August 2013 - Beginners

Type inference in ST monad?
by Aurimas 19 Aug '13

19 Aug '13

I have the following code which does not compile due to explicit type annotation (ST s Double). Error message says usual thing about "s" type variables. ---------------------------------------------------------------------------------------------------------- import Control.Monad.ST import System.Random.MWC (initialize, uniformR, Gen) import Control.Monad.Loops (whileM) import Data.Vector (singleton) import Data.Word(Word32) main :: IO () main = do print $ runSimulation 1 runSimulation :: Word32 -> [Int] runSimulation seed = runST $ do gen <- initialize (singleton seed) whileM (do r1 <- uniformR (-1.0, 1.0) gen :: ST s Double -- does not compile due to this if r1 > 0.0 then return True else return False) (do r2 <- uniformR (0, 10) gen if r2 > 5 then return r2 else return 0) --------------------------------------------------------------------------------------------------------- if I rewrite runSimulation like this (below), everything is OK. --------------------------------------------------------------------------------------------------------- runSimulation :: Word32 -> [Int] runSimulation seed = runST $ do gen <- initialize (singleton seed) whileM (do r1 <- tempFun gen if r1 > 0.0 then return True else return False) (do r2 <- uniformR (0, 10) gen if r2 > 5 then return r2 else return 0) where tempFun :: Gen s -> ST s Double -- this line automatically provides required type annotation tempFun g = uniformR (-1.0, 1.0) g --------------------------------------------------------------------------------------------------------- Ca somebody explain what's wrong with the first version? Best Regards, Aurimas

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Re: [Haskell-beginners] Compiler can't deduce Bool as instance of ToField
by Bryan Vicknair 15 Aug '13

15 Aug '13

> > Bryan Vicknair: > > postgresql-simple declares Bool as an instance of the ToField class. The > > compiler can't deduce that given this simple code however: > > > > import Database.PostgreSQL.Simple.ToField (ToField(..)) > > > > foo :: (ToField a) => a > > foo = True > > > > > > It fails with this error: > > > > Db.hs:64:7: > > Could not deduce (a ~ Bool) > > from the context (ToField a) > > bound by the type signature for foo :: ToField a => a > > at Db.hs:63:8-23 > > `a' is a rigid type variable bound by > > the type signature for foo :: ToField a => a at Db.hs:63:8 > > In the expression: True > > In an equation for `foo': foo = True > > Failed, modules loaded: none. > > > > What am I missing? > > > Oliver Charles > You have declared that foo is *any* type that has a ToField instance, > allowing the caller of foo to determine the type at their will. However, > your implementation of foo is more specific and requires a is actually Bool > and nothing else. > On 14 Aug 2013 10:24, "Bryan Vicknair" <bryanvick(a)gmail.com> wrote: I see. I was confused, because the following works: bar :: (Num a) => a bar = 5 I guess it is because the literal '5' could be an Int or a Float, and the type system doesn't know. When I saw that the following does not work...:: bar :: (Show a) => a bar = False ...it made a bit more sense. It seems that if a literal can be considered part of a typeclass in more than one way, you can declare that literal to be of that typeclass. However, if there is only one way for a literal to belong to a typeclass, then you can't. Are there other literals (besides (Num a) => [a]) that don't have a concrete type like literal numbers? If I understand correctly, -XOverloadedStrings turns all literal strings into IsString instead of [Char], so that seems to be another case. I don't have a need for this, but now I'm curious: is there a way to declare that all literal numbers in a module are of a certain concrete type? Something like -XAllNumbersAreInt8?

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Trouble with layout in wxHaskell
by Nathan Hüsken 15 Aug '13

15 Aug '13

Hey, I am trying to understand how do a layout in wxHaskell. I am trying to make a frame with 2 text entries. One single lined at the top and one multiline below it. Both should expand with the window size horizontal. When the window is expanded vertical, only the multiline text entry should expand. Her is what I wrote: main = start mainFrame mainFrame = do f <- frame [text := "BiVision"] buffer <- textEntry f [] mainArea <- textCtrl f [] set f [layout := expand $ column 2 [hstretch $ widget buffer, stretch $ widget mainArea]] This gives me a window with the desired controls. But they are not expanding. Why? I tried different combinations of expand and stretch (I do not really understand the difference?). But nothing works. I am guessing there is something simple I do not yet understand :). Thanks for any help! Nathan

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Heterogeneous Lists
by harry 14 Aug '13

14 Aug '13

Every OO language which supports generics allows a declaration such as List<Show> alist, where Show is an interface. Any type implementing Show can be put in alist, and any Show operation can be performed on the alist's members. No casts, wrappers, or other special types and plumbing are needed. Why isn't it possible to do this directly in Haskell?

