- Beginners - Haskell.org

Trouble Understanding How to Start
by Keeley Abbott 18 Oct '14

18 Oct '14

Hi All, So, I am taking a Functional Programming class this fall, and I have struggling to understand how to build types and functions correctly. For some reason it just isn’t clicking. I have the following module I am working on, and I need some guidance. — Variable names type Name = String — An environment for looking up the value of a variable type Env = Name -> Maybe Int — An empty environment empty :: Env empty env = Nothing — Set a variable to a value in the environment if it doesn’t already exist set :: Name -> Int -> Env -> Env set x i e = \y -> if x == y then (Just i) else e y — Lookup the value of a variable in the environment get :: Name -> Env -> Int get x env = ?? (I previously had “type Env = Name -> Int”, so this was working with env x, but we had to change empty to be something other than just a runtime error, which is why I made Env = Name -> Maybe Int) —Remove a variable from the environment unset :: Name -> Env -> Env unset x e = \y -> if x == y then empty x else env y At this point I don’t know how to fix get, so I get an int to display (it keeps telling me it can’t match the type ‘Maybe Int’ with the expected type ‘Int’). And I don’t know where to go from there to create getOr, setAll, and mapEnv functions, because I am just not understanding what I am doing. For the most part I have been doing simpler things that I could just mess with the functions until they work, but even at that I’m not getting HOW they work… Any assistance or instruction on how I can GET what I am trying to do would be greatly appreciated. Thanks, Kallarhynn

3 2

Re: [Haskell-beginners] How to override a type class instance ?
by Johannes Joachim Engels 18 Oct '14

18 Oct '14

> You usually wrap the data type in a newtype and use > GeneralizedNewtypeDeriving to derive the classes you want to keep. Then > write custom instances for the type classes you want to override. thank you very much, this worked! Best regards Johannes

1 0

How to override a type class instance ?
by Johannes Joachim Engels 18 Oct '14

18 Oct '14

2 1

QuickCheck Crazy
by Dimitri DeFigueiredo 17 Oct '14

17 Oct '14

Hello Everyone, I am trying to do homework #3 of Stephanie Weirich's programming class. It is about QuickCheck. http://www.seas.upenn.edu/~cis552/current/hw/hw03/index.html However, quickCheck is magically coming up with the wrong answer! I am loading a very simple file (distilled from the homework). Here it is: ~~~~ import Test.QuickCheck ------------------------------------------------------------------------------ prop_const :: Eq a => (a -> a -> a) -> a -> a -> Bool prop_const const' a b = const' a b == a const_bug :: a -> b -> b const_bug _ b = b -- Oops: this returns the *second* argument, not the first. --main = quickCheck (prop_const const) ~~~~ Now, I just wanted to check that 'prop_const' holds for the 'const' function, but fails for 'const_bug' So, I fired up GHCi, but here's what happens: econ1gw-131-21-dhcp:week3 dimitri$ ghci GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :l weird-quickcheck.hs [1 of 1] Compiling Main ( weird-quickcheck.hs, interpreted ) weird-quickcheck.hs:22:1: Warning: The import of `Test.QuickCheck' is redundant except perhaps to import instances from `Test.QuickCheck' To import instances alone, use: import Test.QuickCheck() Ok, modules loaded: Main. *Main> const 1 2 1 *Main> const_bug 1 2 Loading package array-0.4.0.1 ... linking ... done. Loading package deepseq-1.3.0.1 ... linking ... done. Loading package old-locale-1.0.0.5 ... linking ... done. Loading package time-1.4.0.1 ... linking ... done. Loading package random-1.0.1.1 ... linking ... done. Loading package primitive-0.5.0.1 ... linking ... done. Loading package pretty-1.1.1.0 ... linking ... done. Loading package containers-0.5.0.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package tf-random-0.5 ... linking ... done. Loading package transformers-0.3.0.0 ... linking ... done. Loading package QuickCheck-2.7.6 ... linking ... done. 2 *Main> :t prop_const prop_const :: Eq a => (a -> a -> a) -> a -> a -> Bool *Main> quickCheck (prop_const const) +++ OK, passed 100 tests. *Main> quickCheck (prop_const const_bug) +++ OK, passed 100 tests. *Main> quickCheck (prop_const const_bug :: Char -> Char -> Bool) *** Failed! Falsifiable (after 1 test and 2 shrinks): 'a' 'b' *Main> This makes no sense to me! First, I have no idea how quickCheck is able to run any tests at all as the type of 'const' is: *Main> :t const const :: a -> b -> a In other words, it's a polymorphic type. Second, why does GHCi load so many files when I try to evaluate 'const_bug'? Finally and most puzzling, why does quickCheck give out the wrong result if I don't specify the type when testing 'const_bug'? That's really scary. It means that quickCheck can easily fool me if I forget to specify the necessary types. (GHC complains if I uncommented the '--main' line, so I can only do this in GHCi.) I am totally lost here, any pointers would be much appreciated. Thanks, Dimitri

3 6

"Delegating" class instance definition
by Nicolaas du Preez 12 Oct '14

12 Oct '14

data Cycle = Cycle { name :: String, size :: Double } deriving (Eq, Show) instance Ord Cycle where (<) a b = (<) (size a) (size b) -- problem statement! … How can I specify that, on the right-hand side of the problem statement, I refer to (<) :: Double -> Double -> Bool instead of (<) :: Cycle -> Cycle -> Bool? Is there another way to state that "Cycle is an instance of Ord based on size"?

