September 2016 - Beginners

Multiline code in GHCi
by Shishir Srivastava 29 Aug '17

29 Aug '17

Hi, I am trying to run this basic multiline code in GHCi. I am not sure if this is even possible without writing it in a file and compiling it in a traditional way. ------- Prelude> :set +m Prelude> let main1 = do Prelude| a <- readLn Prelude| return a Prelude| Prelude> Prelude> main1 <interactive>:75:1: No instance for (Show (IO b0)) arising from a use of `print' In the first argument of `print', namely `it' In a stmt of an interactive GHCi command: print it ------- Could someone please confirm if the function can be defined and called like this in GHCi or point to where am going wrong. Thanks, Shishir Srivastava

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The meaning of categories constructed from HASK
by Dimitri DeFigueiredo 25 Oct '16

25 Oct '16

In category theory, there are many ways one can make new categories out of an old one. In particular, given a category C one can construct: 1. The arrows category of C: arrows in C become objects and commutative squares in C become arrows 2. The slice category of C given an object A: arrows into a distinguished object A become objects in the slice commutative triangles become arrows There are also functors going from C to these new categories (and back). Are these constructed categories useful when C = `Hask` (the category of haskell types and functions)? What do they represent in programming terms? In other words, is there intuition for what the arrows category of Hask is? What about the slice category of Hask over a specific type? Do the functors between these match some programming abstractions? Any pointers are much appreciated. Thanks, Dimitri

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Re: [Haskell-beginners] how to enter EOF in emacs haskell mode
by Shrivats 06 Oct '16

06 Oct '16

Hi, If it's anything like other interactive modes, does C-c C-d do the job of signalling EOF? Shrivats On Sep 14, 2016 15:53, "Chul-Woong Yang" <cwyang(a)aranetworks.com> wrote: Hi, all. I use emacs haskell mode for editing and testing simple programs. I have no success in finding out how to enter EOF to haskell program in emacs "Interactive-Haskell" buffer. So I am unable to test haskell program which reads from stdin. :-( Any help would be appreciated deeply. Regards, Chul-Woong Yang _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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short-circuit like Maybe monad with a difference
by Dennis Raddle 30 Sep '16

30 Sep '16

I have a function 'evalRule' that applies rules to steps in a backtracking search optimization problem and evaluates the step's fitness or decides that it must be eliminated entirely. In the following, a result of Nothing means "eliminate the step," Just x means the step has fitness score x. evalRule :: Rule -> Step -> Maybe Double I would like to write a function that applies a bunch of rules but short-circuits the computation if it hits Nothing. However, unlike the way the Maybe monad works, I want to know the partial results. In the following, the rules are applied in order and the "Just" results are collected up the point where a rule returns Nothing. evalRules :: [Rule] -> Step -> [Double] What's a nice way to do this? D

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Could not deduce (Frame a r0) arising from a use of `len'
by PICCA Frederic-Emmanuel 30 Sep '16

30 Sep '16

Hello I try to write something like this {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE RankNTypes #-} module Hkl.Frame ( Frame(..) , frames , frames' ) where import Pipes import Control.Monad class (Frame a r) where len :: a -> IO (Maybe Int) row :: a -> Int -> IO r frames :: (Frame a r) => Pipe a r IO () frames = do d <- await (Just n) <- lift $ len d forM_ [0..n-1] (\i' -> do f <- lift $ row d i' yield f) But when I try to compile this I get this error message. Could not deduce (Frame a r0) arising from a use of `len' from the context (Frame a r) bound by the type signature for frames :: Frame a r => Pipe a r IO () at src/Hkl/Frame.hs:17:11-39 The type variable `r0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) In the second argument of `($)', namely `len d' In a stmt of a 'do' block: (Just n) <- lift $ len d In the expression: do { d <- await; (Just n) <- lift $ len d; forM_ [0 .. n - 1] (\ i' -> do { f <- lift $ row d i'; .... }) } What should I do in order to solve this problem. Thanks Frederic

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Why isn't :t (1+1) Integer
by Lai Boon Hui 29 Sep '16

29 Sep '16

Hi, i read that the default type for Num is Integer. If we specify a type for one of the arguments: :t (1 + 1::Double) is Double If we don't, i expect the default type for Num to come into play: However :t (1 + 1) is Num a => a Why is that so? -- Best Regards, Boon Hui

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Haskell Platform on OS X, Sierra
by Dennis Raddle 29 Sep '16

29 Sep '16

I have been running the Haskell Platform on OS X El Capitan for a few months. I just upgraded to Sierra and I notice it broke something. For instance when I run ghci I get xcrun: error: invalid active developer path ... missing xcrun .... What's the best way to recover? Should I uninstall and reinstall Haskell Platform? BTW, how DO you uninstall the Haskell Platform on OS X?

