April 2015 - Beginners

Value from Monad
by Shishir Srivastava 22 Apr '15

22 Apr '15

Hi, Going through the '<-' description in 'learnouahaskell' it says - *" If we have a Maybe String and we bind it with <- to a variable, that variable will be a String"* so if you write do str <- Just "3" str should be of type 'String'. My question is how do I return "34" by concatenating it to "4" because the following line of code fails - --- do str <- Just "3" return str++"4"::Maybe String --- or for that matter how can I return/print just the value of str from this do block ? Thanks, Shishir

3 2

integer to noise function in Haskell
by Florian Gillard 22 Apr '15

22 Apr '15

I am trying to implement a basic pelin noise function, but I have some problem with the integer to noise function used to generate deterministic noise from integer inputs. the function I am trying to implement is defined there: http://libnoise.sourceforge.net/noisegen/index.html#continuousnoise and my code so far look like this: noise2d :: (Int32, Int32) -> Double noise2d (x, y) = let m = x + y * 57 n = (shiftR m 13) ^ m j = (n * (n * n * 15731 + 789221) + 1376312589) .&. 0x7fffffff in 1.0 - (fromIntegral j / 1073741824.0) the code compile but I get the same result for any input, due to the fact that n is evaluated to 0. Is there a better way to do that?

2 2

Return function
by Shishir Srivastava 22 Apr '15

22 Apr '15

Hi, Looking at the definition of return function for a Monad, it is described as - return :: a -> m a However when I do - return (Just 3) The result is Just 3 and not Just (Just 3) If I understand the definition correctly (which doesn't seem to be the case) the return function should wrap the parameter (which in my case is 'Just 3') into the Maybe Monad and therefore should return me 'Just (Just 3)' Please can someone help explain. Thanks, Shishir Srivastava +44 (0) 750 127 5019

3 2

Re: [Haskell-beginners] filterM function
by Shishir Srivastava 22 Apr '15

22 Apr '15

Hi Mike, Thanks for your response. I was aware that 'casting' doesn't really fit in Haskell vocab but for the lack of better word used it. My question was however more towards the usage of [Bool] in the 'if' statement of the filterM function. More precisely - How does 'if [True, False] then x else y' work , because when I do this in GHCi it throws up the following error ? *>>Couldn't match expected type `Bool' with actual type `[Bool]'* Clearly the 'if' construct does not take a list of Boolean but a single Boolean value so how does filterM use it in it's implementation. Hope have made myself clear this time. Thanks, Shishir > From: Mike Meyer <mwm(a)mired.org> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Cc: > Date: Wed, 22 Apr 2015 04:15:31 -0500 > Subject: Re: [Haskell-beginners] filterM function > On Wed, Apr 22, 2015 at 3:31 AM, Shishir Srivastava < > shishir.srivastava(a)gmail.com> wrote: > >> I still don't quite understand how 'flg' being a boolean [] is used in >> the last 'if statement' of implementation because when I try to do the >> same thing outside in GHCi it fails miserably even though I am casting it >> to [Int] - >> >> -- >> return (if [True, False] then "4" else "3")::[Int] >> > > "cast" is a misnomer in Haskell. When you add a type to an expression, you > aren't changing the type of the expression like a C-style cast, but picking > out a type from the set of possible types for that expression. > > Ignoring the if part and and focusing on return, which has a type Monad m > => a -> m a. [Int] is equivalent to [] Int, so [] would be the Monad, and a > is Int. While 3 can be an Int, "3", can't. So you could do return 3 :: > [Int] or equivalently return 3 :: [] Int to get [3], you can't do return > "3" :: [Int], because "3" can't be an Int. You can do return "3" :: > [String], since "3" is a string. Just to show the range of possibilities, > you can do return 3 :: IO Float, and get back 3.0 as an IO action. The > monad in the type is IO, and 3 can be interpreted as a Float. > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > >

