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Need feedback on my Haskell code
by CK Kashyap 31 Jul '09

31 Jul '09

Hi Everyone, I managed to write up the line drawing function using the following links - http://www.cs.helsinki.fi/group/goa/mallinnus/lines/bresenh.html http://rosettacode.org/wiki/Bresenham%27s_line_algorithm#Haskell line :: Point -> Point -> [Point] line (xa,ya) (xb,yb) = line' (x1,y1) (x2,y2) deltax deltay ystep isSteep 0 where isSteep = abs (yb - ya) > abs (xb - xa) (xa',ya',xb',yb') = if isSteep then (ya,xa,yb,xb) else (xa,ya,xb,yb) (x1,y1,x2,y2) = if xa' > xb' then (xb',yb',xa',ya') else (xa',ya',xb',yb') deltax = x2 - x1 deltay = abs (y2 - y1) ystep = if y1 < y2 then 1 else -1 line' :: Point -> Point -> Integer -> Integer -> Integer -> Bool -> Integer -> [Point] line' (x1,y1) (x2,y2) deltax deltay ystep isSteep error = if x1 == x2 then if isSteep then [(y1,x1)] else [(x1,y1)] else if isSteep then [(y1,x1)] ++ line' (newX,newY) (x2,y2) deltax deltay ystep isSteep newError else [(x1,y1)] ++ line' (newX,newY) (x2,y2) deltax deltay ystep isSteep newError where newX = x1 + 1 tempError = error + deltay (newY, newError) = if (2*tempError) >= deltax then (y1+ystep,tempError-deltax) else (y1,tempError) Can someone please provide feedback on this? In terms of, how do I get more Haskell'ism into it. Regards, Kashyap

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Strange memory usage problem
by Ketil Malde 31 Jul '09

31 Jul '09

Hi, I'm observing something I find quite strange. I have a program that normally streams through a file, and it runs in the expected constant space, consuming about 5MB, according to 'top'. When I uncomment a certain block of code in the library it uses, space consumption appears to grow linearly with the input size. This is a bit surprising to me, since the code in question isn't used by the program at all, and yesterday, Cale confirmed this behavior on a recent GHC (6.10.3, I think - mine is 6.8.2 as shipped with Ubuntu 9.04). Since this appears to be reproducible, I put together a test case. The steps are: darcs get http://malde.org/~ketil/biohaskell/biolib-unstable darcs get http://malde.org/~ketil/biohaskell/flower Build them using the usual incantations (./Setup.hs configure/build/install). Download the test input file: wget http://malde.org/~ketil/test.sff Run for instance this command line: fselect "And (Func GT len 100) (Func GT k2 0.5)" test.sff This should now run in constant space. To observe the linear growth space, unpull the last patch from biolib, i.e. this one: Fri Jul 31 00:14:09 CEST 2009 Ketil Malde <ketil(a)malde.org> * comment out recover functionality - otherwise FSelect leaks memory! M ./Bio/Sequence/SFF.hs -1 +7 This uncomments some otherwise unused code, which apparently triggers this behavior. I'm unable to see what causes this, so any suggestions welcome. -k -- If I haven't seen further, it is by standing in the footprints of giants

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gtksourceview link problem with leksah on windows
by Bart De Vylder 31 Jul '09

31 Jul '09

Dear Haskellers, i'm having troubles linking Leksah against gtksourceview on windows (XP). I've installed gtk2hs (using the windows installer) but am not sure whether/where to specify the library search path for ld. Here is the output I get: C:\Documents and Settings\bartdv>cabal install leksah Resolving dependencies... Configuring leksah-0.6.1... Preprocessing executables for leksah-0.6.1... Building leksah-0.6.1... [ 1 of 55] Compiling DeepSeq ( src\DeepSeq.hs, > dist\build\leksah\leksah-tmp\DeepSeq.o ) > ... > [55 of 55] Compiling Main ( src\Main.hs, > dist\build\leksah\leksah-tmp\Main.o ) Linking dist\build\leksah\leksah.exe ... C:\app\Haskell_Platform\2009.2.0.1\gcc-lib\ld.exe: cannot find > -lgtksourceview-2.0 collect2: ld returned 1 exit status cabal: Error: some packages failed to install: leksah-0.6.1 failed during the building phase. The exception was: exit: ExitFailure 1 any help/ tips are very welcome, I'm eager to leave emacs... Bart

