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Network PortNumber deprecation warning
by Jeff C. Britton 16 Aug '19

16 Aug '19

I have just installed Haskell using Stack on Windows. I wanted to try and write some code to do network programming with TCP/IP. I found a simple example on Stack Overflow to start me off. I'm having some problems with PortNumber and deprecated warnings. I don't know how to modify the code to be deprecation free. stack install network - installed version 2.8.0.1 ------------------------- package.yaml network >= 2.8.0.1 --------------------------- The following code gives a deprecation warning, but ultimately my sample compiles and runs. import qualified Network import qualified Network.Socket main = Network.withSocketsDo $ do handle <- Network.connectTo "192.168.1.2" (Network.PortNumber 7200) talk handle `finally` hClose handle D:\work\hsSocket\app\Main.hs:14:46: warning: [-Wdeprecations] In the use of data constructor `PortNumber' (imported from Network): Deprecated: "The high level Network interface is no longer supported. Please use Network.Socket." ----------------------------------------- So, now I try and use Network.Socket as the warning indicates. main = Network.withSocketsDo $ do handle <- Network.connectTo "192.168.1.2" (Network.Socket.PortNumber 7200) talk handle `finally` hClose handle D:\work\hsSocket\app\Main.hs:15:46: error: Not in scope: data constructor `Network.Socket.PortNumber' Module `Network.Socket' does not export `PortNumber'. | 15 | handle <- Network.connectTo "192.168.1.2" (Network.Socket.PortNumber 7200)

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Storable which is only Peekable
by PICCA Frederic-Emmanuel 04 Aug '19

04 Aug '19

Hello, I would like to know what is the best way to create a type which is only peekable and pokable. When I use Storable, I "need" to define peek and poke and ghc complain that I did not defined the poke part. So how can I write something nice. Cheers Frederic #include "hkl-binoculars.h" data Space = Space (ForeignPtr Space) Int deriving Show foreign import ccall safe "fromImage" c_fromImage :: Ptr Double -> Int -> Ptr Double -> Ptr Int -> Int -> Ptr Word16 -> Ptr Int -> Int -> IO (Ptr Space) foreign import ccall unsafe "&space_free" c_space_free :: FunPtr (Ptr Space -> IO ()) instance Storable Space where alignment _ = #{alignment struct space_t} sizeOf _ = #{size struct space_t} peek ptr = Space <$> newForeignPtr c_space_free ptr <*> #{peek struct space_t, ndim} ptr

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Stack doesn't find downloaded module
by Miguel Angel Ordoñez Silis 23 Jul '19

23 Jul '19

Hi! I downloaded the following package https://www.stackage.org/lts-13.29/package/multiset-0.3.4.1 using the following cli (Linux) command: stack install multiset. It was supposedly successfully installed but when I try to import it stack says it couldn't find it ('Could not find module ‘Data.MultiSet’). I've tried reading haskellstack.org's documentation but I've not been able to find a solution. Well, thanks in advance for your help Mike

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binding vs. parameter passing
by James Jones 23 Jul '19

23 Jul '19

One problem in *Programming Haskell from First Principles* confuses me. (So far.) It's one where you start with the declaration and binding i :: Num a => a i = 1 (which of course works because Num a => a is the type of 1) and then change the declaration to i :: a and first try to predict and then see what happens. It fails, with ghci suggesting that you put the Num a => back. That seemed reasonable at first, but then I considered this: - id has type a -> a, and if you try to evaluate id 1 it works without complaint. - In all the work I've done on compilers, parameter passing has effectively been assignment of actual parameters to the corresponding formal parameters. In Haskell, that might mean passing along the appropriate thunk, but the principle is the same, isn't it? So, if I can't bind 1 to i which is declared to have type a, why can I successfully pass 1 to id?

