November 2015 - Beginners

manipulating Map for music application
by Dennis Raddle 07 Nov '15

07 Nov '15

I have a Haskell program that computes music compositions. A composition consists of sounds that happen at locations. A location is data type Loc. At each location is a list of chords. A chord is data type Chord. Each chord contains some chord-specific data and a list of notes. A note is data type Note. So we have data Note = Note ... data Chord = Chord ChordSpecificData [Note] type Composition = Map Loc [Chord] I would like to write a few different functions that operate over all the notes. The following function breaks out all the notes, tupling them with the associated Locs and Chords: compositionToList :: Composition -> [(Loc,Chord,Note)] The following function transforms Notes, keeping only the Just results. If a Chord gets all its notes eliminated, that Chord is removed. If a Loc has all its Chords removed, that Loc is removed from the Map. mapNotesMaybe :: (Loc -> Chord -> Note -> Maybe Note) -> Composition -> Composition Any advice for concise code appreciated. Dennis What's a concise way of doing this? Another useful function would be used for mapping Dennis

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Re: [Haskell-beginners] randmomR produces only even values
by Dominic Steinitz 07 Nov '15

07 Nov '15

> http://article.gmane.org/gmane.comp.lang.haskell.beginners/15925 I do think this is a flaw and catches many people out despite apparently being well documented. And it's something one probably wants e.g. to run multiple Markov Chain Monte Carlo simulations in parallel. Some further information here: https://github.com/haskell/random/issues/30#issuecomment-153647055 Dominic

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strict evaluation and MIDI playback
by Dennis Raddle 06 Nov '15

06 Nov '15

I wrote a program in Haskell that does real-time MIDI playback. This involves computing what are called MIDI "events" (things like notes and volume controls) and then using the PortMidi library to play back the events. PortMidi also provides a time reading function that is accurate to 1 millisecond which is useful for timing the playback accurately. I used 'evaluate' in the IO Monad to make sure that all events were fully computed before playback started. However I was still getting delays (timing errors). I added some code to the program that wrote the data to disk before playing it back, and the timing errors went away. Apparently 'evaluate' was not forcing a complete evaluation, while writing a disk file was. Or at least I think that's what happened. But writing to disk is a hack. What can I do to get 'evaluate' to work? A simplified version of the code is here: (the actual program is pretty long and complex so I posted something which is stripped down but I hope retains the essential features) http://lpaste.net/144762

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mapKeysMaybe
by Dennis Raddle 06 Nov '15

06 Nov '15

Can someone give me a clever way to implement mapKeysMaybe, defined as follows? Ord k => (k -> Maybe k) -> Map k a -> Map k a where the transformed keys that evaluate to Just are the only ones kept? I am thinking this will probably help me understand a new type, like traversable or something.

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Add parenthesis to Sin/Cos expressions
by goforgit . 06 Nov '15

06 Nov '15

Hello! I've written a function that is supposed to add parenthesis to sin/cos expressions. For example - Sin5+3 -> Sin(5) + 3 - SinCos(5+3) -> Sin(Cos(5+3)) - SinCos5 * 3 -> Sin(Cos(5)) * 3 The problem is that it converts the following expression *SinSin(5+1+2)* into *Sin(Sin(5)+1+2)* Could someone point me to the direction of a solution to this? Thanks in advance! convert :: String -> String convert s = convert' s 0 convert' :: String -> Int -> String convert' [] n = replicate n ')' convert' (a:as) n | ((a == 'n' || a == 's') && ((take 1 as) /= "(")) = a : "(" ++ (convert' as (n+1)) | ((a == '+' || a == '*') && (n > 0)) = (replicate n ')') ++ [a] ++ (convert' as 0) | otherwise = a : (convert' as n)

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List of a Data Type
by Thomas Jakway 03 Nov '15

03 Nov '15

I've made a new datatype data Visibility a = Hidden a | Shown a I want to create a type that's a list of hidden and visible integers. So something like this: type possiblyVisibleInts = [Visibility Int] The idea is that some of the ints will be hidden and some will be shown. But it doesn't work. Why not? This seemed even odder to me after poking around in ghci: :t [Just 5, Nothing] gives [Just 5, Nothing] :: Num a => [Maybe a] So clearly it's possible to have a list of a type that has 2 constructors--you're not restricted to having a list that only contains Just, for example. ghci is also OK with e.g. :t [Hidden 5, Shown 6] [Hidden 5, Shown 6] :: Num a => [Visibility a]

