- Haskell-Cafe - Haskell.org

Need help to edit the Haskell script in Winhugs and documentation.
by RAJESH DALSANIYA 28 Mar '09

28 Mar '09

* * *Both sections relate to the case study: Index for a document of text.* * * *SECTION A: * * * *Given the attached Haskell code which produces an index of words, make the following alterations by modifying existing functions and including new functions where necessary – parts 1) to 5):* * * *1) **Where a word occurs N times on the same line, ensure that the line number occurs n times in the index entry for that word. * * * *2) **Allow words to be hyphenated and treat a hyphenated word as a single word. However, for those words which are split over two lines, treat a split word as a single word without the hyphen. * * * *3) **Treat a capitalised word (one or more capitals) as being different from the word in all lower case (but they should still be sorted alphabetically) – unless it is at the start of a sentence with only the initial letter capitalised. A sentence is terminated by a ‘.’, ‘?’ or ‘!’. * * * *4) **Make the output more readable in the form of an index table in columns with appropriate spacing and without brackets. * * * *5) **Include a user-friendly menu, so that the user can choose input/output file names or default files, and choose to rerun or exit. * * * *Parts 1) to 5) may be developed in any order. * * * * * *SECTION B:* * * *6) **For your version of function, makeIndex (only), show how an alternative ordering of the composed functions would provide a more efficient execution of makeIndex. Justify your answer. * * * *7) **For the parts 1) to 5) above that you have attempted, discuss the use you have made of a) higher-order functions, b) list comprehension, c) monadic input/output, d) functional composition, and/or e) partial parameterisation (or Curried functions). Include an evaluation of how useful your use of these concepts has been. * * * *------------------------------------------------------------------- * * * *import Prelude* * * *type Doc = String* *type Line = String* *type Word = String * * * *makeIndex :: Doc -> [ ([Int], Word) ]* * * *makeIndex* * = shorten . -- [([Int], Word)] -> [([Int], Word)]* * amalgamate .-- [([Int], Word)] -> [([Int], Word)]* * makeLists . -- [(Int, Word)] -> [([Int], Word)]* * sortLs . -- [(Int, Word)] -> [(Int, Word)]* * allNumWords .-- [(Int, Line)] -> [(Int, Word)]* * numLines . -- [Line] -> [(Int, Line)]* * splitUp -- Doc -> [Line]* * * *splitUp :: Doc -> [Line]* * * *splitUp [] = []* *splitUp text* * = takeWhile (/='\n') text : -- first line* * (splitUp . -- splitup other lines* * dropWhile (==’\n’) . -- delete 1st newline(s)* * dropWhile (/='\n')) text -- other lines* * * *numLines :: [Line] -> [(Int, Line)]* * * *numLines lines -- list of pairs of * * = zip [1 .. length lines] lines -- line no. & line* * * *-- for each line* *-- a) split into words* *-- b) attach line no. to each word* * * *splitWords :: Line -> [Word] -- a)* * * *splitWords [] = []* *splitWords line * * = takeWhile isLetter line : -- first word in line* * (splitWords . -- split other words* * dropWhile (not.isLetter) . -- delete separators* * dropWhile isLetter) line -- other words* * where* * isLetter ch* * = (‘a’<=ch) && (ch<=’z’)* * || (‘A’<=ch) && (ch<=’Z’)* * * *numWords :: (Int, Line) -> [(Int, Word)] -- b)* * * *numWords (number, line)* * = map addLineNum ( splitWords line) -- all line pairs* * where* * addLineNum word = (number, word) -- a pair* * * *allNumWords :: [(Int, Line)] -> [(Int, Word)]* * * *allNumWords = concat . map numWords -- doc pairs * * * *sortLs :: [(Int, Word)] -> [(Int, Word)]* * * *sortLs [ ] = [ ]* *sortLs (a:x)* * = sortLs [b | b <- x, compare b a] -- sort 1st half* * ++ [a] ++ -- 1st in middle* * sortLs [b | b <- x, compare a b] -- sort 2nd half* * where* * compare (n1, w1) (n2, w2)* * = (w1 < w2) -- 1st word less* * || (w1 == w2 && n1 < n2) -- check no. * * * *makeLists :: [(Int, Word)] -> [([Int], Word)]* * * *makeLists * * = map mk -- all pairs* * where mk (num, word) = ([num], word)* * -- list of single no.* * * *amalgamate :: [([Int], Word)] -> [([Int], Word)]* * * *amalgamate [ ] = [ ]* *amalgamate [a] = [a]* *amalgamate ((n1, w1) : (n2, w2) : rest)-- pairs of pairs* * | w1 /= w2 = (n1, w1) : amalgamate ((n2, w2) : rest)* * | otherwise = amalgamate ((n1 ++ n2, w1) : rest)* * -- if words are same grow list of numbers* * * *shorten :: [([Int], Word)] -> [([Int], Word)]* * * *shorten* * = filter long -- keep pairs >4* * where* * long (num, word) = length word > 4 -- check word >4* * *

