- Beginners - Haskell.org

HXT and missing or null values
by Ian Knopke 13 Jun '12

13 Jun '12

I have an xml file I'm trying to parse using HXT. This is the code (for some reason the list seems to be rejecting the XML example): data Thing = Thing {first :: String, second :: String} getThings = atTag "thing" >>> proc x -> do first <- text <<< atTag "first" -< x second <- text <<< atTag "second" -< x returnA -< Thing {first = first,second = second} atTag tag = deep (isElem >>> hasName tag) text = getChildren >>> getText parseThings str = runX (readString [withValidate no] str >>> getThings) The problem is that sometimes one of the tags is empty. I'd like the code to return something like Thing {first="val",second=""} but instead the entire entry is skipped. Can someone explain to me what I need to do to get a null or empty string value inserted instead? Ian

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Re: [Haskell-beginners] wrapping text in a multiline string
by Chaddaï Fouché 10 Jun '12

10 Jun '12

On Fri, Jun 8, 2012 at 1:33 PM, Rico Moorman <rico.moorman(a)gmail.com> wrote: >> Or you could just use matchM : >> >>> go text = case RE.matchM regex' text of >>> Just (before, match, after) -> >>> before ++ replace' match ++ go after >>> _ -> text >> >> and have match be a string, just like before (since you don't use all >> the power of MatchText anyway). > > Thank you very much for this suggestion. It works correctly. This is > the final function (for reference): > > regexReplace :: String -> (String -> String) -> String -> String > regexReplace pattern replace text = go text > where > regex = RE.makeRegexOpts compOpts RE.defaultExecOpt pattern > compOpts = RE.compMultiline + RE.compDotAll > go text = case RE.matchM regex text of > Just (before, match, after) -> > before ++ replace match ++ go after > _ -> text > > It is really interesting that the result of a function can behave like > this though. I mean that a different structure is returned based on > the type (inferred/given) within the dependant function, e.g. [String] > or Bool ... This is something that's possible with typeclass in Haskell and one of the way they differ from interfaces in the OO world. The classic example is the Read typeclass and "read :: (Read a) => String -> a". > One other thing I saw is that there can be a runtime error using the > option compUTF8 even if the compNoUTF8Check is set too. I get: > > user error (Text.Regex.PCRE.String died: (ReturnCode (-10),"Error in > Text.Regex.PCRE.Wrap: ReturnCode (-10)")) > > which is the error code for bad utf8 (PCRE_ERROR_BADUTF8) according to > http://www.pcre.org/pcre.txt . Luckily I don't seem to need those > options (yet). Would there be some way to "catch"/trace/fix those > error somehow? Fix I don't know since that's an internal error of the PCRE package or library but you can catch exceptions though only in the IO monad. In this particular case there may be an alternative since what fails is the compilation of the regex itself and makeRegexOpts has an alternative which handle errors : makeRegexOptsM . In this case writing your whole replace function in the Maybe monad would probably be a good idea : > regexMaybeReplace :: String -> (String -> String) -> String -> Maybe String > regexMaybeReplace pattern replace text = do > let compOpts = RE.compMultiline + RE.compDotAll > regex <- RE.makeRegexOpts compOpts RE.defaultExecOpt pattern > (before, match, after) <- RE.matchM regex text > return $ before ++ replace match ++ regexReplace after > > regexReplace :: String -> (String -> String) -> String -> String > regexReplace pattern replace text = fromMaybe text (regexMaybeReplace pattern replace text) Or something like that. -- Jedaï

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Scoping within arrow notation (using HXT)?
by Michael Alan Dorman 08 Jun '12

08 Jun '12

Hey, Haskellers, I'm trying to use state threaded through an arrow in some HXT code to avoid passing explicit parameters through several layers of functions, but I think I'm not understanding quite what the arrow notation is doing, because when I try to use a value I'm extracting from the state, I'm getting a scope error. I had many ways I was prepared for the code to be wrong, but that one has me baffled. Any suggestions? Mike. {-# LANGUAGE Arrows, NoMonomorphismRestriction #-} module HXTTest () where import Text.XML.HXT.Core data Info = Info { value :: String } deriving (Show) info = Info { value = "foo" } html = "<html><head></head><body><div class='foo'>llama</div></body>"; -- print (runSLA (getState >>> arr value) info html) -- Div class is static, no reference to state findFoo = proc content -> do (deep (isElem >>> hasName "div" >>> hasAttrValue "class" (== "foo"))) -< content -- print (runSLA (hread >>> findFoo) info html) -- Extract class from state, but don't use it findFoo' = proc content -> do divName <- (getState >>> arr value) -< content content >- (deep (isElem >>> hasName "div" >>> hasAttrValue "class" (== "foo"))) -- print (runSLA (hread >>> findFoo') info html) -- Extract class from state, try to use it: "Not in scope: `divName'" -- findFoo'' = -- proc content -> do -- divName <- (getState >>> arr value) -< content -- content >- (deep (isElem >>> -- hasName "div" >>> -- hasAttrValue "class" (== divName)))