3 5

Connecting telnet connection to a wx widget
by Tilmann 04 Oct '14

04 Oct '14

Hi, I am stuck at this problem. I want to read from a telnet connection and display the result in a wxHaskell widget (textCtrl). I got the widget and I got the connection piped to stdout, but I don´t know how to connect the two. Any help is greatly appreciated! Versions: conduit 1.2.0.2, wx 0.91.0.0 Thanks a lot, Tilmann This is the code I got so far: module Main where import Control.Concurrent (forkIO, killThread) import Control.Monad.IO.Class (MonadIO, liftIO) import Control.Monad.Trans.Resource import Data.Conduit import Data.Conduit.Binary import Network (connectTo, PortID (..)) import System.IO import Graphics.UI.WX main = start gui gui = do f <- frame [] t <- textCtrl f [] e <- entry f [on click := (onClick t "...")] set f [layout := boxed "console" (grid 5 5 [[floatLeft $ expand $ hstretch $ widget t] ,[expand $ hstretch $ widget e]])] onClick t val pt = set t [ text :~ \x -> val] -- http://www.mega-nerd.com/erikd/Blog/CodeHacking/Haskell/telnet-conduit.html telnet :: String -> Int -> IO () telnet host port = runResourceT $ do (releaseSock, hsock) <- allocate (connectTo host $ PortNumber $ fromIntegral port) hClose (releaseThread, _) <- allocate (forkIO $ runResourceT $ sourceHandle stdin $$ sinkHandle hsock) killThread sourceHandle hsock $$ sinkHandle stdout

2 1

Aeson: parsing json with 'data' field
by Miro Karpis 04 Oct '14

04 Oct '14

Hi, please can you help me with this.......I have a json file which contains a field with name "data". Problem is that I can not create a data type with "data", (or can I)? How else can I handle this? I know I can convert all json to Object and then search for the field....but I was hoping for some friendly/easier option. json example: { "data" : { "foo" : "bar" } } below definition returns: parse error on input ‘data’ data Foo = Foo { data :: String } Cheers, Miro

2 2

Error message: "Couldn't match expected type"
by S. H. Aegis 02 Oct '14

02 Oct '14

Hello. I'm learning Haskell on OSX. How can I fix this error message? (Code from The Haskell School of Expression, P40) Sorry for border you... Sok. import Graphics.SOE main() = runGraphics ( do w <- openWindow "My First Graphics Program" (300, 300) drawInWindow w (text(100, 200) "HelloFraphicsWorld") spaceClose w ) spaceClose :: Window -> IO() spaceClose w = do k <- getKey w if k == ' ' then closeWindow w else spaceClose w [1 of 1] Compiling Main ( firstGraphics.hs, firstGraphics.o ) firstGraphics.hs:3:1: Couldn't match expected type `IO t0' with actual type `() -> IO ()' In the expression: main When checking the type of the function `main'

2 2

Different behaviors between GHC 7.6.3 and 7.8.3
by Emilio De Camargo Francesquini 01 Oct '14

01 Oct '14

Hello everyone, I'm quite intrigued about these different behaviors between GHCs 7.6.3 and 7.8.3: GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> let x = 3 + 4 Prelude> :print x x = (_t1::Integer) Prelude> :force x x = 7 Prelude> print _t1 7 Prelude> GHCi, version 7.8.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> let x = 3 + 4 Prelude> :print x x = (_t1::Num a => a) Prelude> :force x x = _ Prelude> print _t1 ghc: panic! (the 'impossible' happened) (GHC version 7.8.3 for x86_64-unknown-linux): tcTyVarDetails a{tv atm} [tv] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Is it really a bug or is it something I really shouldn't be doing? I've found this (fixed) bug (https://ghc.haskell.org/trac/ghc/ticket/8557) with a similar error output, but it does not seem to be the same case. Thanks Emilio

3 3

How to pass configuration to a function further down the call stack?
by Thomas Koch 30 Sep '14

30 Sep '14

Hi, I wrote my first Haskell 'project', a test suite for my personal SIEVE mail filter script using dovecots 'sieve-test' tool: https://github.com/thkoch2001/sieve-test-hs The file Network/Sieve/Test.hs might actually be interesting as a standalone library. The most important function is assertMailActions :: Mail -> Actions -> IO () 'Actions' is a type that describes the result of running a SIEVE filter over the passed mail, e.g. store in a certain folder, discard, reject, ... Somewhere the external process 'sieve-test' is called. At this point I'd like to make certain things configurable: - the path and name of the executable - the parameters for the test-sieve executable - the name of the sieve script - the place where to store the temporary mail files - whether or not to remove the temporary mail files Does anybody has an idea where I could start to look for a solution? Thank you, Thomas Koch

2 1