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ByteStrings, FFI, iovec
by Antoine Latter 28 Mar '09

28 Mar '09

Folks, I'm putting together an FFI layer to a library which uses writev for its IO, and so expects an array of iovec structs as an argument to one of its calls. for more information, check out "man writev"[1]. Since I'm already using Data.Binary, the writev call almost sounds like a perfect match for lazy bytestrings, so I thought I'd put up what I have for comments. I'm using c2hs[2]. The recursive nesting of 'withForeignPtr' put me off at first, but I thought I'd at least give it a shot. I haven't put this into practice yet, so it could be buggy. I'm more looking for comments on the approach. Antoine [1] http://linux.die.net/man/3/writev [2] http://www.cse.unsw.edu.au/~chak/haskell/c2hs/ Foreign/IOVec.chs >>>>> -- -*-haskell-*- module Foreign.IOVec (IOVec() ,withLazyByteString ) where import C2HS import qualified Data.ByteString as S import qualified Data.ByteString.Internal as S import qualified Data.ByteString.Lazy as L #include <sys/uio.h> #c typedef struct iovec hs_iovec; #endc {#pointer *hs_iovec as IOVec newtype#} -- | This is intended for calling into something like writev. -- But I suppose that nothing stops you from calling into -- readv and blowing away the passed-in ByteString. withLazyByteString :: L.ByteString -> (IOVec -> Int -> IO b) -> IO b withLazyByteString b f = let bs = L.toChunks b num = length bs in allocaBytes (num * {#sizeof hs_iovec#}) $ \vecAry -> go vecAry 0 bs where go vecAry off [] = f (IOVec vecAry) off go vecAry off (b:bs) = let vec = vecAry `plusPtr` (off * {#sizeof hs_iovec#}) (fptr, bsOff, bsLen) = S.toForeignPtr b in withForeignPtr fptr $ \bsPtr -> do {#set hs_iovec->iov_base#} vec $ castPtr $ bsPtr `plusPtr` bsOff {#set hs_iovec->iov_len#} vec $ cIntConv bsLen go vecAry (off+1) bs <<<<<

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Re: Exception handling in numeric computations
by Gregory Petrosyan 28 Mar '09

28 Mar '09

On Sat, Mar 28, 2009 at 10:53 AM, Ketil Malde <ketil(a)malde.org> wrote: > So the difference between an exception or an error type is mainly what > you intend to do about it. There's no point in wrapping divisions in > Maybe unless you actually are able to do something useful to recover > from a zero denominator. That is exactly the point I was trying to make. When I write a code, I can't say in advance, in what way it will be used. So, for dealing with errors, I have to choose one way or another, mostly without that knowledge. When I'm using e.g. C++, it's easy: something like mantra "when in doubt, throw an exception" :-) combined with RAII, works good (but not ideal, of course). So, I'll ask again: when I program in Haskell, what mechanism should I use? Gregory

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putting SOE 'on the library path'
by Michael Mossey 28 Mar '09

28 Mar '09

This is a beginners question, which I have posted to beginners(a)haskell.org several times, but because I have gotten no answer, I thought I would try the cafe. I'm working through the "School of Expression" book, and I would like to install the code that comes with the book somewhere. Right now I have to write my exercise programs in the same directory where the SOE code is sitting so that my exercise programs can import SOE modules. My platform is Windows. I tried using the -i options to ghc and ghci, but to no avail. It wasn't clear on Windows in what form -i takes its arguments: backslashes as usual for Windows, forward slashes perhaps? How about paths with spaces in them? Does it need quotes around the filename? I tried all combinations. ghc never gave an error indicating there was something wrong with what I typed. It happily accepted all forms of -i. But it never found the libraries. Thanks, Mike

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Conditional compilation
by Robin Green 28 Mar '09

28 Mar '09

I am writing some code for citation support in gitit, and all the #ifdefs I'm using to do conditional compilation are a bit tiresome. Suppose you have the requirement that a certain feature of your software be disable-able at compile time, to avoid having to pull in certain dependencies (which may not be available on all platforms). Disabling a feature may entail removing certain fields from certain constructors (again, to avoid pulling in certain dependencies), and/or removing certain functions from certain modules. What is the best way to do this in Haskell? This problem description suggests that perhaps insights from Aspect-Oriented Programming and/or Software Product Lines may be relevant. However, I haven't heard of much work that relates these concepts to Haskell. Maybe this would be a good topic for an enterprising student? -- Robin