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Re: [Haskell-beginners] wxHaskell install errors
by Gregory Guthrie 07 Jun '12

07 Jun '12

No, I am not sure of this - I followed the instructions at: http://www.haskell.org/haskellwiki/WxHaskell/Windows#wxWidgets_2.9_and_wxHa… The describe the "install" as the wxWidgets install of source code, and then how to compile it, and then just to do cabal installs, which is what is failing. The cabal install error describes some configuration options, but I don't know where or how they are intended to be used. This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. I see in my cabal directory that it has files : Wx-0.12.1.6, Wxcore-0.12.1.7 Wxdirect-0.12.1.4 Wxdirect-0.90.0.1 I also notice that these instructions require a bunch of PATH fiddling that I never had to do before for wxWidgets /wxHaskell 2.8. Also that it asks to install a path pointer to the cabal directory to \wxc-%WXC_VERSION%\%GHC_VERSION% which file does not exist on my system. ?? > -----Original Message----- > Subject: Re: [Haskell-beginners] wxHaskell install errors > To: beginners(a)haskell.org > > C: >cabal install wx > > Resolving dependencies... ... > > setup\setup.exe ... > > Configuring wxc-0.90.0.3... > > Configuring wxc to build against wxWidgets 2.9 > > > > setup.exe: Missing dependencies on foreign libraries: ... > Are you sure that you installed wxWidgets 2.9 and didn't just compile it from source? It > looks like cabal can't find the wxWidgets libraries on your system.

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wrapping text in a multiline string
by Rico Moorman 07 Jun '12

07 Jun '12

Hello, I have a given piece of multiline HTML (which is generated using pandoc btw.) and I am trying to wrap certain elements (tags with a given class) with a <div>. I already took a look at the Text.Regex.PCRE module which seemed a reasonable choice because I am already familiar with similar regex implementations in other languages. I came up with the following function which takes a regex and replaces all matches within the given string using the provided function (which I would use to wrap the element) import Text.Regex.PCRE ((=~~)) -- Replaces the whole match for the given regex using the given function regexReplace :: String -> (String -> String) -> String -> String regexReplace regex replace text = go text where go text = case text =~~ regex of Just (before, match, after) -> before ++ replace match ++ go after _ -> text The problem with this function is, that it will not work on multiline strings. I would like to call it like this: newBody = regexReplace "<table class=\"sourceCode\".*?table>" wrap body wrap x = "<div class=\"sourceCodeWrap\">" ++ x ++ "</div>" Is there any way to easily pass some kind of multiline modifier to the regex in question? Or is this approach completely off and would something else be more appropriate/haskelly for the problem at hand? Thank you very much in advance.

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wxHaskell path
by Henry Lockyer 07 Jun '12

07 Jun '12

Hi all Trying to work out how to get wxHaskell correctly installed on mac os x (10.6.8 snow leopard). Have decided to try and get the older version working (0.13) so as to use the pre-installed wxWidgets (2.8.8 it seems). I was looking at the wxHAskell install info for mac os x at (1) http://www.haskell.org/haskellwiki/WxHaskell/MacOS_X - which only talks about widgets 2.9, but I have now been reading (2) http://www.haskell.org/haskellwiki/WxHaskell/2.8 Question: The variable "$wxwin" is referred to in the page at link (2) above, where it says: "We assume in this guide that the variable $wxwin points to your wxWidgets installation directory, for example: ~/dev/wxGTK-2.8.10." And in the page at link (1), step 3, it says: "Check your path to make sure you are using your wxWidgets and not the default Mac one" Are these referring to the same thing? ( I had originally interpreted 'path' as referring to $PATH.) The reference to $wxwin is in instructions for building wxWidgets 2.8 which I should not need to do if I am using the already installed 2.8.8 widgets library, but if I echo $wxwin it is blank and does not therefore appear to point to my wxWidgets installation directory. Is this an issue or is it only relevant to the initial build process for wxWidgets? Can anyone throw any light on this? Thanks/ Henry

