use setSocketOption sock NoDelay 1 but then I get the following error: setSocketOption: unsupported operation (Protocol not available) Here is the code if it may help (I'm on linux btw). sock <- socket AF_INET Datagram 17 -- 17 is IPPROTO_UDP addr <- inet_addr "127.0.0.1" bindSocket sock (SockAddrInet 44004 addr) setSocketOption sock NoDelay 1 -- <-- doesn't seem to work addr <- inet_addr "127.0.0.1" forever $ do sent <- sendTo sock "hello" (SockAddrInet 44005 addr) dat <- recv sock 5 -- <-- I'd like it to be non-blocking B.putStrLn dat threadDelay 1000000 -- 1 second Thanks, Thu

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foreign function interface "Invalid type signature"
by Ken Takusagawa 26 Aug '10

26 Aug '10

What am I doing wrong? module Ffi2 where { import Foreign.C.Types; foo :: CInt -> CInt; foo x = x; foreign export ccall foo :: CInt -> CInt; } $ ghc -c Ffi2.hs Ffi2.hs:8:0: Invalid type signature --ken

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Re: Re : Re: Re : Error importing
by Luc TAESCH 26 Aug '10

26 Aug '10

btw, this could be helpful, if you have some time: http://en.wikipedia.org/wiki/Cabal_(software) http://www.haskell.org/cabal/ in there you maay find : http://www.haskell.org/cabal/FAQ.html in which you can see ( hey !): Hidden packages (a)<http://www.haskell.org/cabal/FAQ.html#hidden-packages-a> What is this hidden package? You're writing your own package and you get: Could not find module `Data.Map': it is a member of package containers-0.1.0.0, which is hidden. Hidden packages (b)<http://www.haskell.org/cabal/FAQ.html#hidden-packages-b> You're building some other package and you get: Could not find module `Data.Map': it is a member of package containers-0.1.0.0, which is hidden. this is insteresting but maybe too advaced for now, may side track you. however may give you the big picture http://www.haskell.org/haskellwiki/How_to_write_a_Haskell_program besides, Howard, leksah is a small audience list, you may find more support on the cafe list, as your question as less leksah specific, but more on the "haskell bootstrapping procees" Ill copy you here, in case some good sould know some fast track ( btw , do you know about rwh : http://book.realworldhaskell.org/read/ especially http://book.realworldhaskell.org/read/installing-ghc-and-haskell-libraries.… ) 2010/8/25 HowardBrazee <howard(a)brazee.net> > Boy, am I dense. > > I see two "depends" lines in Reading.cabal: > > pkgconfig-depends: > & > build-depends: base -any > > I uncommented out > import qualified Data.Map as M > > and looked at Lekah's message: > src\Main.hs:19:17: > Could not find module `Data.Map': > It is a member of the hidden package `containers-0.3.0.0'. > Perhaps you need to add `containers' to the build-depends in > your .cabal file. > Use -v to see a list of the files searched for. > > So I put "containers" after one of the depends: > build-depends: containers base -any > > Leksah said: > Setup: .\Reading.cabal has been changed, please re-configure. > ========================================= > Setup: Reading.cabal:9: Parse of field 'build-depends' failed. > ========================================= > Can't activate package C:\Documents and Settings\brazee\My Documents > \Code\Windows\Reading\Reading.cabal: 9: user error (Parse of field > 'build-depends' failed.) > Can't read package file > > So I undid that change and did this instead: > pkgconfig-depends: containers > > I got the following: > Can't read package file > Setup: .\Reading.cabal has been changed, please re-configure. > ========================================= > Configuring Reading-0.0.1... > Setup: pkg-config version >=0.9.0 is required but it could not be > found. > ========================================= > The job now runs with these unused imports. > > I never found "build-depends" in my cabal file. > > I edited this with ultra-edit as I know how to get to it. > > I'm still clueless about what I did, despite being told by two > different people what to do and not being able to follow the > directions, but doing trial and error instead. Even after I > finished, I don't understand the directions. >

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How to generate dependend values with QuickCheck
by Jürgen Nicklisch-Franken 26 Aug '10

26 Aug '10

I want to generate values, so that i have some arbitrary object, which has a certain set of signals, and each set of signals has a certain set of sensor value pairs, were the values are arbitrary. However to demonstrate my problem I have an easy example, were I want to generate an instance of data QCExample = QCExample Int Int deriving Show and the second value should always be the double of the first. So I tried this: instance Arbitrary QCExample where arbitrary = let i1 = arbitrary i2 = fmap (* 2) i1 in liftM2 QCExample i1 i2 but showing some of the generated test cases in ghci does not give me what I expected: let gen :: Gen (QCExample) = arbitrary Test.QuickCheck.Gen.sample gen > QCExample (-2) 0 > QCExample (-4) (-6) > QCExample 3 30 > ... I know that I can filter, but this would be to inefficient. Jürgen

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Low level problem with Data.Text.IO
by Pieter Laeremans 25 Aug '10

