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Is there an idiom for this?
by Mark Carter 16 Nov '15

16 Nov '15

Suppose I want to use an argument twice, as for example in the expression: (\x -> (pred1 x) and (pred2 x)) Is there a shorter way of doing this?

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which Applicative instance?
by Graham Gill 15 Nov '15

15 Nov '15

I'm following the CIS194 lectures and exercises. In particular, in one of the Applicative lectures [0] we're asked to implement mapA and sequenceA, from the name and given type signature alone. I renamed them mapA' and sequenceA' to avoid name clashes, and defined: import Control.Applicative mapA' :: Applicative f => (a -> f b) -> ([a] -> f [b]) mapA' f = sequenceA' . map f sequenceA' :: Applicative f => [f a] -> f [a] sequenceA' = foldr (\fa fla -> (:) <$> fa <*> fla) (pure []) These implementations seem to be correct, though undoubtedly not the nicest. Now, I ask for a type in ghci: > :t mapA' pure [1..5] mapA' pure [1..5] :: (Enum b, Num b, Applicative f) => f [b] And so I try the following: > mapA' pure [1..5] :: [[Int]] [[1,2,3,4,5]] > take 5 . getZipList $ (mapA' pure [1..5] :: ZipList [Int]) [[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5],[1,2,3,4,5]] > mapA' pure [1..5] :: Maybe [Int] Just [1,2,3,4,5] That makes sense. But I don't understand the following: > mapA' pure [1..5] [1,2,3,4,5] Which Applicative instance is being used here? The type test above says my expression should have type f [b], where f is an Applicative and b is a Num, but all I see is a result of type [b] when I compute mapA' pure [1..5]. What's going on? More simply > :t pure pure :: Applicative f => a -> f a > pure [1..5] [1,2,3,4,5] a must match a list of Nums, but then where did f go? Is ghci using some default instance? Actually, after I add the following lines to my source file: ggg = pure [1..5] hhh = mapA' pure [1..5] I get the errors: No instance for (Applicative f0) arising from a use of ‘pure’ No instance for (Applicative f1) arising from a use of ‘mapA'’ respectively. That makes sense to me. Graham [0] http://www.seas.upenn.edu/~cis194/fall14/spring13/lectures/11-applicative2.…

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Query regarding an unusually behaving code
by Abhishek Kumar 15 Nov '15

15 Nov '15

Bellow is my code for returning a list of digits given an integer.I'm unable to find any bug in it but somehow it crashes any computer I run it on when I try to calculate digs 100.Please tell me bug in my code. let digs 0 =[0] let digs x = (digs (x `div` 10)) ++ [(x `rem` 10)] digs 100 Thanks Abhishek

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Generate a list of all constructors
by Kees Bleijenberg 15 Nov '15

15 Nov '15

In this piece of code data Color = RED | BLUE | YELLOW allColors :: [Color] allColors = [RED, BLUE, YELLOW] I have to remeber that if I add a Color, I have to add the color in allColors too. Is there no better way to add all possible colors in allColors? Kees

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Opaque (to me) compilation error
by Dan Stromberg 14 Nov '15

14 Nov '15

In the following code: prefix_md5 :: String -> Data.ByteString.ByteString prefix_md5 filename = do let prefix_length = 1024 file <- System.IO.openBinaryFile filename System.IO.ReadMode :: (IO System.IO.Handle) data_read <- Data.ByteString.hGet file prefix_length :: (IO Data.ByteString.ByteString) _ <- System.IO.hClose file let hasher = Crypto.Hash.MD5.init :: Crypto.Hash.MD5.Ctx let hasher2 = Crypto.Hash.MD5.update hasher data_read :: Crypto.Hash.MD5.Ctx let digest = Crypto.Hash.MD5.finalize hasher2 :: Data.ByteString.ByteString return digest :: (IO Data.ByteString.ByteString) I get the error: Md5s.hs:13:5: Couldn't match type `IO Data.ByteString.ByteString' with `Data.ByteString.ByteString' Expected type: IO System.IO.Handle -> (System.IO.Handle -> IO Data.ByteString.ByteString) -> Data.ByteString.ByteString Actual type: IO System.IO.Handle -> (System.IO.Handle -> IO Data.ByteString.ByteString) -> IO Data.ByteString.ByteString In a stmt of a 'do' block: file <- System.IO.openBinaryFile filename System.IO.ReadMode :: IO System.IO.Handle How should I interpret that error to solve this kind of problem on my own in the future? I don't see where the line in question does anything with ByteString's! How might I correct this function to eliminate the error? Thanks! -- Dan Stromberg

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is this not a better way ? data structure
by Roelof Wobben 14 Nov '15

14 Nov '15

Hello, In the book there is a example of this data-structure. data Shape = Circle float float | Rectangle float float Now I have to change it to add a center point. But I wonder if this is not a better way to describe it Data Schape = Circle Radius | Rectangle Width Height Type Radius = Float type Width = Float type Height = Float Roelof

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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 13 Nov '15

13 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 13 Nov '15

13 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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