December 2008 - Haskell-Cafe

Line noise
by Andrew Coppin 21 Dec '08

21 Dec '08

I hang out on another forum that is populated by various kinds of computer geeks. There's a fair few programmers in there, as well as nerds of every degree. And yet, every time I post anything written in Haskell, everybody complains that it "looks like line noise". What do "normal" programmers find so puzzling about Haskell? You might guess some of the following: - High-order functions. - Lambda functions. - Algebraic data types. - Lazy evaluation. - Recursion. - Monads. Actually, none of these things were mentioned. The things people have *actually* complained to me about are: - Haskell expressions are difficult to parse. - Several standard library elements have unhelpful names such as "elem", "Eq" and "fst". (My favourite has to be "Ix". I mean, WTH?) - Several standard library functions have names which clash badly with the usual meanings of those names - e.g., "break", "return", "id". - Variable names such as "x" and "f" aren't fabulously helpful to lost programmers trying to find their way. If you take a "typical" lump of C or Java code, there's so much redundancy and noise in it that you can *usually* figure out vaguely what's going on, even if it doesn't completely make sense. When presented with a typical Haskell fragment... uh... well, good luck with that. Let's take Haskell's most celebrated example: qsort [] = [] qsort (x:xs) = qsort (filter (x <) xs) ++ [x] ++ qsort (filter (x >=) xs) To a novice, it's not actually real clear what the arguments to qsort are. You and I know that function application binds much more tightly than (++), but a novice isn't going to know that. You might also not immediately realise that "xs" is also an argument to filter, not just (x <) or whatever. The people I spoke to also seemed pretty confused about the usage of (.) and ($), even after I explained it a few times. Several people also voiced being puzzled about Haskell's layout rules. I'm not sure what we *do* with all this data, but I found it interesting so I thought I'd share. ;-) I've spent the last few years trying to convert a few people to The Haskell Way(tm), but so far I haven't succeeded in the slightest. I just get yelled at for pointing white noise. Heh. Oh well!

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Trouble with FFI on Windows2000...
by Serge LE HUITOUZE 21 Dec '08

21 Dec '08

Hi there! I'm trying to FFI-bind a MS Visual dll (under Windows2000) to a Haskell program. I have this little C(++) dll: ******* FILE inc.h: START ******* extern "C" { int pten(int i); } ******* FILE inc.h: END ******* ******* FILE inc.cpp: START ******* #include "inc.h" int pten(int i) { return i+10; } ******* FILE inc.cpp: END ******* ******* FILE cproj1.def: START ******* LIBRARY cproj1 EXPORTS pten ******* FILE cproj1.def: END ******* And this little Haskell program: ******* FILE testFFI_2.hs: START ******* {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign.C -- get the C types foreign import ccall unsafe "pten" c_pten :: CInt -> CInt plus10 :: Int -> Int plus10 i = fromIntegral (c_pten (fromIntegral i)) main = print (map plus10 [1..10]) ******* FILE testFFI_2.lhs: END ******* Using information found at paragraph 1.4.3.4 of http://www.haskell.org/haskellwiki/GHC:FAQ#GHC_on_Windows I do the following: > dlltool -d cproj1.def -l libcproj1.a > ghc --make testFFI_2.hs -optl-lcproj1 -optl-L. This seems fine, since it produces a "testFFI_2.exe". However, executing it, I get a MSWindows error box with a message that looks like (approximate translation): "DLL (null) not found on specified path" At first, I didn't have the "LIBRARY" directive in the ".def" file, so I thought that was the reason for the "(null)" appearing in the message. But adding said directive didn't change anything... [I've checked that my MSVisual dll was in current dir, and that this dir was in the search paths showed in the error box] Can anyone enlighten me? Thanks in advance. --Serge

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Graham Hutton's calculator example for win32
by Ahn, Ki Yung 21 Dec '08

