April 2014 - Beginners

general structuring of "foreach" logic in IO
by John M. Dlugosz 14 Apr '14

14 Apr '14

This works: main = do myargs <- getArgs mapM_ (\s -> putStrLn s ) myargs and imagine that the "body" will be substantial rather than just a putStrLn. My gut instinct is that the code ought to be arranged as: <any needed keywords or punctuation> and <the collection of items> <body to perform for every element ... ... > Meanwhile, there is no need to name the result of getArgs into myargs. So, getArgs is of type IO [String], and I want to apply that in the manner of a list. Without the Monad wrappers, plain map ( blah ) strings could be ( blah ) <$> strings, and in this particular case I don't see a reversed-arg version, although there is one for <*> (as <**>). But, for monad stuff in general there are reversed arrows for (most?) everything, and that's where I'm heading. So the first question is, how do I do the equivalent map-as-nondeterministic-apply when the strings is further wrapped in IO, as is the function being applied. getArgs >>= mapM_ (\s -> putStrLn s ) does double-duty of moving the argument from last place to the left, as it makes use of eta reduction. Because I have two things going on (list applicative and IO monad) I'm losing the slickness of using applicative syntax. Is there a nice way to make these work together? And more generally, how would you write such a construct? I'm naturally biased with my knowledge in other languages, so maybe there's a completely different "normal" way of approaching this? Thanks, -John

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How do I use Guards in record syntax?
by Dimitri DeFigueiredo 14 Apr '14

14 Apr '14

I'm having some trouble understanding where I can or cannot use guards inside record syntax. I'm writing a simple conversion routine, but I am not able to write it without inserting an extra let. Do I need a let expression here? Am I missing something? -------------- data OldTrade = OldTrade { oldprice :: Double , oldamount :: Double , oldbuysell :: String -- "buy", "sell" or "" } deriving( Eq, Show) data BuyOrSell = Buy | Sell | Unknown deriving(Eq, Show) data Trade = Trade { price :: Double , amount :: Double , buysell :: BuyOrSell } deriving( Eq, Show) convert :: OldTrade -> Trade convert ot = Trade { price = oldprice ot, amount = oldamount ot, buysell = let x | oldbuysell ot == "buy" = Buy | oldbuysell ot == "sell" = Sell | otherwise = Unknown in x } -- how do I eliminate the 'let' expression here? -- I wanted to write something like: -- -- buysell | oldbuysell ot == "buy" = Buy -- | oldbuysell ot == "sell" = Sell -- | otherwise = Unknown -------------- Thanks! Dimitri

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set multiplication
by Nishant 14 Apr '14

14 Apr '14

hi, I am trying to implement a set multiplication program in haskell. Input : [1,2,3] [4,5] [6,7] Ouput : [ [1,4,6] , [1,4,7] , [1,5,6] , [1,5,7] ...] I implemented it for constant number of inputs like setMul xs ys zs = [ [x] ++ [y] ++ [z] | x <- xs , y<-ys ,z <- zs] I am not able to generalize this for any number of lists. type signature would be : setMulMany :: [[a]] -> [[a]] Example : Input : [ [1,2,3] , [4,5] , [6,7]] Ouput : [ [1,4,6] , [1,4,7] , [1,5,6] , [1,5,7] ...] Regards. Nishant

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function not working as expected, type issues
by Alexej Magura 14 Apr '14

14 Apr '14

K, so I have a function that I’m writing that takes a *string* and a *count* and prints the *string* to STDOUT *count* times: displayD :: IO () -> String -> Int displayD string count = do (count > 0) && hPutStr stdout string (count > 0) && displayD string (count - 1) However, when I try to compile the file, I get several errors; here’s a pastebin: http://pastebin.com/DEsuAvfz What I’m trying to do here is check to see if *count* is greater than 0, and then if it is, then I’d like to print *string* to STDOUT until *count* equals 0. I tried doing it via pattern matching, like so: displayD string (count > 0) = do But I haven’t seen any examples of how to do pattern matching in functions using *do*, that is *IO*, so I’m unsure if the above is even anywhere near correct. Still very uncomfortable with declaring function types: I have a hard time figuring out which type is returned and what type is expected as an arg. My initial guess is that the *first* type specified is the one returned, but I’m not sure. -- Alexej Magura Sent with Airmail

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parsing exif file: binary, binary-strict, or cereal?
by Emmanuel Touzery 13 Apr '14

13 Apr '14

Hello, I could not find a pure haskell library to parse EXIF so I wrote one (pretty basic so far): https://github.com/emmanueltouzery/hsexif I wrote it with binary-strict. I also considered binary, but I would like the library not to throw an exception if the file that it's given is not a JPEG or is a JPEG without EXIF, but rather return an Either. I didn't manage to do that with binary (I mean I could offer an IO monad wrapper with try* but to do that even if you give me a lazy bytestring, when it could be done only with pure code, is annoying). It annoys me to load the full JPEG in memory just to parse its EXIF though. Then again with binary I'd have to be pretty careful with strictness annotations and so on, although parsing the EXIF is going to be fast and it won't contain that much data. Still, I want the lazy bytestring holding the JPEG file to be released ASAP. It'd be a shame to have it lying in memory, together with holding the JPEG file open until the library caller actually makes use of the EXIF contents... And then I realized that binary-strict was last updated in 2010, it seems cereal is recommended for strict binary file parsing nowadays. So, what should I do? Leave it as it is? Port to cereal? Port to binary? Otherwise I did write this because I'd like to make a program to parse EXIF file which will have to run also on windows. The haskell exif parsing library that I could find uses the C library libexif, which is going to get complicated to get running on windows. Plus it was fun :-) Thanks, Emmanuel

