April 2009 - Beginners

Fetching a URI
by Adam Keys 06 Apr '09

06 Apr '09

I'm trying to write a wrapper for a REST service. However, I find myself stumped by the simplest part: merely performing a GET on a URI. I'm using the HTTP Cabal package. It specifies an example like so: > simpleHTTP (getRequest "http://www.haskell.org/") >>= fmap (take > 100) . getResponseBody However, it seems I'm either importing the library incorrectly or there is a discrepancy between the documentation and the library. I can't seem to call getRequest. > :) ghc-pkg list > ... > /Users/adamkeys/.ghc/i386-darwin-6.8.2/package.conf: > Cabal-1.6.0.1, HTTP-3001.1.3, HTTP-4000.0.4, Shellac-0.9.5, > ... > > GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help > Loading package base ... linking ... done. > Prelude> import Network.HTTP > Prelude Network.HTTP> :type getRequest > > <interactive>:1:0: Not in scope: `getRequest' Am I missing something obvious here? -- ~akk

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Is this overkill?
by Zachary Turner 05 Apr '09

05 Apr '09

I thought I would try to see if it were possible to write, in point-free form using no lambda functions and no intermediate functions, a function that takes 2 lists of Booleans, computes the pairwise logical AND of the two lists, and returns a list containing the 0 based indices of the elements where the logical and of the two was true. I know that at some point it becomes overkill and for the sake of readability one should know when to draw the line. So I want to see if someone with more experience than me can comment on whether or not this is over the line :P trueIndices = curry $ map fst . filter snd . zip [0..] . map (uncurry (&&)) . (uncurry zip) So do all the uncurries and curries make it too hard to understand or is it pretty easy to read this? For me it takes me a while to figure out by looking at it because it's hard to trace all the currying and uncurrying. And is there a more elegant solution?

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Differences between OO-style Interfaces and type classes. Any 'Haskell' way of doing it?
by Александър Л. Димитров 04 Apr '09

04 Apr '09

Hello folks, thanks for being such a very nice and helpful community :-) I have another pretty boring question… Today I noticed where type classes and OO-Interfaces differ. Up until now, I treated the two notions as pretty much equivalent, but knowing that there are some differences I do not yet understand. Now I do understand at least one of them. Say I have several types Type, Subtype, Subsubtype and so on. All are instances of a type class Class. The constructors all look like > data Type = Type (Maybe Subtype) > data Subtype = Subtype (Maybe Subsubtype) and so on. I would now like to build a tree structure with Data.Tree which only holds the type constructors as partial functions and be able to fold over the tree to yield a complete value of type Type. So a user could select any point of the tree (whether leaf or not) and she would get this node's type combined with all the parent's node's types. A small example: 0,Type | `- 1,A | `- 3,C `- 2,B Selecting node 1 would result in the type (Type (Just (A Nothing))) returned and selecting node 0 would return (Type Nothing). Assuming all types implement Class, I thought equipping the nodes of a Data.Tree with a type like (Class a) => Node (Maybe a -> a) would work. Of course it didn't, because it couldn't unify the Subtype with the Type. The reason I need this is because I have a HaXml generated file with data types from a DTD, and would like to display them in a GTK TreeModel so the user could choose a type there and have it turned to XML. The class of all DTD-originated types is, of course, XmlContent. So, my problem here is: I'd like a tree to store partial functions of possibly different types and then be able to compose a leaf's or non-leaf's payload with all the parent's payloads to yield a final result. Writing up my idea made me realise how ludicrous it is from a theoretical point of view, and that Data.Tree might simply not be the correct data structure for this (alas, it's required for TreeModels in GTK.) At least it can serve as an example to people where type classes do not act like Interfaces ;-) And I'd be very glad to hear about a solution to my problem, it would make some things a lot easier for me :-). Thanks again, Aleks

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Event handling in GTK2hs: managing events and global state
by Александър Л. Димитров 04 Apr '09