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Deepest polymorphic functor
by Eduard Sergeev 31 Jul '09

31 Jul '09

I was wondering if it is possible to somehow change "deep" f_map from http://okmij.org/ftp/Haskell/deepest-functor.lhs article in a such a way that it would work not only for monotypes like in the provided example: test1 = f_map (+1) [[[1::Int,2,3]]] But for polymorphic types as well (e.g. behaves like simple map) so the following line would compile as well: test1 = f_map (+1) [[[1,2,3]]] ? -- View this message in context: http://www.nabble.com/Deepest-polymorphic-functor-tp24709303p24709303.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] [Haskell Cafe] Data construction: how to avoid boilerplate code?
by Luke Palmer 30 Jul '09

30 Jul '09

On Wed, Jul 29, 2009 at 10:15 AM, Paul Sujkov<psujkov(a)gmail.com> wrote: > Hi Luke, > > I'm not pretty sure. The thing I don't like is the need to copy-and-paste > all the code with enumeration constructors. So, now I have two types to make > Type data from, but they could be many, so I'll have many almost identical > makeTypeFromFoo-functions. The thing I need is something like (*): > > makeType :: ? -> Type > makeType c = case c of > ("-$-" or 240) -> Status > ("-M-" or 64) -> Message > ("-W-" or 32) -> Warning Well, you could write a helper like this: matchType :: (Eq a) => (a,a,a) -> a -> Type matchType (status,message,warning) x | x == status = Status | x == message = Message | x == warning = Warning To reduce the size of your specifications: makeTypeStr = matchType ("-$-", "-M-", "-W-") makeTypeInt = matchType (240, 64, 32) There are trade-offs to doing something like this. It's smaller, but harder to read as specification. But, because it uses a tuple, it will catch you if you add a new case but forget to add it to one of the makeType*s (providing you remember to change matchType). What you're asking for puts all the conversions in the same place, which forbids them from being split out, decoupled, and modularlized. What if, instead of simple values, you had a more involved parser for these things? Even though it's kind of verbose, I think what you already have is fine. You do have to repeat the names, but it is still content code, and the relationship of the content to the names is explicit. You might find tables like this in a user's manual for your software... Luke > > then I have all this fabric code in one place. I could use something like > Either for this example, but it will scale fine up to two types to build > value from; while I want to have arbitrary numbers of them > > And from another point, it would be great if I could avoid doubling the code > for datatype declaration with empty (no parameter) constructors, and actual > "constructing" code from the different types. So it could look somewhat > alike this (pseudo-code) (**): > > data Type = Status <-- (String "-$-", Integer 240) > | Message <-- (String "-M-", Integer 64) > | Warning <-- (String "-W-", Integer 32) > > however, I'm not sure this one is really needed. Something for the (*) is > much more interesting > > 2009/7/29 Luke Palmer <lrpalmer(a)gmail.com> >> >> On Wed, Jul 29, 2009 at 6:27 AM, Paul Sujkov<psujkov(a)gmail.com> wrote: >> > Hi haskellers, >> > >> > I have a datatype of this sort: >> > >> > data Type = Status >> > | Message >> > | Warning >> > | Error >> > | StepIn >> > | StepOut deriving (Eq, Show) >> > >> > and (at this moment) two fabric-like functions: >> > >> > makeType :: String -> Type >> > makeType c = case c of >> > "-$-" -> Status >> > "-M-" -> Message >> > "-W-" -> Warning >> > "-E-" -> Error >> > "->>" -> StepIn >> > "<<-" -> StepOut >> > otherwise -> error "Uknown Message Type" >> > >> > deduceType :: Integer -> Type >> > deduceType n = case n of >> > 240 -> Status >> > 64 -> Message >> > 32 -> Warning >> > 8 -> StepOut >> > 4 -> StepIn >> > 2 -> Error >> > otherwise -> error "Unknown Message Type" >> > >> > how can I avoid boilerplate code at this stage? The thing that I really >> > need >> > is some n-type constructor, kind of a fabric for a variety of types with >> > the >> > possibility to add them on the fly (I have simple Integer and String >> > here, >> > but they could be much more sophisticated). I don't need the possibility >> > to >> > unpack the original value (e.g. "-$-" or 240) once the Type is created - >> > in >> > this case I can always have some sort of mapping to deduce it at any >> > moment, >> > so it will be redundant to carry it through the code explicitly >> >> I am not sure what you're asking. Are you saying that what you have >> written is boilerplate? What code are you writing that could be >> automatically deduced with enough smartness? >> >> Give an example of what you might like the solution to look like. >> >> Luke >> >> > The example is rather short and simple, but I have some more places in >> > code, >> > where the same problem is observed. Is there any generic solution? >> > Thanks in >> > advance >> > >> > -- >> > Regards, Paul Sujkov >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> > > > > > -- > Regards, Paul Sujkov >