21 Dec '08

In Graham Hutton's "Programming in Haskell" there is an interactive calculator example using ANSI code to implement the UI on the terminal. This example doesn't work on MS Windows XP or other MS OSes based on NT kernel, since their command line does not support ANSI very well. But, thanks to ansi-terminal on Hackage, I was able to extract minimal code from the package to make a win32 version of the calculator example in Hutton's book. calculatorWin32.lhs and Win32ANSI.hs is an implementation for win32. I extracted the win32 console API bindings for setting cursor positions from ansi-terminal project and put them in Win32ANSI.hs. I had to put this in a separate file because I had an issue with ghci. To run this, I had to compile the console API bindings with ghc first and then run ghci as follows C:\> ghc -c Win32ANSI.hs C:\> ghci calculatorWin32.lhs Without compiling the object code, ghci cannot find the proper link for win32 console API FFI bindings. C:\> ghci calculatorWin32.lhs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 3] Compiling Parsing ( Parsing.lhs, interpreted ) [2 of 3] Compiling Win32ANSI ( Win32ANSI.hs, interpreted ) During interactive linking, GHCi couldn't find the following symbol: GetConsoleScreenBufferInfo@8 This may be due to you not asking GHCi to load extra object files, archives or DLLs needed by your current session. Restart GHCi, specifying the missing library using the -L/path/to/object/dir and -lmissinglibname flags, or simply by naming the relevant files on the GHCi command line. Alternatively, this link failure might indicate a bug in GHCi. If you suspect the latter, please send a bug report to: glasgow-haskell-bugs(a)haskell.org Is this a bug or a natural behavior of ghci? This is strange to me since ghci finds the proper link for the functions in the other C libraries such as getch in conio.h. In addition, I am attaching a patched calculator.lhs which works for Unix/Linux on both GHC 6.8.x and GHC 6.10.1. The one currently on the book homepage only works for GHC 6.8.x but not GHC 6.10.1. This is due to the bug fix of hSetBuffering in GHC 6.10.1. To run these calculator example you will also need Parsing.lhs from the book hompage. http://www.cs.nott.ac.uk/~gmh/Parsing.lhs -- Ahn, Ki Yung

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Haskell as a religion
by Hugo Pacheco 21 Dec '08

21 Dec '08

I just found this on the web. Do you like the description of Haskell? It is not that far from true :P http://www.aegisub.net/2008/12/if-programming-languages-were-religions.html -- www.di.uminho.pt/~hpacheco

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Beginner's TH question
by Jeff Heard 20 Dec '08