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reactiva-banana : how to implement a certain type of throttling
by Miguel Negrao 07 Jun '12

07 Jun '12

Hi list, I would like to implement a certain type of throttling of events in reactive-banana. It should work such that an event is not let through if arrives at less then delta seconds from the last event that passed through. If it is not let through then it is stored and is fired after delta seconds from the last fired event. Below is a program that implements this for lists of time stamped numbers. Would it be possible to translate this to reactive-banana ? Also, in reactive-banana how do I fire an event x seconds after some other event comes in. The way I would do it in scala’s reactive-web is to start a timer that fires x seconds after it is started when an event comes in using dynamic event switching . It seems to me this would not be possible in reactive-banana because starting a timer is an IO operation, so I assume it can’t be done inside the event/behavior logic, right ? best, Miguel Negrão module Main where import Data.List -- 1 second throtling -- logic is to never output a value before 1 second has passed since last value was outputed. main :: IO() main = print $ test [ (0.0, 1.0), (1.1, 2.0), (1.5,3.0), (1.7,4.0), (2.2, 5.0) ] --should output [ (0.0, 1.0), (1.1, 2.0), (2.1,4.0), (3.1, 5.0) ] test :: [(Double,Double)] -> [(Double,Double)] test list = g v (concat xs) where (v, xs) = mapAccumL f (-50,Nothing) list g (t, Just x) ys = ys ++ [ (t+1,x) ] g _ ys = ys f (lasttime, Just holdvalue) (t,x) = if t > (lasttime+1) then if t > (lasttime + 2) then ( (t, Nothing), [ (lasttime+1,holdvalue), (t,x)] ) else ( (lasttime+1, Just x) , [ (lasttime+1,holdvalue) ] ) else ( (lasttime, Just x), [] ) f (lasttime, Nothing) (t,x) = if t > (lasttime+1) then ( (t,Nothing) , [ (t, x ) ] ) else ( (lasttime, Just x), [] )

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Strange difference in behaviour between ghc and ghci
by Matthew Moppett 07 Jun '12

07 Jun '12

I have a very simple Haskell file (HelloWorld.hs) that reads like this: main = do putStr "What's your name? " n <- getLine putStrLn $ "Pleased to meet you, " ++ n When I load it into ghci, I get the following result, as expected: [1 of 1] Compiling Main ( HelloWorld.hs, interpreted ) Ok, modules loaded: Main. *Main> main What's your name? Matt Pleased to meet you, Matt *Main> However, when I compile the same file using ghc and run it in a terminal, I get a very different result: matt@matt-Lenovo-G575:~/Haskell$ ghc HelloWorld.hs [1 of 1] Compiling Main ( HelloWorld.hs, HelloWorld.o ) Linking HelloWorld ... matt@matt-Lenovo-G575:~/Haskell$ ./HelloWorld Matt What's your name? Pleased to meet you, Matt matt@matt-Lenovo-G575:~/Haskell$ -- in other words, the getLine action is being run before the putStr action, for some strange reason. Is this a bug? Can anyone enlighten me as to what might be going on? Regards, Matt.

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flatten comma operator
by Kees Bleijenberg 07 Jun '12

07 Jun '12

In 'Real World Haskell' I found code like LiftA2 (,) .... Looks odd. But after some experimenting in winghci I found that (,) 1 2 is valid code and is equal to (1,2). Now I wonder how to create (1,2,3). I think you need a join or a flatten function or ...? Join doesn't work? Kees

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Re: [Haskell-beginners] wxHaskell install errors
by Gregory Guthrie 07 Jun '12

07 Jun '12

I already did this, this is the instruction given in: http://www.haskell.org/haskellwiki/WxHaskell/Windows I copied a summary of this issue(s) to the wxHaskell mailing list, hopefully they can give some assistance. ------------------------------------------- > To install wxWidgets you can find how in the 'Readme' as I did, or you can use what I have > used on windows 7: > > cd c:\wx\build\msw > mingw32-make -f makefile.gcc BUILD=debug > > I'm not sure if the 'BUILD=debug' is really needed, but it worked for me. > > Hope it helps, > Carlos Gomez >

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