- Beginners - Haskell.org

01 Oct '13

HP includes the html package, but everything I've seen that produces html uses blaze-html. Is html included for some historical reason?

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scotty installation error
by Ovidiu D 01 Oct '13

01 Oct '13

I'm trying to write a small web application starting from here: http://adit.io/posts/2013-04-15-making-a-website-with-haskell.html Resolving dependencies... Configuring scotty-0.2.0... Building scotty-0.2.0... Preprocessing library scotty-0.2.0... Web/Scotty.hs:35:8: Could not find module `Control.Monad.Trans.Resource' It is a member of the hidden package `resourcet-0.4.8'. Perhaps you need to add `resourcet' to the build-depends in your .cabal file. Use -v to see a list of the files searched for. Failed to install scotty-0.2.0 My build-depends clause is the following: build-depends: base ==4.5.* , wai ==1.3.* , warp ==1.3.* , http-types ==0.7.* , resourcet , scotty , text , bytestring , blaze-html , persistent , persistent-template , persistent-sqlite , persistent-postgresql , heroku , transformers ==0.3.0.0 , wai-middleware-static ==0.3.2 , wai-extra , time , monad-logger ==0.2.4 Any idea what is wrong with it? I also tried to use the dependencies list without any versions and I get the same error Thanks ovidiu

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Getting Documentation with Command Line Hoogle Results
by Changlin Li 27 Sep '13

27 Sep '13

Hello all, I've installed the command line version of Hoogle and everything seems great. However, I was hoping to get a short documentation blurb with each type signature as is the case when I search on the actual Hoogle website. Is there any way to enable this on the command line (I've already set the Cabal configuration option for documentation to True) and specifically, how can I do so for functions in Prelude and other built-in modules? Thanks! Changlin Li

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:trace seems not to work in ghci
by Nathan Hüsken 26 Sep '13

26 Sep '13

Hey, I have a program I want to debug in ghci because it crashes with an exception. So I do: Prelude> :l Main Ok, modules loaded: Main, PlotDisplay, PlotDiagram, ForexData. Prelude Main> :set -fbreak-on-exception Prelude Main> :trace main (...) Stopped at <exception thrown> _exception :: e = _ [<exception thrown>] Prelude Main> :hist Empty history. Perhaps you forgot to use :trace? [<exception thrown>] Prelude Main> :back no more logged breakpoints [<exception thrown>] Prelude Main> :force _exception _exception = GHC.Exception.SomeException (GHC.Exception.ErrorCall "Prelude.tail: empty list") What could be the reason why :hist/:back/:trace are not working? Thanks! Nathan

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finding the exact instance of a function causing an error
by Nathan Hüsken 25 Sep '13

25 Sep '13

Hey, I have a program, that, when I run it fails with: Main: Prelude.tail: empty list Can I somehow find out which exat instance of tail causes this error? I tried compiling with -prof and running with +RTX -xc, which gives me: *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace: GHC.List.CAF --> evaluated by: SYSTEM.SYSTEM *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace: GHC.List.CAF *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace: GHC.List.CAF Does not really help me ... Thanks! Nathan

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Overwriting wxFrame::ProcessEvent in wxHaskell
by Nathan Hüsken 25 Sep '13

25 Sep '13

Hey, In C++ wxWidgets code, I discovered (by looking at the richtext example) that when I overwritte the ProcessEvent function of my main frame, a few wonderful effects occur: * Cut/Copy/Paste/Undo/Redo menu entries work out of the box for wxTextCtrl and other simply by creating menu items with the correct id. * The menu items are also updated to be enabled or disabled correctly. I want that in wxHaskell to! But I do not know if and how I can overwrite the ProcessEvent function of wxFrame in wxHaskell ... anyone knows if there is a way? There is windowPushEventHandler, which might be what I want. But it takes "EvtHandler a" as argument, and I do not know how to write an EvtHandler with my own event handling function ... Regards, Nathan For completeness, the C++ version of what I want (taken from richedit.cpp): bool MainFrame::ProcessEvent(wxEvent& event) { if (event.IsCommandEvent() && !event.IsKindOf(CLASSINFO(wxChildFocusEvent))) { // Problem: we can get infinite recursion because the events // climb back up to this frame, and repeat. // Assume that command events don't cause another command event // to be called, so we can rely on inCommand not being overwritten static int s_eventType = 0; static wxWindowID s_id = 0; if (s_id != event.GetId() && s_eventType != event.GetEventType()) { s_eventType = event.GetEventType(); s_id = event.GetId(); wxWindow* focusWin = wxFindFocusDescendant(this); if (focusWin && focusWin->GetEventHandler()->ProcessEvent(event)) { //s_command = NULL; s_eventType = 0; s_id = 0; return true; } s_eventType = 0; s_id = 0; } else { return false; } } return wxFrame::ProcessEvent(event); }

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How to understand the type "ShowS"?
by yi lu 25 Sep '13