04 Apr '09

Hello community, I'm just diving into my first bigger Haskell problems, and got stuck at a certain point with gtk2hs. I couldn't find documentation or even small examples for it, no matter where I looked, so I turn to this list. My basic problem is that I want a certain action to happen based on the effects of a side effect; in my case, loading a file into a widget. Together with the file, I load a data structure, and define an event that will translate clicks on the widget into some magic on the data structure. So when I load a file, I do the following: > loadFile :: GladeXML -> FilePath -> IO () > loadFile xml fp = do widget <- xmlGetWidget -- … > -- do stuff with the widget, yield some 'data' > connectId <- widget `on` buttonPressEvent $ handler data > … (this is a minimal trivial example, to show the crucial point.) So, the handler step depends on the particular contents of the file. This means, that for every file I load, I first have to disable the old action, and only then install a new one, since I generate a new handler every time a file is loaded. Every addition of a handler gives me a connectId, which is its true name. And, as the gtk2hs docs inform me, I have to call it by its true name to dispel it. The gtk2hs docs suggest I 'keep around' the connectId, but herein lies my problem. The loadFile function can be called from several spots in the application (and I'd like to keep it that way.) Even if I wrapped loadFile in a State monad, those entry points to load file (command line, menu, button, possibly more) have no way of knowing the state of connectId, lest I make it global (*shudders*) using unsafePerformIO and a Maybe IORef. Forgive me if I'm missing something obvious, but I have thus far only used monads for serialized code flows, and not for asynchronous, event-driven computation. So the other approach would be (and thanks to mmorrow at freenode for the idea) to wrap my widget component (if I understood correctly) in a new type that also keeps track of its connectedId (namely, the event it's been connected to) via an IORef. This way the state gets 'hidden' in the IO monad, via IORef. I'd end up with something like data WW = WW { widget :: SomeWidget , connectId :: IORef (Maybe (ConnectId SomeWidget)) } and of course no monads to indicate there's something state-changing going on (Except the IO monad, which is the underlying monad for the whole brouhaha anyway.) The above call would look more like: > main = do … > widget <- xmlGetWidget … > nothingRef <- newIORef Nothing > openButton `onClicked` $ loadFile xml fp (WW widget nothingRef) > … > > > loadFile :: GladeXML -> FilePath -> WW -> IO () > loadFile xml fp (WW widget cid) = -- do stuff, yield some 'data' > ref <- readIORef cid > case ref of > (Just i) -> signalDisconnect i > Nothing -> return () > connectId <- widget `on` buttonPressEvent $ > handler data widget > writeIORef cid $ Just connectId > … This doesn't strike me as a very elegant way of doing it. I'm sure there is some nice Haskellery that would make it more beautiful, so I'd like to ask this list what that might be? Also, a small plea: the gtk2hs documentation is sorely lacking an example of the usage of "signalDisconnect". I can provide a working minimal example of this solution, if anyone is interested, so maybe that could serve as a documentation. Though I still hope that this isn't the best possible way of doing it. Thanks in advance, Aleks

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beginner's type error
by Ivan Moore 03 Apr '09

03 Apr '09

Hi all, consider this very small function: thing n = n + round(sqrt n) It loads into ghci with no warnings. When I try to run "thing 10" I get: *Main> :load c:\temp\statictype.hs [1 of 1] Compiling Main ( C:\temp\statictype.hs, interpreted ) Ok, modules loaded: Main. *Main> thing 10 <interactive>:1:0: Ambiguous type variable `t' in the constraints: `Integral t' arising from a use of `thing' at <interactive>:1:0-7 `RealFrac t' arising from a use of `thing' at <interactive>:1:0-7 `Floating t' arising from a use of `thing' at <interactive>:1:0-7 Probable fix: add a type signature that fixes these type variable(s) I have tried to add various type signatures (without really knowing what I'm doing!) and haven't been able to get it to work. I am confused about a few things related to this: (a) what type signature fixes it and why it needs any help - it looks like the sort of thing that type inference shouldn't need any help with (b) it looks like a runtime type error and I thought you didn't get runtime type errors in Haskell (c) if I substitute 10 for n and do "10 + round(sqrt 10)" I get the expected answer 13 any help most welcome. cheers, Ivan

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What is "~" ?
by TG 02 Apr '09

02 Apr '09

Looking at the definition of "partition", it calls select :: (a -> Bool) -> a -> ([a], [a]) -> ([a], [a]) select p x ~(ts,fs) | p x = (x:ts,fs) | otherwise = (ts, x:fs) what does the "~" do? Can't seem to find it in the documentation, or see a difference in output without it. Thank you. -- TG cowscanfly(a)airpost.net -- http://www.fastmail.fm - Email service worth paying for. Try it for free