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Question about code I found
by Terry Phelps 17 Jul '19

17 Jul '19

I found this code on the net somewhere. It compiles and works properly: import qualified Data.ByteString as BS import Text.Printf (printf) toHex :: BS.ByteString -> String toHex bytes = do hex <- BS.unpack bytes printf "%02x" hex I cannot understand the 'do' notation is required, because it seems to be a pure function. I guess there's a monad hiding somewhere that my newbie mind can't see. So, I rewrote the code to remove the 'do stuff': -- BAD import qualified Data.ByteString as BS import Text.Printf (printf) toHex :: BS.ByteString -> String toHex bytes = printf "02x" (BS.unpack bytes) WRONG. Ghci says: xx2.hs:5:15: error: • No instance for (Text.Printf.IsChar GHC.Word.Word8) arising from a use of ‘printf’ • In the expression: printf "02x" (BS.unpack bytes) In an equation for ‘toHex’: toHex bytes = printf "02x" (BS.unpack bytes) | 5 | toHex bytes = printf "02x" (BS.unpack bytes) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Can someone please explain to me: 1. Why does this simple-looking code to convert a bytestring to a string of hex characters require 'do' or equivalent code? 2. What is the specific problem with my version of the code. The error message doesn't mean much to me yet.

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Parser using the show method
by PICCA Frederic-Emmanuel 21 Jun '19

21 Jun '19

Hello, I have a data type like this data SpaceGroup = P1 | P2 | P21 | C2 | P222 | P2221 | P21212 | P212121 | C222 | C2221 | F222 | I222 | I212121 | P4 | P41 | P42 | P43 | P422 | P4212 | P4122 | P41212 | P4222 | P42212 | P4322 | P43212 | I4 | I41 | I422 | I4122 | P3 | P31 | P32 | P312 | P321 | P3112 | P3121 | P3212 | P3221 | P6 | P61 | P62 | P63 | P64 | P65 | P622 | P6122 | P6522 | P6222 | P6422 | P6322 | R3 | R32 | P23 | P213 | P432 | P4232 | P4332 | P4132 | F23 | F432 | F4132 | I23 | I213 | I432 | I4132 deriving (Show) has you can see it is quite long. I want to write a method in order to parser a string using the show method. myparser :: Parser SpaceGroup myparser = do ... I do not want to write for each constructor something like do t <- takeText if t == "P21" then P21 else ... thanks for your help Frederic

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How to test "error"?
by Hilco Wijbenga 12 Jun '19

12 Jun '19

Hi all, I have a function that may call "error" if the input represents a bug in my program. (Think something like PositiveInt -1; this should never happen so Maybe or Either are not applicable.) f :: Whatever f = error "This should never happen." spec :: Spec spec = describe "f" $ it "error" $ return f `shouldThrow` anyErrorCall The above does not work, I get "did not get expected exception: ErrorCall". Using "anyException" doesn't work either. How can I write a test for this? And (assuming I can get the test to work), how do I check that the error message is indeed what I expect? Cheers, Hilco

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How can I view a digit on the N-th position of 9^(9^9) in Haskell?
by Sergej KAREL 07 Jun '19

07 Jun '19

Hello, Im looking for setting up a solver for the determination of digit on N-th position of the high value number Best regards Sergej

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Where is my stack ghci?
by Lawrence Bottorff 29 May '19

29 May '19

I've installed Haskell the stack way, but a > which ghc or > whcih ghci comes up with nothing. How can I tell my (Ubuntu 19.04) system where to find ghci? LB

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Emacs org-mode code block problem
by Lawrence Bottorff 27 May '19

27 May '19

I've got this in an Emacs org-mode code block: #+begin_src haskell :results output doubleSmallNumber4 x = if x > 0 then x else x * 2 #+end_src but when I try to run it (with C-c C-c) I get ... Prelude> doubleSmallNumber4 x = if x > 0 then x else x * 2 "org-babel-haskell-eoe" <interactive>:19:23: error: parse error (possibly incorrect indentation or mismatched brackets) Prelude> <interactive>:20:11: error: parse error (possibly incorrect indentation or mismatched brackets) Prelude> <interactive>:21:5-8: error: parse error on input ‘then’ Prelude> <interactive>:22:5-8: error: parse error on input ‘else’ Prelude> "org-babel-haskell-eoe" ... I can run #+begin_src haskell tripleMe x = x + x + x #+end_src just fine though -- which means I've done something right with my Emacs init. The REPL started is 8.6.3 stack-ghci. I've formatted with the org-mode C-c ' LB

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