25 Sep '13

Prelude> :i ShowS type ShowS = String -> String -- Defined in `GHC.Show' It is a type of a function? I cannot understand this type, and don't know how to create functions of this type. And this function "shows" Prelude> :i shows shows :: Show a => a -> ShowS -- Defined in `GHC.Show' I don't know how this function works. Yi

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Evaluation Order Semantics
by Mike Meyer 24 Sep '13

24 Sep '13

> > > Date: Tue, 24 Sep 2013 12:01:51 +0800 > From: ??? <wbbtiger(a)gmail.com> > > Yes, if the spec does not state the evaluation order I can't predict the > performance definitely. > Well, depending on how definite you want to be, an inability to predict the performance definitely is inherent in using compiled languages. That the performance can change if you change the compilation options shows that. On the other end of the scale, if you correctly implement an O(n) algorithm, then it's definitely going to take at most O(n) steps to print the results from it whether the language is strict or lazy. Being more definite does get harder in the face of lazy evaluation, but it also gets harder in the face of optimization. A lazy language will not evaluate expressions whose value is never used. So will a good optimizing compiler for a strict language. Can you be a bit more explicit in what about performance you're trying to predict? Note to Keshav - yes, evaluating expressions you don't have to can change the results. But I was talking about the evaluation order for expressions you actually evaluate, though maybe I wasn't clear about it. Thanks for pointing that out, though.

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Re: [Haskell-beginners] vim - quickly take suggestions from ghc, in case of compile errors
by Rene＠gmail 24 Sep '13

24 Sep '13

Hi Nathan, try ghc-mod (cabal install ...) and syntastic (https://github.com/scrooloose/syntastic/) cheers, René On 24.09.2013 16:30, beginners-request(a)haskell.org wrote: > Message: 4 > Date: Tue, 24 Sep 2013 16:08:24 +0200 > From: Nathan H?sken <nathan.huesken(a)posteo.de> > To: Haskell Beginners Mailinglist <beginners(a)haskell.org> > Subject: [Haskell-beginners] vim - quickly take suggestions from ghc > in case of compile errors > Message-ID: <52419CD8.8010206(a)posteo.de> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > Hey, > > ghc has a nice feature, when one compiles a haskell program and has made > a spelling error, it suggest similar names that could be what one meant. > > Example: > Data.hs:24:44: > Not in scope: `mont_' > Perhaps you meant `month_' (line 10) > > Is there a vim plugin that takes advantage of this? So that I can > compile my haskell project, and for errors where ghc has a suggestion > insert the suggestion with a simple key press? > > Thanks! > Nathan

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Split list by list using Continuations
by Dmitriy Matrosov 24 Sep '13

24 Sep '13

Hi. I want to write a function to split list by list. E.g. if i have input list "aXYbc" and list "XY" is separator, then result should be ["a", "bc"]. And i want to write it using Continuations. Here is my version, which implements following scheme: .. >>= f x(k+1) >>= f x(k+2) >>= f x(k+3) >>= f x(k+4) >>= .. ....(match to sep) ..........>+ | (failed) +<= (continuation backward) <=+ (add to word) | \------->..(match to sep).....+ | (succeed) +------>+.. (match to sep) .. > import qualified Data.Foldable as F > import Control.Applicative > import Control.Monad.Cont > > nullF :: F.Foldable t => t a -> Bool > nullF = null . F.toList > > addToHeadA :: Alternative f => a -> [f a] -> [f a] > addToHeadA x [] = [pure x] > addToHeadA x (y : ys) = (pure x <|> y) : ys > > type Sep a = [a] -- Word separator. > type Res5 f a = [f a] -- Result. > data SplitState5 m f a = MaybeSep5 (Sep a) (Res5 f a) > (() -> m (SplitState5 m f a)) > | Word5 (Res5 f a) > > split5M :: (Eq a, F.Foldable t, Alternative f, MonadCont m) => > Sep a -> t a -> m (Res5 f a) > split5M ks0 xs > | nullF xs = return [] > | otherwise = F.foldrM go (Word5 [empty]) xs >>= finalize > where > ksR = reverse ks0 > --go :: (Eq a, MonadCont m) => > -- a -> SplitState5 m f a -> m (SplitState5 m f a) > go _ (MaybeSep5 [] _ h) = h () > go x (MaybeSep5 [k] zs _) > | x == k = return (Word5 (empty : zs)) > go x (MaybeSep5 (k : ks) zs h) > | x == k = return (MaybeSep5 ks zs h) > | otherwise = h () > go x (Word5 zs) = callCC $ \r -> do > callCC $ \h -> go x (MaybeSep5 ksR zs h) >>= r > return (Word5 (x `addToHeadA` zs)) > finalize :: (Alternative f, MonadCont m) => > SplitState5 m f a -> m (Res5 f a) > finalize (Word5 zs) = return zs > finalize (MaybeSep5 _ _ h) = h () >> return undefined > And i have several questions about this implementation: - Is it good CPS implementation? Or there is much simpler and better one? - Can it be improved? - Can i make it more generic? - Would non-CPS implementation be better or simpler, than this one? -- Dmitriy Matrosov

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