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the equivalent of OO class data and methods
by Michael Mossey 02 Apr '09

02 Apr '09

I'm making the transition from OO to Haskell, and a major problem for me is that I'm used to OO, where member variables are distinct from other variables and functions, so I can use totally generic member variable names and never have a conflict with other member variables or local variables. For example, if I want to create LayoutItem and name its coordinates x and y, I do this: (Python) class LayoutItem : def __init__( self, x, y ) : self.x, self.y = (x, y ) Then later, p = LayoutItem( 10, 20 ) Then later I can refer to p.x and p.y. I can also create other classes and name *their* member variables x and y, with no confusion. Now, I know that in Haskell I can do data LayoutItem = LayoutItem { x, y :: Int } Later, let p = LayoutItem { x = 10, y = 20 } I have read that this automatically creates "accessor" functions called x and y, so I can refer to (x p) and (y p) let foo = (x p)^2 + (y p)^2 Now let's say I create another type that also uses x and y: data Notehead = Notehead { x, y :: Int } n = Notehead { x = 100, y = 200 } This doesn't work. Apparently the accessor functions x and y (for LayoutItem) have global scope and conflict with those for Notehead. Which means I have to name them different things, which is awkward. For example, data LayoutItem = LayoutItem { layoutItemX, layoutItemY :: Int } data Notehead = Notehead { noteheadItemX, noteheadItemY :: Int } Also, I tried using some of these accessor function names as variables, and the compiler doesn't distinguish between them. For example, myfunc :: Notehead -> LayoutItem -> Int myfunc n p = let layoutItemY = layoutItemX p z = layoutItemY n in z^2 This doesn't work, because declaring layoutItemY as a variable hides its defintion as an accessor function. Is there a way to get what I want from Haskell? Thanks, Mike

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making translation from imperative code
by Michael Mossey 02 Apr '09

02 Apr '09

I'm translating a Python program into Haskell, and running into a problem---a type of code where I don't know how to make the conceptual shift. The Python code does a graphical layout of a music score. There is a loop in which it add items (chords, symbols, text, etc.) one at a time, moving to the right, until it reaches the end of the line. So in typical imperative code, we have a bunch of loop state, such as - xpos of last chord or symbol placed - time of last chord or symbol placed - several StaffData objects, each containing the symbols that went on that particular staff - a cache of miscellaneous information about each symbol for later reference So imperative code is pretty simple: loop, and each time update the state. Much of this state consists of lists to which symbols are added once per loop. I'm not sure how to conceive of this problem in Haskell. Without using mutable variables, I would have to "rebuild" the state on each loop. I can do that, but I don't want to see an ugly proliferation of variables. I would like to encapsulate the state in a single object so I can pass it around as a single variable, like LoopState = LoopState { lastXPos :: Int, lastTime :: Double, staffDataLists :: [ ( String, StaffData ) ], chunkCache :: [ ( Double, Chunk ) ] } So lets say I have an instance of this call x. Let's say I want to create y, by "updating" lastXPos. Do I have to do something like this: newLastXPos = 25 y = LoopState ( newLastXPos, lastTime x, staffDataLists x, chunkCache x ) Obviously this is verbose compared to an imperative language in which one would say: newLastPos = 25 x.setLastXPos( newLastXPos ) I am aware that Haskell provides some mutable structures, but part of what drew me to Haskell was the benefit of avoiding mutable data (reducing bugs and making it easier to reason about my program's behavior). Are there any shortcuts for doing things like the above? Thanks, Mike

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Can we make this program better (and shorter)?
by Zachary Turner 02 Apr '09