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A seemingly simple use-case for Template Haskell
by Mario Lang 29 Sep '16

29 Sep '16

Hi. As a long-term Lisp fan, and someone who always admired compile-time code-generation, I wanted to at least do something simple with Template Haskell once. In a small project of mine, I have this basically auto-generated data type: -- Braille music code only uses the old 6-dot system. We enumerate all -- possible dot patterns to use the type system to avoid accidentally -- specifying invalid dot patterns in the source code. -- -- gen :: String -- gen = -- "data Braille = " ++ intercalate " | " ctors ++ " deriving (Enum, Eq)" where -- ctors = "NoDots" : map ctorName [1..63] where -- ctorName :: Int -> String -- ctorName = (++) "Dot" . concatMap (show . succ) . flip filter [0..5] . testBit data SixDots = NoDots | Dot1 | Dot2 | Dot12 | Dot3 | Dot13 | Dot23 | Dot123 | Dot4 | Dot14 | Dot24 | Dot124 | Dot34 | Dot134 | Dot234 | Dot1234 | Dot5 | Dot15 | Dot25 | Dot125 | Dot35 | Dot135 | Dot235 | Dot1235 | Dot45 | Dot145 | Dot245 | Dot1245 | Dot345 | Dot1345 | Dot2345 | Dot12345 | Dot6 | Dot16 | Dot26 | Dot126 | Dot36 | Dot136 | Dot236 | Dot1236 | Dot46 | Dot146 | Dot246 | Dot1246 | Dot346 | Dot1346 | Dot2346 | Dot12346 | Dot56 | Dot156 | Dot256 | Dot1256 | Dot356 | Dot1356 | Dot2356 | Dot12356 | Dot456 | Dot1456 | Dot2456 | Dot12456 | Dot3456 | Dot13456 | Dot23456 | Dot123456 deriving (Bounded, Enum, Eq, Read, Show) So, while actually quite simple, this looks like an opportunity to use Template Haskell for me. In other words, I want to figure out what is necessary to generate this data type with TH, instead of the gen function that basically generates a piece of plain Haskell code. I have been reading "A practical Template Haskell Tutorial"[1] but I find it a little bit too terse to actually solve this very little riddle on my own. For one, I find it confusing that some TH functions return "Q Dec" while others just return Dec. I am aware that this is some sort of Monad for the TH system, but I have never seen it explained anywhere. Also, all the examples I can find seem to be mostly focused in generating Q Exp or similar, but I didn't really find an example for Q Dec. I realize this should be simple to figure out on my own, but it apparently is not. I have tried to wrap my head around this on my own at least three times now, but always stopped after an hour or two due to frustration. Is there some comprehensive TH documentation I haven't seen yet? Could you please give me enough of a head-start that I actually manage to write something which can generate this simple data type above? [1] https://wiki.haskell.org/A_practical_Template_Haskell_Tutorial -- CYa, ⡍⠁⠗⠊⠕

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ftp-client for Windows
by Kees Bleijenberg 27 Sep '16

27 Sep '16

Is there a library with a ftp client that works on Windows? I tried ftphs but I get segmentation faults. Kees

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Exception/Error Handling Best Practices
by Hillary Ryan 25 Sep '16

25 Sep '16

Dear Haskellers, I've been working on variants of the following code: connect_from_file :: String -> IO Connection connect_from_file fi = do s <- BL.readFile fi cred <- (decode s :: Maybe Db_cred) c <- connect . reformat_cred $ cred return c What this code intends to do is read a config from a file in JSON, decode the JSON, and then open a connection based the decoded info. This code doesn't compile for several reasons, but I've left it in this form because I believe it is the most convenient for approaching my question: How should I handle errors here? Both readFile (from Bytestring) and connect (from mysql.simple) throw exceptions when things don't go their way, and decode uses maybe. I see four ways of handling these errors: 0) *All typed errors: *I could "capture" all the exceptions in Either types and use an EitherIO monad or transformer to automatically abort when a Left value appears. Now my function looks like: String -> IO (Either MyErrorType Connection) 1) *All exceptions: *I could throw an exception if Nothing appears during decoding. The function looks the same: String -> IO Connection 2) *A mix*: I could leave the connect and readFile functions alone, but return IO (Nothing) if the decoding doesn't work out. Now we have: String -> IO (Maybe Connection) 3) Not 0)-2) So what is the proper way to do things? Many Thanks, Hillary PS For what it is worth, I have read: 0) https://www.schoolofhaskell.com/user/commercial/content/ exceptions-best-practices . I'd love to get feedback on how to apply this article to my question if it is appropriate. 1) https://lexi-lambda.github.io/blog/2016/06/12/four-months-with-haskell/ . From this article and others, I get the sense that there is great confusion in the community as to how to handle errors correctly, and worse yet, that different approaches are not easily compatible.

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