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can somebody explain the type of this expression?
by Ovidiu Deac 27 Oct '14

27 Oct '14

Prelude> let f x = x * 2 Prelude> :t f f :: Num a => a -> a The typeclass Num is defined like this: class Num a where (+), (-), (*) :: a -> a -> a ... ...which means that the operator (*) expects two parameters of type a and returns a value of type a. Since the definition of expr looks like this: Prelude> let f x = x * 2 ...and 2 is an Int, I would expect that the type inferred for (*) is (Int -> Int -> Int) and thus f should be (Int -> Int) Can somebody explain this? Thanks!

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Lazy state + IO not lazy?
by Jan Snajder 27 Oct '14

27 Oct '14

Dear Haskellers, I'm confused with the behaviour of a lazy state monad when combined with an IO monad. Look at this: ghci> Control.Monad.State.Lazy.evalState (head `fmap` mapM return (1:undefined)) 0 1 ghci> Control.Monad.State.Lazy.evalStateT (head `fmap` mapM return (1:undefined)) 0 *** Exception: Prelude.undefined I would have expected the second case to behave identically to the first case. Why is this not the case? Cheers, Jan

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Haskell typing question
by Cui Liqiang 26 Oct '14

26 Oct '14

Hi, I am doing an exercise in Haskell, which is converting a string like “$123.312” to double value. Below is my code: module Main where import Data.Char caluInt l = foldl1 (\acc x -> acc * 10 + x) (map digitToInt l) caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)) convert(x:xs) = let num = [e | e <- xs, e /= ','] intPart = takeWhile (/='.') num decimalPart = tail(dropWhile (/='.') num) in (caluInt intPart) + (caluDecimal decimalPart) And I got an error in this line: caluDecimal l = (foldr1 (\x acc -> acc / 10.0 + x) (map digitToInt l)), which says: No instance for (Fractional Int) arising from a use of `/' Possible fix: add an instance declaration for (Fractional Int) In the first argument of `(+)', namely `acc / 10.0' In the expression: acc / 10.0 + x In the first argument of `foldr1', namely `(\ x acc -> acc / 10.0 + x)' Why Haskell insists that 10.0 is a Int? How can I explicitly tell Haskell I want a Fractional? -- Cui Liqiang

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Use mysql-simple (OS X and dymanic library trouble)
by Christian Sperandio 26 Oct '14

26 Oct '14

Hi, I’d like to use Haskell for some processing on a MySQL database. Currently, I use Java program to do that but I’d like to change.And an important information, I’m working on OS X Maverick and I use EclipseFP. I installed mysql-simple package. I had some troubles. First, I had to download pcre to get a pcre.h I put into the /usr/include dir. After, to link the mysql-simple package I did an export LD_LIBRARY_PATH=/usr/local/mysql-5.6.11-osx10.7-x86_64/lib. Thus, the library libmysqlclient.18.dylib was found. All stuff above were done by command line. Now, I want to code a simple program: module Main where import Database.MySQL.Simple main::IO() main = undefined hello :: IO Int hello = do conn <- connect defaultConnectInfo [Only i] <- query_ conn "select 2 + 2" return i But when I try to launch a repl from EclipseFP, I’ve got this error: Loading package mysql-0.1.1.7 ... <command line>: can't load .so/.DLL for: /Users/batman/Library/Haskell/ghc-7.8.3-x86_64/lib/mysql-0.1.1.7/libHSmysql-0.1.1.7-ghc7.8.3.dylib (dlopen(/Users/batman/Library/Haskell/ghc-7.8.3-x86_64/lib/mysql-0.1.1.7/libHSmysql-0.1.1.7-ghc7.8.3.dylib, 9): Library not loaded: libmysqlclient.18.dylib Referenced from: /Users/batman/Library/Haskell/ghc-7.8.3-x86_64/lib/mysql-0.1.1.7/libHSmysql-0.1.1.7-ghc7.8.3.dylib Reason: image not found) How can I set the place of this library ? I read some stuff on the web but I don’t understand everything :( Thanks for your help. Chris

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Question on syntax
by Rohit Sharma 25 Oct '14

25 Oct '14

All, I started learning haskell very recently and have a question on the pattern matching style for lists. In the below snippet i.e. "(x:xs)" why do we went with round braces and not square? I know we are using cons that tells this is not a tuple but would it not make more sense to write something like [x:xs] instead of (x:xs), i thought round braces was used for pair/tuples? safeHead [] = Nothing safeHead (x:xs) = Just x Thanks, Rohit

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GHCi error msgs
by Julius Gedvilas 24 Oct '14

24 Oct '14

How difficult would it be to change format of GHCi compile-time error-messages of type "couldn't match expected type"?