02 Apr '09

Be warned, this code is really ugly. I need some help to make it better. I was in this situation this week where there was a sample of some numbers. I knew the valid possible range that numbers could lie in, and I knew about 30% of the actual values from the sample. I also knew the original average and the original standard deviation. I wanted more information about the other values in the sample, so I sat down and derived two formulas: One that, given an average, the number of items in the average, and 1 number that you know was used somewhere in the calculation of the average, calculates what the average would be if that number had not been there. Another that does the same for standard deviation. The second formula is fairly insane, if someone can figure out how to simplify it let me know, it took forever to derive it without making any mistakes. Actually I didn't even know it was possible to do a running standard deviation like that until I sat down and derived it. So anyway the program asks the user how large the original sample was, and the average and standard deviation. Then it runs in a loop asking it for a value to remove from the sample set and calculate the updated values. For certain values however, even if they are actually in the range of valid values for the range of the sample space, they might be impossible to achieve given a certain sample size, average, and stdev (for example, if you have two numbers in the range [0..100] and the average is 100, and you remove 1 number, none of the remaining numbers can be 0 obviously). So I wanted to detect this if the user inputs such a value, that way for one thing I can calculate exact upper and lower bounds for the actual range of the numbers just by trying all possible values. I know that the nature of the program mandates that there's going to be a lot of IO Monadic code, but I still feel like my code sucks. For one thing, I really tried to make the removePoints method be pure, and just accept a list as the argument and do the I/O in another function. I imagined some kind of lazy list comprehension, where each time the removePoints method asked for the next item, it would ask for the input and do all that stuff. I couldn't figure out how to make this work. Perhaps it's not even possible, since by definition a list comprehension that asked for input when forced isn't pure. But I still think there's a better way to do it. I also really dislike having to do manual recursion for getting input, there's got to be a way to use lists and built-in functions to do it for you, using some kind of takeWhile() perhaps. Anyway if anyone has any suggestions for making this code look better, more elegant, or more Haskelly, let me know :) import Control.Arrow promptUntilValid :: (Read a, Show a, Eq a) => String -> IO a promptUntilValid prompt = do putStr prompt putStr " " result <- getLine case (reads result) of [(parsed,_)] -> return parsed invalid -> do putStrLn $ "Invalid input! You entered " ++ result ++ ", and results length is " ++ (show (length invalid)) promptUntilValid prompt newStdev :: Int -> (Int,Int,Double,Double,Double) -> Double newStdev removeValue (old_count, new_count, old_stdev, old_avg, new_avg) = sqrt.(/ (n-1.0)) $ n * σ_x^2 + 2.0*old_avg*new_avg*(n-1.0) - (n-1.0)*old_avg^2 - (a_n - old_avg)^2 - (n-1.0)*new_avg^2 where σ_x = old_stdev n = fromIntegral old_count a_n = fromIntegral removeValue removeSinglePoint :: Int -> (Int,Double,Double) -> (Int,Double,Double) removeSinglePoint value (count,stdev,avg) = (new_count,new_stdev,new_avg) where new_count = count-1 new_avg = (avg*(fromIntegral count) - (fromIntegral value)) / (fromIntegral new_count) new_stdev = newStdev value (count, new_count, stdev, avg, new_avg) removePoints :: (Int,Double,Double) -> IO () removePoints (count,stdev,avg) = do putStr "Enter a number to remove from the sample set (Enter to stop): " putStr " " result <- getLine case (result,reads result) of ("",_) -> return () (_, [(parsed,_)]) -> do if (any (uncurry (||) . (isInfinite &&& isNaN)) [new_stdev, new_avg]) then do putStrLn "That number could not have been there! The new values are NaN!" removePoints (count,stdev,avg) else do putStrLn $ "New count: " ++ (show new_count) ++ ", new stdev: " ++ (show new_stdev) ++ ", new avg: " ++ (show new_avg) removePoints (new_count,new_stdev,new_avg) where (new_count,new_stdev,new_avg) = removeSinglePoint parsed (count,stdev,avg) (_,invalid) -> do putStrLn $ "Invalid input! You entered " ++ result ++ ", and results length is " ++ (show (length invalid)) removePoints (count,stdev,avg) main = do num_values::Int <- promptUntilValid "Enter the initial number of values: " stdev::Double <- promptUntilValid "Enter the initial standard deviation: " average::Double <- promptUntilValid "Enter the initial average: " putStrLn ("num_values = " ++ (show num_values)) putStrLn ("stdev = " ++ (show stdev)) putStrLn ("average = " ++ (show average)) removePoints (num_values, stdev, average) return ()

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short function to remove and replace an item
by Michael P Mossey 01 Apr '09

01 Apr '09

What if I have a list xs, and I want to remove one item and replace it with another? The item to remove can be detected by a predicate function. The order of the items doesn't matter. How about this: replaceItem :: (a -> Bool) -> a -> [a] -> [a] replaceItem p new xs = new : snd (partition p xs) This will actually replace all items that match the predicate with one copy of 'new'. It will also prepend 'new' even if no items match the predicate. For another challenge, can someone explain to me how to write this in point-free style? Thanks, Mike

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