- Beginners - Haskell.org

Functions as Applicatives
by Olumide 24 Aug '16

24 Aug '16

Hi List, I'm struggling to relate the definition of a function as a function instance Applicative ((->) r) where pure x = (\_ -> x) f <*> g = \x -> f x (g x) with the following expression ghci> :t (+) <$> (+3) <*> (*100) (+) <$> (+3) <*> (*100) :: (Num a) => a -> a ghci> (+) <$> (+3) <*> (*100) $ 5 508 From chapter 11 of LYH http://goo.gl/7kl2TM . I understand the explanation in the book: "we're making a function that will use + on the results of (+3) and (*100) and return that. To demonstrate on a real example, when we did (+) <$> (+3) <*> (*100) $ 5, the 5 first got applied to (+3) and (*100), resulting in 8 and 500. Then, + gets called with 8 and 500, resulting in 508." The problem is that I can't relate that explanation with the definition of a function as an applicative; especially f <*> g = \x -> f x (g x) . Is (g x) the second argument to f? Regards, - Olumide

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putting together monadic actions
by Dennis Raddle 24 Aug '16

24 Aug '16

Is there a function foo that does foo :: a -> [a -> m a] -> a So foo 3 [x,x,x] = return 3 >>= x >>= x >>= x I don't think replicateM and sequence do this. At least I can't figure it out.

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Re: [Haskell-beginners] No instance for Show arising from a use in “main” level
by T. Andrea Moruno Rodriguez 24 Aug '16

24 Aug '16

Hi! Yes. Thanks I resolved my problem. The explanation is here: http://stackoverflow.com/questions/39112380/no-instance-for-show-arising-fr… 2016-08-23 20:23 GMT-04:00 Imants Cekusins <imantc(a)gmail.com>: > Hello Tatiana, > > these signatures do not look good: > > main :: (Show (IO J2s)) => IO() > > test :: (Show (IO J2s)) => String -> IO() > > testParser :: (Show (IO J2s)) => IO() > > > constraints are used to specify type variables. IO J2s looks like * (a > fixed type). > > what errors do you see without these constraints? >

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No instance for Show arising from a use in “main” level
by T. Andrea Moruno Rodriguez 24 Aug '16

24 Aug '16

I have a code that reads files and parses using UU.Parsing lib that returns an abstract sintax tree and shows on the screen. I received the error message "No instance for Show" in my functions originated intokensParserToByteString and applyParser using parseIO (of UU.Parsing lib) and inherited signatures until main. I fixed the signatures but my problem is in the main function. I added the instance Show in the signature but I have the next compilation error: No instance for (Show (IO J2s)) arising from a use of ‘main’ In the expression: main When checking the type of the IO action ‘main’ Some idea, about the problem? Main module ----------------------------------------------------------------------- {-# LANGUAGE FlexibleContexts #-} module Main where import UU.Parsing ... import Content main :: (Show (IO J2s)) => IO() main = do f <- getLine let command = test f command test :: (Show (IO J2s)) => String -> IO() test "testparser" = testParser Test module ----------------------------------------------------------------------- {-# LANGUAGE FlexibleContexts #-} module J2s.Parser.Test where import Content import J2s.Ast.Sintax import J2s.Parser import UU.Parsing ... testParser :: (Show (IO J2s)) => IO() testParser = (runSafeIO $ runProxy $ runEitherK $ contentsRecursive "path/of/my/tests" />/ handlerParser) :: (Show (IO J2s)) => IO() Content module ----------------------------------------------------------------------- {-# LANGUAGE FlexibleContexts #-} module Content where import Control.Monad(forM, liftM) import System.Directory (doesDirectoryExist, getDirectoryContents) import System.FilePath ((</>), splitExtension, splitFileName) import J2s.Parser import J2s.Ast.Sintax import UU.Parsing import Control.Monad (when, unless) import Control.Proxy import Control.Proxy.Safe hiding (readFileS) import J2s.Scanner.Token import Text.Show import UU.Parsing contentsRecursive :: (CheckP p) => FilePath -> () -> Producer (ExceptionP p) FilePath SafeIO () contentsRecursive path () = loop path where loop path = do contents path () //> \newPath -> do respond newPath isDir <- tryIO $ doesDirectoryExist newPath let isChild = not $ takeFileName newPath `elem` [".", ".."] when (isDir && isChild) $ loop newPath applyParser :: (Proxy p, Show (IO J2s)) => String -> Consumer p B.ByteString IO () applyParser path = runIdentityP loop where loop = do bs <- request () let sc = classify (initPos path) (B8.unpack bs) lift $ B8.putStrLn (tokensParserToByteString sc) tokensParserToByteString :: (Show (IO J2s)) => [Token] -> B.ByteString tokensParserToByteString tokens = B8.pack(show (parseIO pJ2s tokens)) handlerParser :: (CheckP p, Show (IO J2s)) => FilePath -> Session (ExceptionP p) SafeIO () handlerParser path = do canRead <- tryIO $ fmap readable $ getPermissions path isDir <- tryIO $ doesDirectoryExist path isValidExtension <- tryIO $ evaluate ((snd (splitExtension path) == ".java" || snd (splitExtension path) == ".mora") && (snd (splitFileName path) /= "EncodeTest.java") && (snd (splitFileName path) /= "T6302184.java") && (snd (splitFileName path) /= "Unmappable.java")) when (not isDir && canRead && isValidExtension) $ (readFileSP 10240 path >-> try . applyParser) path readFileSP :: (CheckP p) => Int -> FilePath -> () -> Producer (ExceptionP p) B.ByteString SafeIO () readFileSP chunkSize path () = bracket id (openFile path ReadMode) hClose $ \handle -> do let loop = do eof <- tryIO $ hIsEOF handle unless eof $ do bs <- tryIO $ B.hGetSome handle chunkSize respond bs loop loop -- Tatiana Andrea Moruno Rodriguez