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Building GHC under cygwin
by Zachary Turner 28 Mar '09

28 Mar '09

Has anyone had any luck building a windows version of GHC under cygwin? I followed the instruction on the building guide to build it under MinGW/msys, which works, but when I try to build it under cygwin instead of msys I get this: cabal-bin.exe: Cannot find the program 'ghc' at '/cygdrive/c/dev/haskell/ghc/ghc-6.10.1/bin/ghc' or on the path make[1]: *** [bootstrapping.conf] Error 1 make[1]: Leaving directory `/cygdrive/c/dev/haskell/ghc/ghc-6.10.1-src/libraries' make: *** [stage1] Error 2 Fairly early in the build process. Looking in that folder, there is a ghc.exe there, but not a ghc. I'm not too familiar with cygwin so I feel like this is something fairly obvious, but I didn't see it covered on the wiki anywhere. A google search indicated that I might have to override the host mode in the configure state but didn't elaborate, and also I'm not sure how to do that. I also want to make sure that I get a *windows* version of GHC out of the build, as the post I found suggested that might not be the case.

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Haxr doesn't compile from cabal (HTTP > 4000 breaks it)
by Jeff Heard 28 Mar '09

28 Mar '09

The haxr cabal library dependencies seem to be off. I wonder, since haxr would benefit highly from the HTTP 4k series of performance improvements, is it trivial to make it compatible with the latest library? -- Jeff

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Re: [Haskell-cafe] Definition of "tail recursive" wrt Folds
by Brent Yorgey 28 Mar '09

28 Mar '09

On Wed, Mar 25, 2009 at 05:54:17PM +1030, Mark Spezzano wrote: > > What, strictly speaking, is the definition of ”tail recursive” as opposed to > just “recursive”? A recursive function is tail recursive if the final result of the recursive call is the final result of the function itself. If the result of the recursive call must be further processed (say, by adding 1 to it, or consing another element onto the beginning of it), it is not tail recursive. With that said, tail recursion is not that useful of a concept in a lazy language like Haskell. The important concept to know in Haskell is 'guarded recursion', where any recursive calls occur within a data constructor (such as foldr, where the recursive call to foldr occurs as an argument to (:)). This allows the result of the function to be consumed lazily, since it can be evaluated up to the data constructor and the recursive call delayed until needed. -Brent

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Haxr doesn't compile from cabal (HTTP > 4000 breaks it)
by Henning Thielemann 28 Mar '09

28 Mar '09

Most breakage caused by new HTTP package is due to the renaming from Response to Response_String and Request to Request_String. I think it was not a good idea to do that. There could have well be two types named Response from different modules, one with a type parameter and the other one without. Or the parametrized variant could have be named ResponseGeneric or so. That would have allowed packages to work both with version 3001 and 4000.

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Lazy counting and printing
by Anakim Border 28 Mar '09

28 Mar '09

Hi, I have written a code like this: import Control.Monad.State countedRecords :: [Int] -> StateT Int IO [Int] countedRecords [] = return [] countedRecords (r:rs) = do modify (+1) rest <- countedRecords rs return $ r:rs printEven :: [Int] -> StateT Int IO () printEven records = do let even = filter (\n -> n `mod` 2 == 0) records lift $ mapM_ (putStrLn . show) even combine :: [Int] -> StateT Int IO () combine records = countedRecords records >>= printEven main = do let numbers = [1..1000000] count <- execStateT (combine numbers) 0 putStrLn $ "Processed " ++ show count ++ " numbers." My intent is that of processing a large collection of records (in this example, integers) doing two things at once: count their total number and print those that satisfy a certain condition. I would like to keep the definitions of the two functions performing such tasks independent. As a result I've used the State monad to hide intermediate counts. Unfortunately the code above breaks laziness, in that records are first completely counted (and stored in memory) and only then filtered and printed. If I redefine main as: main = do let numbers = [1..] .. i.e. I produce an unbounded number of records, I get a stack overflow. Moreover, I don't like the way countedRecords and printEven are defined, because their type signatures are too "broad". I would prefer them to be: countedRecords :: [Int] -> State Int [Int] printEven :: [Int] -> IO () and let "combine" do some sort of lifting into the StateT monad. Is there any way to perform both operations lazily? Thanks

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