20 Dec '08

Two things... can I add fields to records using Template Haskell, like: data T = T { $fields, myfield :: Field, ... } I assume the answer there is no, and then what's wrong with this? I get: Illegal instance declaration for `UIState t' (All instance types must be of the form (T a1 ... an) where a1 ... an are type *variables*, and each type variable appears at most once in the instance head. Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `UIState t' In the expression: [d| instance UIState t where { setSizeY v a = setSizeY v . uist $ a setSizeX v a = setSizeX v . uist $ a setDrawing v a = setDrawing v . uist $ a setKey v a = setKey v . uist $ a .... } |] In the definition of `deriveUIState': deriveUIState uist t = [d| instance UIState t where { setSizeY v a = setSizeY v . uist $ a setSizeX v a = setSizeX v . uist $ a setDrawing v a = setDrawing v . uist $ a .... } |] in this module: -# LANGUAGE TemplateHaskell #-} module Graphics.Rendering.Thingie.TH where import Language.Haskell.TH import Graphics.Rendering.Thingie.UIState import qualified Graphics.Rendering.Thingie.BasicUIState as S deriveUIState uist t = [d| instance UIState t where mousePosition a = S.mousePosition . uist $ a mouseLeftButtonDown a = S.mouseLeftButtonDown . uist $ a mouseRightButtonDown a = S.mouseRightButtonDown . uist $ a mouseMiddleButtonDown a = S.mouseMiddleButtonDown . uist $ a mouseLeftButtonClicked a = S.mouseLeftButtonClicked . uist $ a mouseRightButtonClicked a = S.mouseRightButtonClicked . uist $ a mouseMiddleButtonClicked a = S.mouseMiddleButtonClicked . uist $ a mouseWheel a = S.mouseWheel . uist $ a keyCtrl a = S.keyCtrl . uist $ a keyShift a = S.keyShift . uist $ a keyAlt a = S.keyAlt . uist $ a key a = S.key . uist $ a drawing a = S.drawing . uist $ a sizeX a = S.sizeX . uist $ a sizeY a = S.sizeY . uist $ a setMousePosition v a = setMousePosition v . uist $ a setMouseLeftButtonDown v a = setMouseLeftButtonDown v . uist $ a setMouseRightButtonDown v a = setMouseRightButtonDown v . uist $ a setMouseMiddleButtonDown v a = setMouseMiddleButtonDown v . uist $ a setMouseLeftButtonClicked v a = setMouseLeftButtonClicked v . uist $ a setMouseRightButtonClicked v a = setMouseRightButtonClicked v . uist $ a setMouseMiddleButtonClicked v a = setMouseMiddleButtonClicked v . uist $ a setMouseWheel v a = setMouseWheel v . uist $ a setKeyCtrl v a = setKeyCtrl v . uist $ a setKeyShift v a = setKeyShift v . uist $ a setKeyAlt v a = setKeyAlt v . uist $ a setKey v a = setKey v . uist $ a setDrawing v a = setDrawing v . uist $ a setSizeX v a = setSizeX v . uist $ a setSizeY v a = setSizeY v . uist $ a |]

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I hate constructing strings with ++
by Thomas Hartman 20 Dec '08

20 Dec '08

I hate constructing strings with ++. Especially icky when you start dealing with escape characters and stuff. cabal install HStringTemplateHelpers ..... import Text.StringTemplate.Helpers putStrLn $ render1 [("name",name)] "Why, hello, $name$" just thinkin out loud :) (originally at http://www.reddit.com/r/programming/comments/7khkd/the_fun_of_programming_a…)

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"non-functions like unsafePerformIO are not technically part of the haskell language!"
by Jason Dusek 20 Dec '08

20 Dec '08

From an old thread: > non-functions like unsafePerformIO are not technically part > of the haskell language! How is this true, exactly? -- _jsn |...an old thread.| http://www.nabble.com/Re%3A-Re%3A-Parsers-are-monadic--p11390440.html

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Re: [Haskell] ANN: HLint 1.0
by Gwern Branwen 20 Dec '08

