October 2015 - Beginners

Fwd: Space consumption and strictness in strict map
by John Lusk 24 Oct '15

24 Oct '15

---------- Forwarded message ---------- From: John Lusk <johnlusk4(a)gmail.com> Date: Sat, Oct 24, 2015 at 3:25 PM Subject: Space consumption and strictness in strict map To: beginners(a)haskell.org Hey, I have a problem I "solved" by sprinkling !s around like pixie dust, but I'd like to know what's going on. I'll attach my program, in all its messy glory, but the whole ball of wax is at https://github.com/JohnL4/PassphraseGenerator. It takes as input one of the 2012 Google ngram (specifically, 1-gram) raw data files. The expected input format is documented in several places. (For testing, I took the first million lines.) The bang that worked is on the 3rd argument of 'wordCount', the map. I tried commenting out the "(Map.insertWith (+) ngram matchCount aMap)" part (and just returning the input map). When I did that, boy was space consumption small, so that line is part of the problem. So, what's going on without the bang? I guess I'm just building up a bunch of thunks w/that Map.insertWith call, but what kind of thunks? Unevaluated calls to splitOn and (!!) and read and (+) and lines? I thought a strict Map would avoid that. I guess WHNF isn't enough? Looking at https://wiki.haskell.org/Weak_head_normal_form, I don't *think* I have any constructors in there, but profiling with -hd shows me that (:) is the most frequently occurring closure. Where is *that* coming from? Is the occurrence of (:) so high because it's either a built-in function applied to too few arguments or a lambda abstraction? Which one? (Also, is "lambda abstraction" the same as "lambda expresson"?) And then... what does that bang on the map argument do? Does it force evaluation of the passed argument all the way down to primitives, so that we truly get a data structure containing only strings and ints (and no thunks)? What does that do to time complexity? Does it have to traverse the entire map looking for thunks, even though I only added one at some random location in the map? Is there a better way? I guess I need to force strictness somewhere else, but I'm not sure how. I tried using seq (a little half-heartedly), and ($!), but I guess I did it wrong and only wound up with more thunks to seq and id ($!), right? So, I wound up with lazy strictness? Thanks for any help. John.

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Ignore in import
by Mike Houghton 24 Oct '15

24 Oct '15

Hi, I’m reading Typeclassopdedia and an exercise is "A good exercise is to implement Functor instances for Either e, ((,) e), and ((->) e).” so I have instance Functor ((,) a) where fmap = undefined which gives compiler error Duplicate instance declarations: instance Functor ((,) a) -- Defined at /Users/mike/haskell/FingerTrees/M.hs:65:10 instance Functor ((,) a) -- Defined in ‘GHC.Base’ so I added import GHC.Base hiding ((,)) but I still get the error. Where am I going wrong? Many thanks Mike

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Can't cast an IO String to s Astring
by chanti houda 23 Oct '15

23 Oct '15

Hello, I'm writing a Haskell code which consists to read a text file, parse it and thansform the parsing result on a specific language.I have a function affiche which takes a data type Ctrl and returns a String. This is the transformation funtion.I have also anothe function parsctrl, which parse the contents of a text file ("ctrl.txt") and after looks for a specific value ("A01") in the parse result (function cherchectrl). I need to use the result of the parsectrl function in another function fc. The code is composed of three functions parsectrl = do f <- readFile "ctrl.txt" let Right r = parse parseCtrl " " f let rez =cherchectrl ( r) "A01" return (rez) fc[] =[] fc((door,[(all,v1),(alt,v2),(lint,v3),(w,v4),(r,v5),(loc,v6),( etat,v7),(ruin,v8)]):ls ) = ("&OUV ID='"++door ++"', ALLEGE="++show((moi v1)/1000)++", LINTEAU="++show((moi v3)/1000)++", LARGEUR="++show((moi v4)/1000)++", COEF=0.7, ALT="++show((moi v2)/1000)++", LOCIDS='"++v6++"', CTRLID='"++ v7++"', CTRLID_RUIN='"++ v8++" /" ++"\n" ++"&CTRL ID='"++v7++"', " ++ "ON_INI=.FALSE., DURATION=0 / \n"++"&CTRL ID='"++v8++"', LOCID='"++ ((parses("'"++v6++"'"))!!0) ++affiche(parsectrl)++ " / \n\n" )++fc(ls) The parsectrl returns an IO String, but the function affiche needs a String as input and even when I tried to adapt the affiche function to take an IO String -> String I can't.the result of fc must be a String too. The IO String comes from the parsectrl function. Can you help me to solve this problem: how can I transform an IO String to a String. Thank you by advance.

