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Help with expressing polymorphic return type
by Marco Yuen 12 Dec '15

12 Dec '15

Hi all, {-# LANGUAGE TypeFamilies #-} data CB = CB deriving (Show) data CO = CO deriving (Show) data CBParseResult = CBParseResult deriving (Show) data COParseResult = COParseResult deriving (Show) class ParseResult a where type EntityType a :: * toEntity :: a -> EntityType a instance ParseResult CBParseResult where type EntityType CBParseResult = CB toEntity result = CB instance ParseResult COParseResult where type EntityType COParseResult = CO toEntity result = CO getParseResult :: (ParseResult r) => String -> IO r getParseResult i = do if someCond then return CBParseResult else return COParseResult where someCond = null i Couldn't match expected type ‘r’ with actual type ‘COParseResult’ ‘r’ is a rigid type variable bound by the type signature for getParseResult :: ParseResult r => String -> IO r at Parse.hs:19:19 Relevant bindings include getParseResult :: String -> IO r (bound at Parse.hs:20:1) In the first argument of ‘return’, namely ‘COParseResult’ In the expression: return COParseResult What I want to express is getParseResult to be able to return any instances of ParseResult type class. But what I understand from the error is that r has been specialized to CBParseResult, but in the next expression I'm returning COParseResult. Is my understanding correct? And, How would I express what I described? Thanks, Marco

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Re: [Haskell-beginners] How can I lift my function taking a function to a Monad?
by Simon Peter Nicholls 11 Dec '15

11 Dec '15

Hiya, That would certainly explain why I found it difficult. Here's the original definition: liftMLabel :: Monad m => a :-> b -> m a :-> m b liftMLabel l = label (liftM $ get l) (liftM2 $ set l) Are here's a sample iteration of me hacking around: liftMLens :: Monad m => a :-> b -> m a :-> m b liftMLens l = lens getter modifier where getter = liftM $ get l modifier mf mm = mf >>= \ f -> modify l f `liftM` mm I only ever seem to replicate liftM2 characteristics when trying to brute force an implementation. And the implementations get more and more brutish as time goes on! It makes me wonder if the implications of fclabels `m a :-> m b` are radically different now, bearing in mind the original code worked. The fclabels :-> type operator has convinced the compiler that `m b -> mb` is required for the actual value modifying function. Does `m a :-> m b` make sense for the latest releases of fclabels? Is it just the lifting that's a problem? Si On Thu, 10 Dec, 2015 at 3:02 PM, Daniel Bergey <bergey(a)teallabs.org> wrote: > On 2015-12-10 at 07:43, Simon Peter Nicholls <simon(a)mintsource.org> > wrote: >> So I figure I need: >> >> neededLift :: (Monad m) => ((b -> b) -> a -> a) -> (m b -> m b) -> >> m a >> -> m a > > I don't believe `neededLift` is possible. The type says that given > any > function `m b -> m b`, you can turn that function into one of type `b > -> > b` (as input to the lens).

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problem with SDL-gfx
by Martín Villagra 11 Dec '15

11 Dec '15

Hello! I hope you can help me. I'm trying to use SDL-gfx but it seems it isn't linking it correctly. When I try to run this simple file with runhaskell: import Graphics.UI.SDL.Primitives main = putStrLn "Hello" It gives: sdl.test.hs: <command line>: can't load .so/.DLL for: /home/username/.cabal/lib/i386-linux-ghc-7.10.2/SDL-gfx-0.6.0.1-6rhSP5mw8M00K9S8yASTVF/ libHSSDL-gfx-0.6.0.1-6rhSP5mw8M00K9S8yASTVF-ghc7.10.2.so (/home/username/.cabal/lib/i386-linux-ghc-7.10.2/SDL-gfx-0.6.0.1-6rhSP5mw8M00K9S8yASTVF/ libHSSDL-gfx-0.6.0.1-6rhSP5mw8M00K9S8yASTVF-ghc7.10.2.so: undefined symbol: SDL_initFramerate) Notes: * SDL works fine in all cases. * My system is i386, using Arch Linux. I tried with another computer with x86_64 with Arch Linux and it worked. * It works if I compile it with ghc and run the executable. * I installed the package using cabal install SDL-gfx with no errors. * I installed the library in the OS just in case, and double checked that it exist in /usr/lib/libSDL_gfx.so and that it contains the symbol SDL_initFramerate. * The long *.so library reported by runhaskell also constains SDL_initFramerate. * GHCi gives the same error (when executing main). * I'm trying to make a cabal package that uses this library, and cabal gave the same error within the sandbox.