20 Dec '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 On Sat, Dec 20, 2008 at 4:55 AM, Neil Mitchell wrote: > Hi, > > I am pleased to announce HLint, a tool for making suggestions to > improve your Haskell code. Previously this tool was called Dr Haskell > and depended on a working installation of Yhc - both of those have now > changed. HLint requires GHC 6.10*, and to install simply type: > > cabal update && cabal install hlint > > If you haven't yet installed the cabal command, instructions are here: > http://ghcmutterings.wordpress.com/2008/11/10/bootstrapping-cabal-install/ > > As an example of what HLint does, running "hlint darcs-2.1.2" over the > latest stable release of darcs gives 385 suggestions, including: > > darcs-2.1.2\src\CommandLine.lhs:46:1: Use a string literal > Found: > [' ', '\t', '"', '%'] > Why not: > " \t\"%" > > darcs-2.1.2\src\CommandLine.lhs:49:1: Eta reduce > Found: > quotedArg ftable > = between (char '"') (char '"') $ quoteContent ftable > Why not: > quotedArg = between (char '"') (char '"') . quoteContent > > darcs-2.1.2\src\CommandLine.lhs:94:1: Use concatMap > Found: > concat $ map escapeC s > Why not: > concatMap escapeC s > > To see all the hints in a nice interactive document visit > http://www-users.cs.york.ac.uk/~ndm/hlint/hlint-report.htm > (recommended if you are thinking of trying out hlint) > > All necessary links, including a manual, hackage links, bug tracker > and source code can be found from the tool website: > http://www-users.cs.york.ac.uk/~ndm/hlint/ > > Acknowledgements: Niklas Broberg and the haskell-src-exts package have > both been very helpful. The darcs users mailing list gave many good > suggestions which I've incorportated. > > Please direct any follow up conversations to haskell-cafe@ > > Thanks > > Neil > > * Why GHC 6.10? View patterns. Hi, Neil. Thanks for the tool; it found some worthwhile changes in my projects. But a few points: 1) How does one actually use the CLI tool? You didn't say. It *seems* that one just feeds it a list of random filepaths, so on Mueval, say, the command would be 'hlint main.hs Mueval/*.hs' (and not, say, 'hlint mueval.cabal'). But I'm not actually sure. 2) I think I found a parsing bug. One line in Mueval/Interpreter.hs runs: > fmap (take n exceptionMsg ++) $ render' (n-length exceptionMsg) s which gives the error: > Mueval/Interpreter.hs:145:59: Parse failure, Parse error > No relevant suggestions Adding spaces between 'n' and 'length', so it reads: > fmap (take n exceptionMsg ++) $ render' (n - length exceptionMsg) s lets hlint parse and suggest for it. - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAklM+kwACgkQvpDo5Pfl1oKDhwCfRgQutbvr9QbKvbLJlbhdla2Z absAniWC+ksXHhfbunsdOsnmG69lppFZ =X9uv -----END PGP SIGNATURE-----

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Re: [Haskell] ANN: HLint 1.0
by Neil Mitchell 20 Dec '08

20 Dec '08

Hi > I noticed that you convert point-wise into point-free. > Perhaps you could add some point-free transformations to remove redundancy > in certain cases. > Is that a goal of the library? It does some transformations of th at nature, but the idea isn't to remove redundancy, its to make the code clearer. Do you have any suggestions for things it could easily do to remove redundancy? It already does a few relating to if expressions. > a suggestion for a rule: dollar eta: f $ x -> f x That can easily be added. > Why not: > ("TAG " `isPrefixOf` just_name pinfo) That's an open bug: http://code.google.com/p/ndmitchell/issues/detail?id=109 In general, most of the hints are "replace this with this" - evaluating the new bit (even to remove easy constants) is beyond it for the moment. Some people theorised it might be too hard, non-terminating, or incorrect. As it happens this rule only fires when "TAG " is either a string literal or a list literal, and when the number is a constant, so its always safe and the guard could always be eliminated. > 1) How does one actually use the CLI tool? You didn't say. It *seems* > that one just feeds it a list of random filepaths, so on Mueval, say, > the command would be 'hlint main.hs Mueval/*.hs' (and not, say, 'hlint > mueval.cabal'). But I'm not actually sure. The filenames are either filenames, or directories which are recursively searched, i.e cd mueval && hlint . is probably sufficient. I should document it more clearly in the manual. Thanks Neil

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Re: [Haskell] ANN: HLint 1.0
by Martijn van Steenbergen 20 Dec '08

20 Dec '08

Hi Neil, This is awesome, thank you. :-) It found a 'use liftM', 'use on' and three eta-reduces in Yogurt. It seems like the line numbers could be a bit more accurate: ./Network/Yogurt/IO.hs:54:3: Use liftM Found: rec >>= return . (c :) Why not: liftM (c :) rec Where the code is: 50 -- Waits for input, but once the first character is read, waits 51 -- no longer than the specified number of ms before giving up. 52 hGetImpatientLine :: Handle -> Int -> IO String 53 hGetImpatientLine h patience = rec where 54 rec = do 55 c <- hGetChar h 56 if c == '\n' 57 then return [c] 58 else do 59 b <- hWaitForInput h patience 60 if b 61 then rec >>= return . (c:) 62 else return [c] I imagine it could have told me to look at line 61 right away. Thanks, Martijn.

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