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type T' = T a b c
by Imants Cekusins 22 Aug '16

22 Aug '16

*problem*: given type T with a few parameters e.g. T a b c some functions where this type is passed to, are agnostic to a, b, c i.e. accept T with any of a, b, c processT::T a b c -> () when either number of these parameters or T change (c is dropped or d is added: T a b c d), signatures of every function where T is passed to, need to be updated. *question*: is this possible to specify type synonym T' so that it may be passed to parameter (a,b,c) - agnostic functions so: processT::T' -> () ?

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help writing a simple api which queries for a resource and displays this info
by xu xiut 21 Aug '16

21 Aug '16

Hello, I am wanting to store something in a database, then retrieve it. That is my entire ambition for the day. Technologies chosen: spock: https://github.com/agrafix/Spock rethinkdb driver: https://github.com/AtnNn/haskell-rethinkdb I chose to go with Rethinkdb, but i'm open to using Postgresql. If someone requests for /companies, I want to list all the companies in the "companies" table. ``` -- ghci λ> companies <- run h $ table "companies" :: IO [Datum] λ> companies [{"name":"tesla","id":"7781ee7e-1e43-4608-bb96-fe10cac3b53a"}] λ> firstCompany <- run h $ table "companies" ! "name" :: IO [Datum] λ> firstCompany ["tesla"] λ> :info Datum data Datum = Null | Bool Bool | String Text | Number Double | Array Database.RethinkDB.Datum.Array | Object Database.RethinkDB.Datum.Object | Time time-1.5.0.1:Data.Time.LocalTime.LocalTime.ZonedTime | Point LonLat | Line GeoLine | Polygon GeoPolygon | Binary bytestring-0.10.6.0:Data.ByteString.Internal.ByteString -- Defined in ‘Database.RethinkDB.Datum’ instance Eq Datum -- Defined in ‘Database.RethinkDB.Datum’ instance Ord Datum -- Defined in ‘Database.RethinkDB.Datum’ instance Show Datum -- Defined in ‘Database.RethinkDB.Datum’ instance Expr Datum -- Defined in ‘Database.RethinkDB.ReQL’ instance Result Datum -- Defined in ‘Database.RethinkDB.Driver’ instance ToDatum Datum -- Defined in ‘Database.RethinkDB.Datum’ instance FromDatum Datum -- Defined in ‘Database.RethinkDB.Datum’ 15:40 < lpaste> xuxu revised “investigating the Datum data type”: “investigating the Datum data type” at http://lpaste.net/179391 ``` I don't know how to display the result. >From Spock's readme, here's an example where I don't need to query the database: ``` main = runSpock 3000 $ spockT id $ do get "companies" $ text $ T.concat ["tesla", "google"] ``` How to convert that into something a little more practial where I do utilize a db? Here's what I have right now, but I'm sure it doesn't compile ``` {-# LANGUAGE OverloadedStrings #-} import Web.Spock import qualified Data.Text as T import qualified Database.RethinkDB as R import qualified Database.RethinkDB.NoClash default (Datum, ReQL, String, Int, Double) main :: IO () main = -- 1. i don't know if the following line will work let h = connect "localhost" 28015 Nothing in runSpock 3000 $ spockT id $ -- 2. list all companies do get "companies" $ text $ T.concat -- something ``` I don't exactly know how "text" works. If I'm reading https://github.com/agrafix/Spock/blob/1ee54503ad67f62af795a31772040f56f7ae0… correctly, spockT is using a reader monad. I don't know how it works with the Text type, but I have a suspicion I need to convert Rethink's Datum type into a Text type. I would be absolutely thrilled if anyone is able to help with this.