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learning lazy evaluation
by Dimitri DeFigueiredo 21 Oct '14

21 Oct '14

I found an interesting situation while making recursive calls that I am trying to understand. I re-wrote the 'minimum' function from the prelude as such: minimum_bug :: (Ord a) => [a] -> a minimum_bug [x] = x minimum_bug (x:xs) | x > minimum_bug xs = minimum_bug xs | otherwise = x (I understand I should be using something like "minbug xs = foldr1 min xs" for this, but bare with me) The interesting thing is that the function has exponential time behavior. In a way, that makes sense as I am making two recursive calls. But I was expecting GHC to be smart enough to transform it into something like: minimum_bug' :: (Ord a) => [a] -> a minimum_bug' [x] = x minimum_bug' (x:xs) | x > y = y | otherwise = x where y = minimum_bug' xs (This one always works as expected) I understand that lazy evaluation implies memoization in some cases. When does GHC only use memoization to avoid this kind of behavior? Another interesting fact is that in my simple tests the exponential behavior does NOT occur when I pass the function a list in sorted order. main = do putStrLn "started - in order list" putStrLn $ show $ minimum_bug [1..30000] -- no problem putStrLn "started - out of order list" putStrLn $ show $ minimum_bug [27,26..1] -- don't do this with large numbers! putStrLn "done!" It's not clear to me how the sorted list is able to escape the exponential slow down. Cheers, Dimitri

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GhcDynamic and DYNAMIC_GHC_PROGRAMS
by Jeremy 20 Oct '14

20 Oct '14

mk/config.mk contains GhcDynamic and DYNAMIC_GHC_PROGRAMS variables. What is the difference between them? From the names and comments, it looks like DYNAMIC_GHC_PROGRAMS controls whether all the GHC binaries will be static or dynamically linked, but GhcDynamic only controls the GHC executable?

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Is working with Haskell easier on Linux than on a Mac?
by Jeffrey Brown 19 Oct '14

19 Oct '14

I have read that Haskell is easier to work with on Linux than on Windows. Is Haskell on Linux also easier than Haskell on OS X? I'm trying to do realtime OSC output from Haskell, because concurrency in Python is hard. Brandon Allbery on the haskell-cafe list, told me "chrt" and "sched_setscheduler" would be helpful. At the shell prompt I found that my system (OS X 10.9) does not recognize "chrt". I found a library on Hackage, "posix-realtime", that claims Mac compatibility and has a "sched_setscheduler" function, but my attempts to install it fail: sh-3.2# cabal install posix-realtime Resolving dependencies... Downloading unix-2.3.2.0... Configuring unix-2.3.2.0... Building unix-2.3.2.0... Failed to install unix-2.3.2.0 Last 10 lines of the build log ( /var/root/.cabal/logs/unix-2.3.2.0.log ): Building unix-2.3.2.0... Preprocessing library unix-2.3.2.0... dist/build/System/Posix/Signals.hs:124:10: fatal error: 'Signals.h' file not found #include "Signals.h" ^ 1 error generated. Even if I found a solution to this particular problem, I'm worried I'll keep running into similar ones, because I'm sure I'll keep trying new packages. Would this kind of work be substantially easier if I were using, say, Linux Mint?

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Functors and Applicatives; I'm just not getting it ...
by Frank 19 Oct '14

19 Oct '14

I've had a go at LYAH and CIS 194 and the Typeclassopedia and I just don't get get functors and applicatives. I'm simply not understanding them, what the various symbols/keywords mean, what they represent, how to think of them, etc. Nothing. Is there any kind of documented model I should be considering? Is there a "functors and applicatives for Dummies" I should read? Should I just give it up, not bother with Haskell and just stick to scheme/ruby/C++? -- P.S.: I prefer to be reached on BitMessage at BM-2D8txNiU7b84d2tgqvJQdgBog6A69oDAx6

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