- Beginners - Haskell.org

Design help to add imports to a (Java) template
by Hilco Wijbenga 03 Jul '18

03 Jul '18

Hi all, I'm working on a Haskell (Eta) tool to generate Java ADTs. I have the following definitions: data Line = Line Int Text [JavaImport] -- The Int represents the indentation level of this line. data Template = None | One Line | Many [Line] | Template (Reader Configuration Template) I then run it like this (incomplete, I hope you get the idea): main :: IO () main = putStr $ replace (importsGoHereKey `append` "\n") importsAsText templateAsText where config = Configuration (Copyright 2018 "Me") USE_GETTERS point' = javaClassTemplate point importsAsText = runReader (mapReader javaImportsToText (javaImports point')) config templateAsText = runReader (toText tab point') config Essentially, I use the template twice: once to extract the Java imports and once to generate the result Text. Then I replace a magic string in the result Text with the import code. It works but makes me sad. ;-) I will probably pull out the JavaImport list from Line and use something like State to keep track of the necessary imports. (Now that I understand Reader, maybe State is no longer a bridge too far.) My main issue is how to avoid the magic string. I thought about adding an extra entry to Template [something like Imports ((Set JavaImport) -> Text or maybe Template)] but then I don't know how to implement "instance Monoid Template". Similarly if I use something Reader like. Any design advice or ideas would be appreciated. If it's of any help, the full code is at [1] (it's tiny). Any and all comments are welcome, by the way: I'm still a Haskell beginner. Cheers, Hilco [1] https://github.com/hilcode/java-algebraic-data-types

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How to start Haskell
by Saurabh Sihag 20 Jun '18

20 Jun '18

Hi all, I am a newbie in Haskell and I am interested to explore Haskell. Kindly help me from where I start. I have prior experience in Python and C. Thank you.

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How to define macro var with stack/cabal/etc?
by PY 20 Jun '18

20 Jun '18

Hello all, I'm using conditional compilation: #ifdef BLAH blahBlah #endif And I want to define BLAN, but no in *.hs file. For example, with `stack` option, in yaml file, in cabal file, in environment variable... How can I do it? For example, `stack -DBLAH=1 build` or something else. What are the options? === Best regards, Paul

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Difficulty understanding how to use filterM to compute powerset
by Olumide 18 Jun '18

18 Jun '18

Dear List, I'm trying to apply the following definition of filterM (from Brent Yorgey's blog https://byorgey.wordpress.com/2007/06/26/deducing-code-from-types-filterm/) import Control.Monad -- for liftM filterM' :: (Monad m) => (a -> m Bool) -> [a] -> m [a] filterM' p [] = return [] filterM' p (x:xs) = let rest = filterM' p xs in do b <- p x if b then liftM (x:) rest else rest in order to understand how filterM can be used to compute the power set of a list, as follows filterM' (const [False,True]) [1,2,3] Where p in the filterM' is (const [False,True]). What confuses me is that p x in filterM'. Based on my very limited understanding p x returns b = [False, True]. How b be tested in the subsequent if-statement if it is indeed a list? What am I getting wrong? Regards, - Olumide

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life with Hutton 2ed
by Sylvester Hazel 17 Jun '18

17 Jun '18

I wonder why life/10.8 runs nonsensically on my PC even with Language.Haskell.HsColour.ANSI import? These cls and goto reappear in 11.7 that spoil the pleasure of understanding. Also putGrid/11.4 is unintuitive backsword engineering concoction that teaches nothing, while it can be very intuitively written: putGrid g = sequence_ [putStrLn j| j<-weave [' ','|',' ','|',' '] (interleave (replicate 6 '-') [(interleave '|' i)| i<-g])] interleave :: a->[a]->[a] interleave x [] = [] interleave x [y] = [y] interleave x (y:ys)= y:x:interleave x ys weave :: a->[a]->[a] weave x [] = [x] weave x [y] = x:[y,x] weave x (y:ys)= x:y:weave x ys Prelude> putGrid [[' ','O','O'],['O','X','O'],['X','X','X']] | | |O|O | | ------ | | O|X|O | | ------ | | X|X|X | |

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fmap composition
by Marco Turchetto 13 Jun '18

13 Jun '18

I'm reading the "Haskell programming" book and in chapter 16 about functors they combine two "fmap" with the "(.)" function. I really don't understand how the type of "fmap . fmap" is "(Functor f2, Functor f1) => (a -> b) -> f1 (f2 a) -> f1 (f2 b)". The thing that really freaks me out is that I thought that "(.)" arguments are ONLY two unary function. Maybe there is some curring magic underline, there is someone that can explain to me how the type of "fmap . fmap" is derived from the type of "fmap" and "(.)"?

