July 2019 - Beginners

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

2 1

binding vs. parameter passing
by James Jones 22 Jul '19

22 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?

2 1

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.

3 4