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Convert my own types to the Haskell types using typeclass
by ke dou 11 Apr '14

11 Apr '14

Hi, I want to define a typeclass that can convert my own types like MyBool, MyInt, MyOption to according Haskell types -- Bool, Int, Maybe. Currently I can convert the first two types, but for MyOption, I don't how to do, since it is a polymophic type. Here is my toy program: -------------------------------------------------------------- {-# LANGUAGE TypeFamilies #-} module Coercible where import qualified Prelude data MyBool = MyTrue | MyFalse data MyInt = Zero | One | Two data MyOption a = Some a | None class Coercible a where type Return a :: * toHaskell :: a -> (Return a) instance Coercible MyBool where type Return MyBool = Prelude.Bool toHaskell MyTrue = Prelude.True toHaskell MyFalse = Prelude.False instance Coercible MyInt where type Return MyInt = Prelude.Int toHaskell Zero = 0 toHaskell One = 1 toHaskell Two = 2 instance Coercible (MyOption a) where type Return (MyOption a) = Prelude.Maybe a toHaskell (Some a) = Prelude.Just a toHaskell None = Prelude.Nothing -------------------------------------------------------------------------- The problem occurs when I am trying to use "toHaskell (Some MyBool)", the error message is "Not in scope: data constructor `MyBool'". Any hints will be appreciated !! Best, --Ke

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Windows x64 ?
by John M. Dlugosz 11 Apr '14

11 Apr '14

The Haskell Platform has a single download link for "Windows" which seems to be 32-bit only. But GHC is available for Win x64. Their site dissuades downloading their installation and recommends getting Haskell Platform instead. So, is there an easy way to get 64-bit versions of compiler or compiled executables so produced?

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Guards problem
by Stijn Muylle 10 Apr '14

10 Apr '14

All, I ran into a small problem using guards, and I don't really understand what I'm doing wrong. I have this code: ---------------------------------- import Data.List.Split type Cell = Bool data Board = Board [[Cell]] deriving Show trueList = True : trueList createBoard :: Int -> Int -> Board createBoard x y = Board (chunksOf x (take (x * y) trueList)) getCellAt :: Int -> Int -> Board -> Cell getCellAt x y (Board b) | x >= (length b) = False | y >= (length (b !! 0)) = False | otherwise = (b !! x) !! y ----------------------------------- When I try to execute this: getCellAt (-1) 0 $ createBoard 3 3 I get a negative index exception. However, as -1 is smaller than 3 (length of the board), I would expect False to be returned. Am I overlooking something? Thanks! Stijn

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Nonsensical behavior with http-conduit, httpLbs
by Christopher Allen 10 Apr '14

10 Apr '14

Repro'd on GHC 7.8.1 and 7.6.3 Original code is: https://github.com/bitemyapp/bloodhound/ The code only works if I manually (out of band) run insertData (in a REPL) and then run main with the insertData invocation stripped out. If I run main with the insertData invocation included, it throws an exception (head) because the search results are empty. The behavior is as if queryTweet was executing after insertData deleted the index, but before it inserted the new data. The following is a stripped down example: insertData :: IO () insertData = do let encoded = encode exampleTweet _ <- deleteExampleIndex created <- createExampleIndex docCreated <- indexDocument (Server "http://localhost:9200") "twitter" "tweet" exampleTweet "1" print "test" return () el queryTweet :: IO (Either String Tweet) queryTweet = do let queryFilter = BoolFilter (MustMatch (Term "user" "bitemyapp") False) <||> IdentityFilter let search = Search Nothing (Just queryFilter) reply <- searchByIndex testServer "twitter" search let result = eitherDecode (responseBody reply) :: Either String (SearchResult Tweet) let myTweet = fmap (hitSource . head . hits . searchHits) result return myTweet main :: IO () main = do _ <- insertData myTweet <- queryTweet print myTweet further up the call chain, the http call is getting dispatched with http-conduit's gear, the result returned with the expression: withManager $ httpLbs req Included this in case it affects the semantics. Can anybody help? This has stumped me for a couple hours and I couldn't get anything clueful on IRC. Thanks to any that help. --- Chris

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Re: [Haskell-beginners] function not working as expected, type issues (David McBride)
by Alexej Magura 10 Apr '14

10 Apr '14

Thanks displayD :: String -> Int -> IO () displayD string 0 = return () …. is exactly what I was looking for. -- Alexej Magura Sent with Airmail

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