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Why can't I return a partially applied function in my example?
by Umair Saeed 23 Oct '15

23 Oct '15

Hello all, I'm learning Haskell, and started to go through a set of intermediate exercises ( https://www.fpcomplete.com/user/DanBurton/20-intermediate-exercises) I am a bit puzzled about one of the exercises, and hope someone can help me understand why one of my solutions doesn't work. We have a typeclass, Misty (only the relevant banana function shown) as: class Misty m where banana :: (a -> m b) -> m a -> m b The exercise asks to implement this typeclass for the type ‘((->) t)’. I started off by filling in the relevant types, and I get: banana :: (a -> ((->) t b) ) -> ((->) t a) -> ((->) t b) banana :: (a -> (t -> b)) -> (t -> a) -> (t -> b) Based on this, I decided to implement banana as: banana f g = (\x -> f (g x)) Here is my thought process: - The type of f is ‘(a -> t -> b)’, and the type of g is ‘(t -> a)’ - g converts an argument of type ‘t’ into a result of type ‘a’. - I then pass the result of ‘(g x)’ (which is of type ‘a’) as an argument to ‘f’. - At this point, ‘f’ would be partially applied, and I *expect* to get a result of type ‘(t -> b)’ However, when I try to build my solution, I get the following error (code is in a file called intermediate-help.hs): [1 of 1] Compiling Main ( intermediate-help.hs, interpreted ) intermediate-help.hs:7:25: Couldn't match expected type ‘b’ with actual type ‘t -> b’ ‘b’ is a rigid type variable bound by the type signature for banana :: (a -> t -> b) -> (t -> a) -> t -> b at intermediate-help.hs:7:5 Relevant bindings include x :: t (bound at intermediate-help.hs:7:20) g :: t -> a (bound at intermediate-help.hs:7:14) f :: a -> t -> b (bound at intermediate-help.hs:7:12) banana :: (a -> t -> b) -> (t -> a) -> t -> b (bound at intermediate-help.hs:7:5) In the expression: f (g x) In the expression: (\ x -> f (g x)) Failed, modules loaded: none. So here's my confusion: The compiler is complaining that it cannot match expected type ‘b’ with actual type ‘t -> b’. However, as I reasoned above, when I wrote this code, I expected to get type ‘t -> b’. Clearly, my thought process has a hole, and I need help/advice from more experienced Haskellers to identify what I am missing. Thank you for any help, ~Umair

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duplicate monoids...
by Mike Houghton 20 Oct '15

20 Oct '15

Hi, I’m looking at a blog post on Monoids and finger trees at http://apfelmus.nfshost.com/articles/monoid-fingertree.html <http://apfelmus.nfshost.com/articles/monoid-fingertree.html> and would appreciate a bit of advice I have type Size = Int type Priority = Int instance Monoid Size where mempty = 0 mappend = (+) instance Monoid Priority where mempty = maxBound mappend = min and I get compiler error Duplicate instance declarations: instance Monoid Size -- Defined at /Users/mike/haskell/FingerTrees/Ftree.hs:60:10 instance Monoid Priority -- Defined at /Users/mike/haskell/FingerTrees/Ftree.hs:64:10 Which I can sort of understand as Size and Priority are both Int but on the other hand, internally, the monoids are different. Is this genuinely incorrect code or is there a language extension to get around this? Thanks Mike

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Why can't I print an IO Integer?
by Dan Stromberg 20 Oct '15

