November 2015 - Beginners

Best MD5 module?
by Dan Stromberg 13 Nov '15

13 Nov '15

Hi again folks. What's the best module for doing an MD5 hash? More specifically, I want to hash the first few dozen K of a file, and then the entire file - to get two different hashes. I'm seeing at least 3 modules for it: https://hackage.haskhell.org/package/cryptohash-0.11.6/docs/Crypto-Hash-MD5… https://hackage.haskell.org/package/pureMD5-2.1.2.1/docs/Data-Digest-Pure-M… https://hackage.haskell.org/package/MissingH-1.3.0.1/docs/Data-Hash-MD5.html Which is best? Thanks! -- Dan Stromberg

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question about list processing
by Dennis Raddle 12 Nov '15

12 Nov '15

What would be an elegant way of writing this computeHead :: (a -> [b]) -> [a] -> [b] Where when [a] is null, it returns a null list, but when [a] contains one or more elements, it applies the given function to the head of a and returns that? Is there some existing typeclass operator that facilitates this? You can write computeHead _ [] = [] computeHead f (x:_) = f x But that first line seems suspicious to me... it makes me think about how in the list Monad, an empty list falls through. But I can't quite make it work.

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My First Haskell Project--Please give feedback!
by Thomas Jakway 12 Nov '15

12 Nov '15

Hi everyone! I'm trying to write a blackjack simulator and would really appreciate advice on the code. I wrote about lots of specific things in an /r/haskell post: https://www.reddit.com/r/haskell/comments/3sjs0y/my_first_haskell_projectpl… And the code is here: https://github.com/tjakway/blackjack-simulator/blob/master/src/Cards.hs (sorry if it's annoying to have to follow a link, thought it'd be better than just copy + pasting a huge blob here) Thanks in advance!

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OrdList implementation
by Mark Carter 12 Nov '15

12 Nov '15

I am trying to implement "OrdList", which is a list that is guaranteed to be ordered. Here's my implementation so far: module OrdList (empty, fromList, insert, toList, OrdList(OrdList)) where import Data.List as L (span) data OrdList a = OrdList [a] deriving (Show) empty :: OrdList a empty = OrdList [] insert :: (Ord a) => a -> OrdList a -> OrdList a insert x ol = let -- OrdList xs = ol (before, after) = L.span (\el -> el <= x) $ toList xs in OrdList $ before ++ [x] ++ after fromList :: Ord a => [a] -> OrdList a fromList xs = if null xs then empty else insert (head xs) (fromList $ tail xs) toList :: OrdList t -> [t] toList xs = let OrdList xs' = xs in xs' It "works", but no doubt there's plenty to dislike about it. Some specific questions I have in mind: 1. Should OrdList be best implemented as a class, data, or newtype? If so, what changes need to be made? 2. How comes I can't refer to OrdList as a data type? This is a problem because I can't say that I want the type of something to be OrdList Int, for example.

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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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