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Compiler can't deduce Bool as instance of ToField
by Bryan Vicknair 14 Aug '13

14 Aug '13

postgresql-simple declares Bool as an instance of the ToField class. The compiler can't deduce that given this simple code however: import Database.PostgreSQL.Simple.ToField (ToField(..)) foo :: (ToField a) => a foo = True It fails with this error: Db.hs:64:7: Could not deduce (a ~ Bool) from the context (ToField a) bound by the type signature for foo :: ToField a => a at Db.hs:63:8-23 `a' is a rigid type variable bound by the type signature for foo :: ToField a => a at Db.hs:63:8 In the expression: True In an equation for `foo': foo = True Failed, modules loaded: none. What am I missing?

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Sorting Bank Accounts problem in Haskell
by Nadav Chernin 14 Aug '13

14 Aug '13

Hi all, I learn Haskell and i try to solve questions from SPOJ. Currently, i try to solve problem *Sorting Bank Accounts*<http://www.spoj.com/problems/SBANK/> Here is my code: *import qualified Data.Map as M* * * *extractTest []=[]* *extractTest (x:xs)=(take n xs):(extractTest (drop (n+1) xs)) where* * n=read x* * * *emp=M.empty* * * *collect m []=m* *collect m (x:xs)* * |M.member x m =collect (M.insert x (amount+1) m) xs* * |otherwise=collect (M.insert x 1 m) xs* * where* * Just amount=M.lookup x m* * * *getList m=map (\(x,k)->x++" "++(show k)) (M.toList m)* *f x=unlines$(getList (collect emp x))++[""]* *main=getLine>>=(\x->interact$unlines.map f.take (read x).extractTest.lines) * * * The problem is that i can't finish it during 7 sec. So i want to know is there more quick solution/method for this problem. Thanks, Nadav

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cabal binary packages
by harry 13 Aug '13

13 Aug '13

I need to build some software on a server, but it doesn't have enough spare memory for compiling all the dependencies. I would like to create a binary distribution of e.g. snap including all of its dependencies (other than those which come with ghc), so that the server can just install them. I found http://www.haskell.org/cabal/users-guide/installing-packages.html#creating-…, but this seems to have two deficiencies - the destination prefix needs to be know in advance (which might be OK if it can be a relative path), and it has to be done individually for each package.

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Text.XML.writeFile question
by Alan Buxton 13 Aug '13

13 Aug '13

Hi I am trying to write an XML file where the filename is created based on a timestamp. Simplified version below. This won't compile - I get this error in doWrite2 filepathtest.hs|24 col 17 error| Couldn't match expected type `system-filepath-0.4.7:Filesystem.Path.Internal.FilePath' || with actual type `String' || In the second argument of `writeFile', namely `t1' || In a stmt of a 'do' block: writeFile def t1 doc || In the expression: || do { t1 <- tsString; || writeFile def t1 doc } Somehow the String "text.xml" in doWrite1 is converted into a FilePath, but not the String t1 in doWrite2. What am I doing wrong? {-# LANGUAGE OverloadedStrings #-} module Filepathtest where import Text.XML import Data.Time.Clock.POSIX (utcTimeToPOSIXSeconds) import Data.Time.Clock (getCurrentTime) import Prelude hiding (writeFile, FilePath) tsString :: IO String tsString = do x <- getCurrentTime let x' = show $ floor $ utcTimeToPOSIXSeconds x return x' doWrite1 :: Document -> IO () doWrite1 doc = writeFile def "test1.xml" doc doWrite2 :: Document -> IO () doWrite2 doc = do t1 <- tsString writeFile def t1 doc

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cabal logs
by vold 12 Aug '13

12 Aug '13

If I set build-summary in cabal's config file, cabal will create duplicate logs - the one specified in build-summary and the default build-reports.log in the packages directory. There's also a log-dir option in the config file which doesn't seem to do anything. Are these bugs, or intended behaviour?

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HTTP Download -> Save File - non-strict
by Dan Krol 11 Aug '13

11 Aug '13

Hi, I'm working on an rss file getter. I was wondering if I could get some help getting files to download and save without holding the entire file in memory in between. I chose Conduit's version of SimpleHttp only because it was recommended, and it was the quickest thing I could get to work correctly because I was eager to get started on this project, so I'd be happy to switch. Here's where I define the download and save functions: https://github.com/orblivion/feedGetter/blob/master/rss.hs#L107 And here's where I use them, getting multiple at a time with async: https://github.com/orblivion/feedGetter/blob/master/rss.hs#L208 What happens when I run this is that it outputs that it's "Getting" the file, waits a while (presumably to download the whole thing), then says it's "Saving". And I checked the file system, it's not there during the pause. I'm not entirely sure why. Is it my choice of libraries, or the way I'm using them? Perhaps something to do with async? I just tried content <- simpleHttp "http://google.com" in ghci, and it does pause for a second, so I'm guessing this is strict from the getgo. But I've done almost no I/O before. Is there a straightforward, canonical option? It seems like there perhaps should be. But if it comes down to using pipes or conduit, what the heck I'll try it out, I'd like to learn pipes eventually. Thanks a lot, Dan

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