20 Oct '15

Here's a small program that replicates the compilation issue I'm seeing: import qualified System.Posix.Files get_size :: String -> IO Integer get_size filename = do file_status <- System.Posix.Files.getFileStatus filename let file_size = System.Posix.Files.fileSize file_status let integer_file_size = fromIntegral file_size return integer_file_size main :: IO () main = do let filenames = ["/etc/services"] let sizes = map get_size filenames mapM_ print sizes The compilation error I get is: ghc -Wall --make -o stat2 stat2.hs [1 of 1] Compiling Main ( stat2.hs, stat2.o ) stat2.hs:15:11: No instance for (Show (IO Integer)) arising from a use of `print' Possible fix: add an instance declaration for (Show (IO Integer)) In the first argument of `mapM_', namely `print' In a stmt of a 'do' block: mapM_ print sizes In the expression: do { let filenames = ...; let sizes = map get_size filenames; mapM_ print sizes } make: *** [stat2] Error 1 I've googled quite a bit, and guessed quite a bit, and added type declarations some, but I'm still not converging on a solution. Why can't I print an IO Integer? Thanks! -- Dan Stromberg

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ghci: inconsistent return values for succ
by Frothy Bits 19 Oct '15

19 Oct '15

Greetings, Absolutely brand new to Haskell. Taking ghci v7.10.2 out for a spin, and I find I get inconsistent return values for succ n: ghci> succ 3.14 4.1400000000000001 for example instead of the expected 4.14 succ 2.14 and 4.14 give the expected results. but succ 2.14 returns 2.1399999999999997. This anomalous behavior runs through the range of n.nn; in the n.01 range, for example, 16.01 and 63.01 return wonky results per above. I tested this on Windows and Linux (various flavors) and I get the same results there and in the interactive test code space on haskell.org. I'm not familiar enough with the language, yet, to go debugging this on my own, but it would seem to be at least a problem with how succ is implemented, if not how values are handled in general.....which could potentially be bad if you were trying to do anything requiring precise calculations....

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State Monad: how to use other Stateful computation on sub-state inside
by martin 17 Oct '15

17 Oct '15

Hello all, I found myself writing this piece of code: -- | Create and add a new 'Item' to the system sysAddItem :: ItmLabel -> ItmVolume -> ItmCapacity -> Position -> Instant -> State System ItmId sysAddItem lbl vol cap pos t = do sys <- get id <- sysNextId :: State System Id itms' <- return $ execState (itmAdd' (Itm id lbl vol cap pos t)) (sysItems sys) --** modify (\sys -> sys{sysItems=itms'}) return id In the lonely line ** in the middle I use itmAdd' :: Item -> State ItemDb (), where the ItemDb is part of the 'System' and can be extracted via sysItems. I believe this code is correct, but I don't like it. The expression to the right of <- must have the type State System ItemDb But ItemAdd' has the type Item -> State ItemDb () So I need to transform (Item -> State ItemDb ()) to (State System ItemDb). There is no question that I have to pass the Item, but the transformation of the States is quite noisy. Is there a better way to make this transformation, given a function System->ItemDb (i.e. sysItems)?

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Reader and ReaderT
by Grzegorz Balcerek 16 Oct '15

16 Oct '15

Hi, I have the following program: import Control.Monad.Reader first :: Reader [String] String first = do strings <- ask return $ if (null strings) then "empty" else head strings printFirst :: ReaderT [String] IO () printFirst = do strings <- ask let theFirstString = runReader first strings liftIO $ putStrLn theFirstString main = runReaderT printFirst ["first","second"] It compiles and works. However, in the printFirst function I am explicitly using ask and I am calling runReader. Can I somehow avoid doing that? The following version of the printFirst function does not compile. printFirst :: ReaderT [String] IO () printFirst = do theFirstString <- first liftIO $ putStrLn theFirstString Program2.hs:11:21: Couldn't match type `Data.Functor.Identity.Identity' with `IO' Expected type: ReaderT [String] IO String Actual type: Reader [String] String In a stmt of a 'do' block: theFirstString <- first In the expression: do { theFirstString <- first; liftIO $ putStrLn theFirstString } Can I somehow call first without using ask and runReader ? Regards Grzegorz Balcerek

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Can't install Haskell platform on Mac OS X El Capitan
by Daniel Hegyi 12 Oct '15

12 Oct '15

Hi, I’m trying to install Haskell on Mac I have OSX El Capitan so I downloaded from https://www.haskell.org/platform/ the Download (64 bit) (10.11). The installer runs, however, when I type into terminal ghci I get ghci -bash: ghci: command not found “which ghci” it just doesn’t return anything. Thanks, Daniel

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