3 2

filterM function
by Shishir Srivastava 22 Apr '15

22 Apr '15

Hi, Continuing on from my previous question about powerset using filterM - Thanks to Alexey and Chaddaï. filterM is implemented as below - ---- filterM _ [] = return [] filterM p (x:xs) = do flg <- p x ys <- filterM p xs return (if flg then x:ys else ys) --- I still don't quite understand how 'flg' being a boolean [] is used in the last 'if statement' of implementation because when I try to do the same thing outside in GHCi it fails miserably even though I am casting it to [Int] - -- return (if [True, False] then "4" else "3")::[Int] -- Offtopic : Also if someone can explain how do I reply to individual mails/responses to my queries because I only get a digest on my gmail and there is no way to isolate and reply to individual mails either this list's page or from digest. Thanks, Shishir

3 2

Powerset function
by Shishir Srivastava 21 Apr '15

21 Apr '15

Hi, In the 'learnyouahaskell' online book the powerset function is described like this - ---- powerset :: [a] -> [[a]] powerset xs = filterM (\x -> [True, False]) xs ---- And the filterM function is defined like this filterM :: (Monad m) => (a -> m Bool) -> [a] -> m [a] -- The filterM function is said to be an extension of 'filter' function which maps the monadic predicate over the given list. Now in powerset function the predicate returns a monad [True,False] for every element of the given list. So for e.g. if the input list was only [1] the output of filterM will understandably be the cartesian product of [True, False] x [1] = [[1], []]. Extending this if the given input list contains two elements [1,2] the predicate would be mapped one by one on each of the elements and the result combined which means the output should be [[1], [], [2], []] But the powerset of [1,2] is definitely not that. Please could someone help me in getting my head around with how filterM is working in this case. Thanks, Shishir

3 2

What's the mean of >>=
by 김명신 19 Apr '15

19 Apr '15

It could be stupid question because i'm very beginner of haskell. I found strange literals when i read haskell code. Does anybody explan what the mean of >>= below ocode ? And additionally, What is mean of >> itself? putStrLn' [] = putChar '\n' putStrLn' xs - putStr' xs >>= \ x -> putChar '\n' putStr' [] = return () putStr' (x : xs) = putChar x >> putStr' n (because of i18n problem, back slash literal could be shown as '\') Thank you in advance Myung Shin Kim

2 2

IO and ghci
by Mike Houghton 19 Apr '15

19 Apr '15

Hi, I have this in a .hs file and it works fine. That is I see lines printed to the screen when the .hs file is loaded and run from ghci. main = do file <- readFile "poem.txt" mapM_ putStrLn (lines file) Experimenting at the ghci command line with let f = readFile “poem.txt” the above is fine. But doing lines f gives <interactive>:52:7: Couldn't match type ‘IO String’ with ‘[Char]’ Expected type: String Actual type: IO String In the first argument of ‘lines’, namely ‘f’ In the expression: lines f In my simplistic beginners way I think this is because when I run main = do … everything is or is within an ‘impure’ area but at ghci command line it is not and I’m mixing pure/impure functions. So my questions are 1. Is this simplistic view any where near correct? 2. How can I progress at the command line in ghci? Many thanks Mike

2 2

dropWhile returning last dropped element
by martin 18 Apr '15

18 Apr '15

Hello all Just curious: is there a library function like dropWhile, but which returns last dropped element along with the remainder of the list? If not, how can I write one - without explicit recursion - without traversing the list more than once - without traversing more of the list than necessary

4 5

StateT MyState IO a vs ReaderT (IORef MyState) IO a
by Konstantin Saveljev 18 Apr '15

18 Apr '15

Hello, I'm working on a project where I use StateT MyState IO a to keep track of the state. MyState is deeply nested and I use lens to help me more easily access and modify the state. Just about yesterday I found someone mentioning about using ReaderT (IORef MyState) IO a instead of StateT MyState IO a because we are already in IO. Can someone explain to me what are the benefits of these different approaches? What about Garbage Collection in both cases (if there is any difference at all)? Can you use lens package to access and update the state which is hidden behind IORef ? Thanks, Konstantin Saveljev

2 1