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Getting Started
by Abhishek Kumar 03 Nov '15

03 Nov '15

Hello Everyone I am new to haskell (learning right now). I want to contribute to data structure and algorithm libraries in haskell.Please guide me with appropriate links and how to start with.I have previous implementation experience in other languages. Thanks Abhishek Kumar

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let , equations, and monads
by PATRICK BROWNE 03 Nov '15

03 Nov '15

{- From Learn Haskell Fast and Hard : 4.3.1. Maybe is a monad http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way/#maybe-monad Concerning the code below I have the following questions: 1) Are eligibleLet and eligibleEquational operationally equivalent (i.e. perform the same operations) and/or semantically equivalent(i.e. Church-Rosser)? 2) Apart from the return type of Maybe instead of Bool how does eligibleMonad differ from eligibleLet? Regards, Pat -} deposit value account = account + value withdraw value account = account - value -- You are eligible for a bonus only if your sequence of transactions stays out of the red. eligibleLet :: (Num a,Ord a) => a -> Bool eligibleLet account = let account1 = deposit 100 account in if (account1 < 0) then False else let account2 = withdraw 200 account1 in if (account2 < 0) then False else let account3 = deposit 100 account2 in if (account3 < 0) then False else let account4 = withdraw 300 account3 in if (account4 < 0) then False else let account5 = deposit 1000 account4 in if (account5 < 0) then False else True -- No let expressions, hence intermediate calculations are performed multiple times. -- Should terminates on first point of failure eligibleEquational :: (Num a,Ord a) => a -> Bool eligibleEquational account = if (deposit 100 account) < 0 then False else if (withdraw 200 (deposit 100 account)) < 0 then False else if (deposit 100 (withdraw 200 (deposit 100 account))) < 0 then False else if (withdraw 300 (deposit 100 (withdraw 200 (deposit 100 account)))) < 0 then False else if (deposit 1000 (withdraw 300 (deposit 100 (withdraw 200 (deposit 100 account))))) < 0 then False else True -- Monadic version depositM :: (Num a) => a -> a -> Maybe a depositM value account = Just (account + value) withdrawM :: (Num a,Ord a) => a -> a -> Maybe a withdrawM value account = if (account < value) then Nothing else Just (account - value) eligibleMonad :: (Num a, Ord a) => a -> Maybe Bool eligibleMonad account = do account1 <- depositM 100 account account2 <- withdrawM 200 account1 account3 <- depositM 100 account2 account4 <- withdrawM 300 account3 account5 <- depositM 1000 account4 Just True main = do print $ eligibleLet 300 -- True print $ eligibleLet 299 -- Falsen print $ eligibleEquational 300 -- True print $ eligibleEquational 299 -- False print $ eligibleMonad 300 -- Just True print $ eligibleMonad 299 -- Nothing -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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Partition a list recursively
by Alexandre Delanoë 03 Nov '15

03 Nov '15

Hello, I am looking for such function: function :: Ord a => [a] -> [a] -> [[a]] function [1,3,6] [0..6] == [[0,1],[2,3],[4,5,6]] Question: does it exist already ? If no, continue: Let: partIt :: Ord a => a -> [a] -> [[a]] partIt m xs = [takeWhile (<= m) xs, dropWhile (<= m) xs] example: partIt 1 [0..5] == [[0,1],[2,3,4,5]] Question: How to define recursively partIt 1) with a list as parameter ? 2) recursively on the second part of the list ? Thanks for help, -- Alexandre Delanoë

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randmomR produces only even values
by martin 01 Nov '15

01 Nov '15

Hello all When I read: mkStdGen :: Int -> StdGen The function mkStdGen provides an alternative way of producing an initial generator, by mapping an Int into a generator. Again, distinct arguments should be likely to produce distinct generators. I thought, that > fst $ R.randomR (1,10) (R.mkStdGen s) should get me a random value between 1 and 10 and that I get different values depending on the seed s. But this > length $ filter even $ map (\s -> fst $ randomR (1::Int,10) (mkStdGen s))[1..1000] gives my 1000, i.e. all random numbers are even numbers. However, when I use random instead of randomR > length $ filter even $ map (\s -> fst $ random (mkStdGen s) ::Int)[1..1000] I get 499 (okay) Why is that so?

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