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Automatic Differentiation
by Jonas Béal 17 Aug '16

17 Aug '16

Hello, I have some questions about the Automatic Differentiation package in Haskell (https://hackage.haskell.org/package/ad-4.3.2.1/docs/Numeric-AD.html ) I need to compute the jacobian with that method and I have a type problem, here a simplified example in order to focus on the error: According to documentation I need to write "*jacobian function values*". So I built the input *function. *Please notice this is important for me that my function may depend on an external parameter (named factor here) testFunctionForJacobian :: Fractional a => a -> [a] -> [a] testFunctionForJacobian factor inputs = [(sum inputs) * factor] Then, using *jacobian* function of Numeric.AD in terminal, as a test, it works perfectly >jacobian (testFunctionForJacobian 2) [1,2] < [[2.0,2.0]] No apparent type problem here > :t (testFunctionForJacobian 2) (testFunctionForJacobian 2) :: Fractional a => [a] -> [a] But, I would like to insert that in a bigger function computing the jacobian testJacobian :: Fractional a => a -> [a] -> [[a]] testJacobian factor inputs = jacobian (testFunctionForJacobian factor) inputs This time it generates an error message about *factor* Couldn't match expected type ‘Numeric.AD.Internal.Reverse.Reverse <http://numeric.ad.internal.reverse.reverse/> s a’ with actual type ‘a’ ‘a’ is a rigid type variable bound by the type signature for testJacobian :: Fractional a => a -> [a] -> [[a]] s a’ All in all, my type seems implicitly correct in the terminal example but I did not manage to write it explicitly in my function. Here the *jacobian* function signature to help you answer me: -- Jonas Béal -- *This message is intended only for the personal and confidential use of the designated recipient(s) named above. If you are not the intended recipient of this message you are hereby notified that any review, dissemination, distribution or copying of this message is strictly prohibited. Email transmission cannot be guaranteed to be secure or error-free. Therefore, we do not represent that this information is complete or accurate and it should not be relied upon as such. All information is subject to change without notice.*

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Bool newtype
by Imants Cekusins 14 Aug '16

14 Aug '16

for a Bool-like newtype: newtype B = B Bool , is there an easy way to use this newtype B in place of Bool? e.g. let b1 = B True in if b1 then 1 else 0 ?

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Problem getting haskell-mode working with emacs on windows
by George Taylor 13 Aug '16

13 Aug '16

Hello, Environment: Win7 32bit, Emacs 24.5.1, ghci 7.8.3 I have had Haskell working with Windows 7 and emacs previously, but have had issues, and am attempting to get everything working again. Problems. I searched for "Install Haskell Emacs", and found a link to the page https://wiki.haskell.org/Emacs/Installing_haskell-mode But this page was blanked on 11 April 2016, with no information as to where to go. I looked at the previous version of the page, and it said to "place the basic mode haskell-mode.el and the modules you want to use in [a suitable directory, I used "~/haskell-extensions"], add the following command to your init file (~/.emacs): (load "~/lib/emacs/haskell-site-file")" I did the above (but just with haskell-mode.el) and launched emacs, and got a message: "File error: Cannot open load file, no such file or directory, ~/haskell-extensions/haskell-site-file" in emacs, as an experiment, I did M-x Load file: C:\home/haskell-extensions/haskell-mode.el And got the message "Cannot open load file: no such file or directory, haskell-customize" I downloaded haskell-customize.el and placed it in ~/haskell-extensions, and tried again, but no joy. Questions: 0. Is haskell-mode depracated? It no longer seems to be available at MELPA. 1. Why was the page "https://wiki.haskell.org/Emacs/Installing_haskell-mode" blanked? 2. Is a similar, current set of instructions available? 3. Should I be able to load haskell-mode using M-x load-file? If so, what other .el (or other) files are required, and where should they be placed? 4. Is there a dissimilar set of instructions available? Any help will win thousands in prizes.

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My Continuation doesn't typecheck
by martin 08 Aug '16

08 Aug '16

Hello all, in order to gain some intuition about continuations, I tried the following: -- two functions accepting a continuation f1 :: Int -> (Integer->r) -> r f1 a c = c $ fromIntegral (a+1) f2 :: Integer -> (String -> r) -> r f2 b c = c $ show b -- combine the two functions into a single one run1 :: Int -> (String -> r) -> r run1 a = f1 a f2 -- *Main> run1 9 id -- "10" So far so good. Then I tried to write a general combinator, which does not have f1 and f2 hardcoded: combine a f g = f a g -- This also works -- *Main> combine 9 f1 f2 id -- "10" What confuses me is the the type of combine. I thought it should be combine :: Int -> (Int -> (Integer->r) -> r) -> -- f1 (Integer -> (String -> r) -> r) -> -- f2 ((String -> r) -> r) but that doesn't typecheck: Couldn't match expected type ‘(String -> r) -> r’ with actual type ‘r’ Can you tell me where I am making a mistake?

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