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How can I lift my function taking a function to a Monad?
by Simon Peter Nicholls 10 Dec '15

10 Dec '15

Hi All, Can anyone help me lift a function that takes a function, so it can be used in a Monad? The function I am given looks like "((b -> b) -> a -> a)", and viewing the definition of liftM2, it's almost what I need: liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r liftM2 f m1 m2 = do x1 <- m1 x2 <- m2 return (f x1 x2) but I get the error: Expected type: (m b -> m b) -> m a -> m a Actual type: m (b -> b) -> m a -> m a which makes sense, as liftM2 just wraps my function arg, whereas the function itself needs to operate on monadic values. So I figure I need: neededLift :: (Monad m) => ((b -> b) -> a -> a) -> (m b -> m b) -> m a -> m a Despite a rough appreciation of how liftM2 works ("unpack" desired monadic versions of the args, apply the function, then return to put result back in the Monad), my mind keeps hazing over regarding the changes I need to make in this case. I've tried implementing neededLift by composing other functions, and also by manual Monad wrangling, but the intuitive leap remains elusive. I posted the specific problem over at SO (http://stackoverflow.com/questions/34152747/how-can-i-lift-an-fclabels-lens…) but I think the specifics may be clouding what is probably a straightforward problem. Cheers, Si

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More type errors I'm having trouble with
by Dan Stromberg 10 Dec '15

10 Dec '15

I'm continuing my now-and-then exploration of Haskell. I'm getting a new crop of type errors that I'm pulling my hair out over. The errors I'm getting are: $ make below cmd output started 2015 Tue Dec 01 04:05:17 PM PST # --make will go out and find what to build ghc -Wall --make -o dph dph.hs Split0.hs [1 of 3] Compiling Split0 ( Split0.hs, Split0.o ) [2 of 3] Compiling Md5s ( Md5s.hs, Md5s.o ) [3 of 3] Compiling Main ( dph.hs, dph.o ) dph.hs:13:13: Couldn't match type `IO' with `(,) (IO String)' Expected type: (IO String, String) Actual type: IO String In a stmt of a 'do' block: hash <- prefix_md5 filename :: IO String In the expression: do { hash <- prefix_md5 filename :: IO String; (hash, filename) } In an equation for `do_prefix_hash': do_prefix_hash filename = do { hash <- prefix_md5 filename :: IO String; (hash, filename) } dph.hs:14:6: Couldn't match type `[Char]' with `IO String' Expected type: IO String Actual type: String In the expression: hash In a stmt of a 'do' block: (hash, filename) In the expression: do { hash <- prefix_md5 filename :: IO String; (hash, filename) } dph.hs:24:23: Couldn't match type `[]' with `IO' Expected type: IO (IO String, String) Actual type: [(IO String, String)] In a stmt of a 'do' block: io_hash_tuples <- map do_prefix_hash filenames :: [(IO String, String)] In the expression: do { buffer <- (hGetContents stdin) :: IO String; let filenames = ...; io_hash_tuples <- map do_prefix_hash filenames :: [(IO String, String)]; hash_tuples <- sequence io_hash_tuples :: [(String, String)]; .... } In an equation for `main': main = do { buffer <- (hGetContents stdin) :: IO String; let filenames = ...; io_hash_tuples <- map do_prefix_hash filenames :: [(IO String, String)]; .... } dph.hs:25:20: Couldn't match type `[a0]' with `(String, String)' Expected type: [(String, String)] Actual type: [[a0]] In the return type of a call of `sequence' In a stmt of a 'do' block: hash_tuples <- sequence io_hash_tuples :: [(String, String)] In the expression: do { buffer <- (hGetContents stdin) :: IO String; let filenames = ...; io_hash_tuples <- map do_prefix_hash filenames :: [(IO String, String)]; hash_tuples <- sequence io_hash_tuples :: [(String, String)]; .... } dph.hs:25:20: Couldn't match type `[]' with `IO' Expected type: IO (String, String) Actual type: [(String, String)] In a stmt of a 'do' block: hash_tuples <- sequence io_hash_tuples :: [(String, String)] In the expression: do { buffer <- (hGetContents stdin) :: IO String; let filenames = ...; io_hash_tuples <- map do_prefix_hash filenames :: [(IO String, String)]; hash_tuples <- sequence io_hash_tuples :: [(String, String)]; .... } In an equation for `main': main = do { buffer <- (hGetContents stdin) :: IO String; let filenames = ...; io_hash_tuples <- map do_prefix_hash filenames :: [(IO String, String)]; .... } dph.hs:25:29: Couldn't match expected type `[[a0]]' with actual type `(IO String, String)' In the first argument of `sequence', namely `io_hash_tuples' In a stmt of a 'do' block: hash_tuples <- sequence io_hash_tuples :: [(String, String)] In the expression: do { buffer <- (hGetContents stdin) :: IO String; let filenames = ...; io_hash_tuples <- map do_prefix_hash filenames :: [(IO String, String)]; hash_tuples <- sequence io_hash_tuples :: [(String, String)]; .... } dph.hs:26:39: Couldn't match expected type `[(String, String)]' with actual type `(String, String)' In the second argument of `map', namely `hash_tuples' In the expression: map tuple_to_string hash_tuples :: [String] In an equation for `strings': strings = map tuple_to_string hash_tuples :: [String] make: *** [dph] Error 1 above cmd output done 2015 Tue Dec 01 04:05:18 PM PST dph.hs looks like: import Md5s import Split0 import System.IO get_filenames :: String -> [String] get_filenames buffer = do -- Let's hope this doesn't give locale-related roundtrip problems. Split0.split0 '\0' buffer :: [String] do_prefix_hash :: String -> (IO String, String) do_prefix_hash filename = do hash <- Md5s.prefix_md5 filename :: (IO String) (hash, filename) tuple_to_string :: (String, String) -> String tuple_to_string (first, second) = do (show first) ++ " " ++ (show second) main :: IO () main = do buffer <- (System.IO.hGetContents System.IO.stdin) :: IO String let filenames = (get_filenames buffer) :: [String] io_hash_tuples <- map do_prefix_hash filenames :: [(IO String, String)] hash_tuples <- sequence io_hash_tuples :: [(String, String)] let strings = map tuple_to_string hash_tuples :: [String] mapM_ putStrLn strings And Md5s.hs looks like: module Md5s where import qualified System.IO import qualified Text.Printf -- cabal install cryptohash import qualified Crypto.Hash.MD5 import qualified Data.ByteString import qualified Data.ByteString.Lazy -- http://stackoverflow.com/questions/10099921/efficiently-turn-a-bytestring-i… byte_string_to_hex :: Data.ByteString.ByteString -> String byte_string_to_hex = concatMap (Text.Printf.printf "%02x") . Data.ByteString.unpack prefix_md5 :: String -> IO String prefix_md5 filename = do let prefix_length = 1024 file <- System.IO.openBinaryFile filename System.IO.ReadMode :: IO System.IO.Handle data_read <- Data.ByteString.hGet file prefix_length :: IO Data.ByteString.ByteString _ <- System.IO.hClose file let hasher = Crypto.Hash.MD5.init :: Crypto.Hash.MD5.Ctx let hasher2 = Crypto.Hash.MD5.update hasher data_read :: Crypto.Hash.MD5.Ctx let binary_digest = Crypto.Hash.MD5.finalize hasher2 :: Data.ByteString.ByteString let hex_digest = byte_string_to_hex binary_digest :: String return hex_digest :: IO String full_md5 :: String -> IO String full_md5 filename = do file <- System.IO.openBinaryFile filename System.IO.ReadMode :: IO System.IO.Handle data_read <- Data.ByteString.Lazy.hGetContents file :: IO Data.ByteString.Lazy.ByteString let binary_digest = Crypto.Hash.MD5.hashlazy data_read :: Data.ByteString.ByteString let hex_digest = byte_string_to_hex binary_digest :: String -- Does this get closed for us later? -- strace shows the file getting closed without our explicit close. -- _ <- System.IO.hClose file return hex_digest :: IO String It might be easier to view these at http://stromberg.dnsalias.org/svn/equivalence-classes/trunk/equivs3-haskell/ , so the line numbers are precise. What is the deal? Can anyone tell me what should be running through my head to fix this kind of problem on my own in the future? Thanks! -- Dan Stromberg