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incoherent instance question
by Graham Gill 09 Jun '18

09 Jun '18

Please see the paste: https://pastebin.com/zBim7Zkx I'm experimenting with defining UpperBounded and LowerBounded typeclasses. An example type belonging to the latter that is not also Bounded would be type Natural from Numeric.Natural. I want to say that if a type is Bounded, then it is also UpperBounded and LowerBounded. If a type is both UpperBounded and LowerBounded, then it is also Bounded. To express the constraints, I need FlexibleInstances and UndecidableInstances extensions. These allow the module to load into ghci (8.4.2) with only a warning, but, without the INCOHERENT pragmas, I get an overlapping instance error if I try to evaluate minBound, maxBound, upperBound or lowerBound instantiated to either of the types Foo or Bar. A solution is to apply the INCOHERENT pragma to the instances at lines 11, 14 and 17. Reading over section 10.8.3.6. Overlapping instances in the GHC User Guide, I believe I understand. (Is there a better solution?) In the paste, I have INCOHERENT pragmas only at lines 11 and 17. This gives me the following behaviour in ghci: 1. minBound, maxBound, upperBound and lowerBound instantiated to type Foo all function as expected, evaluating to the appropriate lower or upper bound. 2. upperBound and maxBound instantiated at Bar give overlapping instance errors for UpperBounded, as expected. 3. lowerBound :: Bar evaluates to C, as expected. 4. minBound :: Bar gives an overlapping instance error for UpperBounded: *UpperLowerBounded> minBound :: Bar <interactive>:141:1: error: • Overlapping instances for UpperBounded Bar arising from a use of ‘minBound’ Matching instances: instance [safe] Bounded a => UpperBounded a -- Defined at UpperLowerBounded.hs:14:10 instance [safe] UpperBounded Bar -- Defined at UpperLowerBounded.hs:31:10 • In the expression: minBound :: Bar In an equation for ‘it’: it = minBound :: Bar It's #4 that I don't understand. An explanation would be very much appreciated. (Also, what's a [safe] instance?) Regards, Graham

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How to get IO String from Network.Socket.ByteString.recv method
by Dinesh Amerasekara 06 Jun '18

06 Jun '18

Hi, I am unable to compile the below code. import Network.Socket hiding(recv) import Network.Socket.ByteString as S (recv) import qualified Data.ByteString.Lazy.Char8 as Char8 getMessage :: Socket -> IO String getMessage sock = Char8.unpack <$> S.recv sock 8888 It gives the below error. Couldn't match type ‘Data.ByteString.Internal.ByteString’ with ‘ByteString’NB: ‘ByteString’ is defined in ‘Data.ByteString.Lazy.Internal’ ‘Data.ByteString.Internal.ByteString’ is defined in ‘Data.ByteString.Internal’ Expected type: IO ByteString Actual type: IO Data.ByteString.Internal.ByteString In the second argument of ‘(<$>)’, namely ‘recv sock 8888’In the expression: unpack <$> recv sock 8888In an equation for ‘getMsg’: getMsg sock = unpack <$> recv sock 8888 Can somebody tell me how I can return the IO String using Network.Socket.ByteString.recv? Best Regards,Dinesh.

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ghc-8.4.3 at Ubuntu
by Andreas Röhler 04 Jun '18

04 Jun '18

While installing ghc-8.4.3-i386-deb8-linux.tar.xz at Debian8 went smoothly, can't se how to install ghc-8.4.3 at Ubuntu resp. Xubuntu: in dir ghc-8.4.3/ ./configure checking for gfind... no checking for find... /usr/bin/find checking for sort... /usr/bin/sort checking for GHC Git commit id... given 51abb1c88b53e2989a2a8c2939ac4abc04bef194 checking for ghc... /usr/bin/ghc checking version of ghc... 7.10.3 configure: error: GHC version 8.0 or later is required to compile GHC. Did cabal update and tried haskell-platform for ubuntu https://www.haskell.org/platform/linux.html#linux-ubuntu which seems broken Tried Haskell-platform from source: ./platform.sh *** *** Building hptool *** cabal: unrecognised command: new-build (try --help) Any help? Cheers, Andreas

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Conflict with global lib from sandbox
by Andreas Röhler 28 May '18

28 May '18

Hi, called "cabal sandbox init" from inside an empty directory. When running cabal install... see errors like that: /home/my-name/.cabal/lib/i386-linux-ghc-7.6.3/unix-2.7.2.2/libHSunix-2.7.2.2.a(HsUnix.o):(.text+0x20): first defined here /usr/lib/ghc/unix-2.6.0.1/libHSunix-2.6.0.1.a(HsUnix.o): In function `__hsunix_grantpt': Shouldn't /usr/lib/... be hidden from inside a sandbox? Thanks, Andreas

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