- Beginners - Haskell.org

figured out use for join!
by Dennis Raddle 12 Nov '15

12 Nov '15

I'm starting to get the hang of certain aspects of typeclasses, particularly with Maybe and list types. For instance I needed to write a function as follows: Ord k => k -> Map k [a] -> Maybe a which evaluates to "Nothing" if there is no such key in the map, or Just the first element of [a] if there is such a key, or Nothing if there is such a key but [a] is null. So I could write import qualified Data.Map as M import Control.Monad import Data.Maybe f k m = case M.lookup k m of Nothing -> Nothing Just xs -> listToMaybe xs But the case "Nothing -> Nothing" is suspicious... seems like that's always a clue some typeclass could simplify it. Eventually I figured out f k = join . fmap listToMaybe . M.lookup k

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assistance please ?
by Roelof Wobben 11 Nov '15

11 Nov '15

Hello, I have this exercise : Define a function fibTable :: Integer -> String which produces a table of Fibonacci numbers. For instance, the effect of putStr (fibTable 6) should be n fib n 0 0 1 1 2 1 3 2 4 3 5 5 6 8 1) Can somone give me any pointers how to calculate the fibb numbers with list comprehension. I know that the fib numbers are (n -1) + n 2) Can someone give me any pointers how to write the outcome of every run Roelof

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Python's collections.defaultdict(list) in Haskell?
by Dan Stromberg 10 Nov '15

10 Nov '15

I'm spending a little time here and there to learn some Haskell. I'm coming from a chiefly Python/C/bash background. I want to build a Data.Map where the keys are strings, and the values are lists of strings. In Python, collections.defaultdict(list) makes this pretty straightforward. It gives a hash table ("dict") that has values that default to an empty list, since list() produces an empty list. More info here: https://docs.python.org/3/library/collections.html#collections.defaultdict Is there an equivalent in Haskell? Thanks! -- Dan Stromberg

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Calling "zipWith" using "^" function.
by Venu Chakravorty 09 Nov '15

09 Nov '15

Hello there, This is just some toy program I was trying. I fail to understand why this works: ============================= Prelude> zipWith (\ x y -> ((x ^ y) / (product [1..x]))) [1..3] [2,2,2] [1.0,2.0,1.5] ============================= and this does not: ============================= Prelude> zipWith (\ x y -> ((y ^ x) / (product [1..x]))) [1..3] [2,2,2] <interactive>:1:19: Ambiguous type variable `a' in the constraints: `Fractional a' arising from a use of `/' at <interactive>:1:19-44 `Integral a' arising from a use of `^' at <interactive>:1:20-24 Probable fix: add a type signature that fixes these type variable(s) ============================ Note that the "^" function has "x" and "y" flipped. :t (^) says: ============================ (^) :: (Num a, Integral b) => a -> b -> a ============================ Could somebody please throw some light? Thanks.

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(no subject)
by Olivier TOURNAIRE 08 Nov '15

08 Nov '15

Hello I would like to implement Haskell with a samsung tablet SM T800. How can i do that ? Thank you very much.

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Why does floor not consider its argument an Integral?
by Jeffrey Brown 08 Nov '15

08 Nov '15

Tidal [1] defines these data types: type Time = Rational type Arc = (Time, Time) I want to write a function "splitMultiCycArc" which divides an Arc into mostly-integer segments, so that, for instance, splitMultiCycArc (0,1) = [(0,1)] splitMultiCycArc (0,2) = [(0,1),(1,2)] splitMultiCycArc (0,3) = [(0,1),(1,2),(2,3)] splitMultiCycArc (1%2,2) = [(1%2,1),(1,2)] splitMultiCycArc (1,5%2) = [(1,2),(2,5%2)] I thought I had solved the problem with this code: splitMultiCycArc:: Arc -> [Arc] splitMultiCycArc (a,b) = let ceiling_ish = floor a + 1 in if b <= a then [] else if b <= ceiling_ish then [(a,b)] else (a,ceiling_ish) : splitMultiCycArc (ceiling_ish,b) When I try to load that, I get this single error: > :reload [12 of 13] Compiling Sound.Tidal.JBB ( Sound/Tidal/JBB.hs, interpreted ) Sound/Tidal/JBB.hs:16:44: No instance for (Integral Time) arising from a use of ‘floor’ In the first argument of ‘(+)’, namely ‘floor a’ In the expression: floor a + 1 In an equation for ‘ceiling_ish’: ceiling_ish = floor a + 1 Failed, modules loaded: Sound.Tidal.Strategies, Sound.Tidal.Dirt, Sound.Tidal.Pattern, Sound.Tidal.Stream, Sound.Tidal.Parse, Sound.Tidal.Tempo, Sound.Tidal.Time, Sound.Tidal.Utils, Sound.Tidal.SuperCollider, Sound.Tidal.Params, Sound.Tidal.Transition. > And yet under other conditions, "floor" is perfectly happy operating on a Time value: > let a = (1%2,1) :: Arc > floor (fst a) + 1 1 > [1] https://hackage.haskell.org/package/tidal -- Jeffrey Benjamin Brown

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manipulating Map for music application
by Dennis Raddle 08 Nov '15

08 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 08 Nov '15

08 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 07 Nov '15

07 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 07 Nov '15

07 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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