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Netwire and event accumulator
by Martín Villagra 10 Dec '15

10 Dec '15

Hi. I'm trying to do a simulation using Netwire 5 and SDL but as I've just started with Netwire I'm not sure if I'm doing it correctly regarding the input events. Suppose I have a game which consists in moving a single pixel around the screen. To make this game, I created these simple wires: readEvent :: Wire s e IO a SDL.Event (SDL.Event can be SDL.NoEvent or an event just like Netwire's events, so I guess they can be easily converted) inputLogic :: Wire s m SDL.Event Position (it keeps the position as an internal state and updates it according to the event) render :: Wire s e IO Position ( ) (just writes the pixel to the screen) And finally the game can be expressed using Arrow notation as: readEvent>>>inputLogic>>>render It works. But the thing here is that events like mouse movement are generated way faster than frames. To solve this, inputLogic should read all the input events until there are none and only then render the frame. I'm unable to do this using Wires. I've tried to make some kind of loop with ArrowLoop but I failed. I did came up with this solution, creating different wires: readEvents :: Wire s e IO a [SDL.Event] (it populates the list until there are no more events) inputLogic' :: Wire s m [SDL.Event] Position And the game is: readEvents>>>inputLogic'>>>render Which does the expected behaviour but it doesn't seem a good approach to me. These new wires aren't as basic as the first ones. Is this the best way to do it? Does anybody has a better idea? Any advice will be considerably appreciated. Martín

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how to get started?
by Abhijit Patel 10 Dec '15

10 Dec '15

i want to start contributing for the haskell community how and from where should i start?