25 Aug '10

Hello, I have a strange (low level) problem with the Data.Text library. Running the simple program below on a certain text file causes a low level error. runghc ./ReadFiles.hs testfile ghc(16402,0xb0103000) malloc: *** error for object 0x2501710: pointer being freed was not allocated *** set a breakpoint in malloc_error_break to debug ghc(16402,0xb0103000) malloc: *** error for object 0x2501710: pointer being freed was not allocated *** set a breakpoint in malloc_error_break to debug ReadFiles.hs: testfile: hGetContents: invalid argument (Illegal byte sequence) module Main where import qualified Data.Text.IO as TI import qualified Data.Text as T import System main = do args <- getArgs let fileName:_ = args txt <- TI.readFile fileName putStrLn $ show txt Unfortunately I can 't post the specific data file. But according to the file program it is a text file with : Non-ISO extended-ASCII text, with very long lines, with CRLF, LF line terminators encoding. How can I debug this problem ? What would you guys do ? Trying gdb ? thanks in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

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ANN: Bugfix release for timezone-olson
by Yitzchak Gale 25 Aug '10

25 Aug '10

Version 0.1.1 of the timezone-olson package is now available on Hackage. http://hackage.haskell.org/package/timezone-olson The timezone-olson package provides a parser and renderer for binary Olson timezone files whose format is specified by the tzfile(5) man page on Unix-like systems. These files are often found in /usr/share/zoneinfo on Unix-like systems. Functions are provided for converting the parsed data into TimeZoneSeries objects from the timezone-series package. Now that "Version 2 format" Olson files are beginning to appear in the wild, we can see the full details of how the specification in the tzfile(5) man page is applied in real life. This release fixes several bugs related to parsing and rendering Version 2 format Olson files, as well as a few other small issues. Updating is recommended. A new version of the timezone-series package, also version 0.1.1, is also now available on Hackage. http://hackage.haskell.org/package/timezone-olson The timezone-series package endows Data.Time, from the time package, with several data types and functions for enhanced processing of timezones. Among them is TimeZoneSeries,which represents all of the clock settings that occur in a time zone, and functions to convert between UTCTime and LocalTime without knowing in advance what clock setting is in effect at the given moment. This release of timezone-series only adds a few type signatures and more documentation for isValidLocalTime and isRedundantLocalTime and friends. Aristid Breitkreuz provided a proof for the basic fact about time change gaps and overlaps that is used to implement them. The proof can be seen here: http://projects.haskell.org/time-ng/gaps_and_overlaps.html Many thanks to Aristid who, in addition to providing the above proof, assisted in finding bugs and preparing for the release of these two updates. Regards, Yitz

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Type families - how to resolve ambiguities?
by DavidA 25 Aug '10

25 Aug '10

Hi, The code below defines a type synonym family: {-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-} {-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-} data Vect k b = V [(b,k)] deriving (Eq,Show) data TensorBasis a b = T a b deriving (Eq, Ord, Show) type family Tensor k u v :: * type instance Tensor k (Vect k a) (Vect k b) = Vect k (TensorBasis a b) class Algebra k v where -- "v is a k-algebra" unit :: k -> v mult :: Tensor k v v -> v instance Algebra Integer (Vect Integer [Int]) where unit 0 = V [] unit x = V [([],x)] mult (V ts) = V [(g++h,x) | (T g h, x) <- ts] Naively I think of the type instance declaration as saying that the two types are synonyms, so I should be able to use one in a context where the other is expected. However, when I try to use the code, I don't seem to be able to get the type inferencer to recognise an object as being of both "types" at the same time. For example: *> mult $ (V [(T [1] [2],3)] :: Vect Integer (TensorBasis [Int] [Int])) <interactive>:1:8: Couldn't match expected type `Tensor k v v' against inferred type `Vect Integer (TensorBasis [Int] [Int])' NB: `Tensor' is a type function, and may not be injective In the second argument of `($)', namely `(V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int]))' In the expression: mult $ (V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int])) In the definition of `it': it = mult $ (V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int])) *> mult $ (V [(T [1] [2],3)] :: Tensor Integer (Vect Integer [Int]) (Vect Integer [Int])) <interactive>:1:8: Couldn't match expected type `Tensor k v v' against inferred type `Vect Integer (TensorBasis [Int] [Int])' NB: `Tensor' is a type function, and may not be injective In the second argument of `($)', namely `(V [(T [1] [2], 3)] :: Tensor Integer (Vect Integer [Int]) (Vect Integer [Int]))' In the expression: mult $ (V [(T [1] [2], 3)] :: Tensor Integer (Vect Integer [Int]) (Vect Integer [Int])) In the definition of `it': it = mult $ (V [(T [1] [2], 3)] :: Tensor Integer (Vect Integer [Int]) (Vect Integer [Int])) Are type families the right mechanism to express what I'm trying to express? If so, what am I doing wrong, and how do I fix it?

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