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Replacing equals with equals not working
by martin 09 Dec '15

09 Dec '15

Hello all, with this code import Data.List import Data.Function data Tree a = B a (Tree a) (Tree a) | L a Char deriving Show get (B a _ _) = a get (L a _) = a tcmp = compare `on` get build :: [Tree Int] -> [Tree Int] build (t:[]) = [t] -- build t = let xs = sortBy (compare `on` get) t -- < -- build t = let xs = sortBy tcmp t in build (merge (xs!!0) (xs!!1) : drop 2 xs) The commented line -- < -- does not work, though I am just replacing equals with equals. I get No instance for (Ord b0) arising from a use of ‘compare’ The type variable ‘b0’ is ambiguous Relevant bindings include tcmp :: Tree b0 -> Tree b0 -> Ordering (bound at /home/martin/projects/haskell/exercises/99_questions/xxx.hs:10:1) Note: there are several potential instances: instance Integral a => Ord (GHC.Real.Ratio a) -- Defined in ‘GHC.Real’ instance Ord a => Ord (Control.Applicative.ZipList a) -- Defined in ‘Control.Applicative’ instance Ord Integer -- Defined in ‘integer-gmp:GHC.Integer.Type’ ...plus 24 others In the first argument of ‘on’, namely ‘compare’ In the expression: compare `on` get In an equation for ‘tcmp’: tcmp = compare `on` get Why is that so?

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how to write this function
by Dennis Raddle 09 Dec '15

09 Dec '15

In my music translation program, I have a ton of data and configuration that goes into every note computation, so I've decided to use the State monad to pass it around. Also, the generated output is just a list of MIDI events, so I think I'll use Writer to gather the output. I will be dealing with several different MIDI synthesizers, which have different specifications and manner of control. But there is enough in common that I can write a single function to do the translation, provided that I provide that function with some data that is specific to the synth, and also I need to configure it with some functions specific to the synth. Let's say my State/Writer type is called Tr. type Tr a = StateT Configuration (Writer [MidiEvent]) a This is the data I provide to StateT: data Configuration = Configuration { t1 :: SynthSpecificData , f1 :: Tr SynthSpecificComputationResult , score :: WholeMusicalScore } So I need to write translate :: Tr () computeMidiEvents = runWriter (runStateT translate theConfig) So inside 'translate' I want to call 'f1' among many other things. Let's just consider the f1 call first. I wrote this first: translate = do f <- gets f1 f That works but looks a little weird. I think this works also: translate = join (gets f1) Is that better? D

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Getting Crypto package imported
by Dustin Lee 09 Dec '15

09 Dec '15

I used cabal to install Crypto and it seemed to work fine Here is what I see at the prompt when I try to import the module $ ghci GHCi, version 7.4.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :m +Crypto <no location info>: Could not find module `Crypto' It is not a module in the current program, or in any known package. Prelude> :m +Crypto.Hash <no location info>: Could not find module `Crypto.Hash' It is not a module in the current program, or in any known package. Prelude> :m +Crypto.Hash.Data <no location info>: Could not find module `Crypto.Data' It is not a module in the current program, or in any known package. Here is my package list. $ ghc-pkg list /var/lib/ghc/package.conf.d Cabal-1.14.0 array-0.4.0.0 base-4.5.0.0 bin-package-db-0.0.0.0 binary-0.5.1.0 bytestring-0.9.2.1 containers-0.4.2.1 deepseq-1.3.0.0 directory-1.1.0.2 extensible-exceptions-0.1.1.4 filepath-1.3.0.0 ghc-7.4.1 ghc-prim-0.2.0.0 haskell2010-1.1.0.1 haskell98-2.0.0.1 hoopl-3.8.7.3 hpc-0.5.1.1 integer-gmp-0.4.0.0 old-locale-1.0.0.4 old-time-1.1.0.0 pretty-1.1.1.0 process-1.1.0.1 rts-1.0 template-haskell-2.7.0.0 time-1.4 unix-2.5.1.0 /home/dlee/.ghc/i386-linux-7.4.1/package.conf.d Crypto-4.2.5.1 HUnit-1.3.0.0 QuickCheck-2.8.1 primitive-0.6.1.0 random-1.1 tf-random-0.5 transformers-0.4.3.0 I'd appreciate tips on how to troubleshoot what's going on. -- Dustin Lee qhfgva=rot13(dustin) ᕕ( ᐛ )ᕗ

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