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[Haskell Cafe] Data construction: how to avoid boilerplate code?
by Paul Sujkov 29 Jul '09

29 Jul '09

Hi haskellers, I have a datatype of this sort: data Type = Status | Message | Warning | Error | StepIn | StepOut deriving (Eq, Show) and (at this moment) two fabric-like functions: makeType :: String -> Type makeType c = case c of "-$-" -> Status "-M-" -> Message "-W-" -> Warning "-E-" -> Error "->>" -> StepIn "<<-" -> StepOut otherwise -> error "Uknown Message Type" deduceType :: Integer -> Type deduceType n = case n of 240 -> Status 64 -> Message 32 -> Warning 8 -> StepOut 4 -> StepIn 2 -> Error otherwise -> error "Unknown Message Type" how can I avoid boilerplate code at this stage? The thing that I really need is some n-type constructor, kind of a fabric for a variety of types with the possibility to add them on the fly (I have simple Integer and String here, but they could be much more sophisticated). I don't need the possibility to unpack the original value (e.g. "-$-" or 240) once the Type is created - in this case I can always have some sort of mapping to deduce it at any moment, so it will be redundant to carry it through the code explicitly The example is rather short and simple, but I have some more places in code, where the same problem is observed. Is there any generic solution? Thanks in advance -- Regards, Paul Sujkov

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Re: Re: Proposal: TypeDirectedNameResolution
by Johannes Waldmann 29 Jul '09

29 Jul '09

While the discussion centers around overload resolution let me re-iterate a point that (e.g.,) Java does nicely: for their "x.f", if x :: T, then you write .f (unqualified) instead of .T.f (qualified), even if you needed qualification to declare the type of x, as in "foo.bar.T x;" This requires a mixture of type checking (for x) and name resolution (for f), which probably wouldn't fit with Haskell's current module and type system. Too bad ... Best - Johannes.

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RFC: Unicode support in Alex
by Jean-Philippe Bernardy 29 Jul '09

29 Jul '09

Hello, I have modified the Alex lexer generator to support unicode. The general idea is that the state-machine works on the UTF8 representation of the text. I submit my work here for review in order to off-load the maintainer (Simon Marlow) as far as possible. The prototype is available on github: git://github.com/jyp/Alex.git Be sure to * checkout the "utf8" branch (so "git diff master" shows the changes) * Do a 2-stage bootstrapping before testing Caveats: * The generated code depends on some utf8 packages; * There is no attempt to fix the bytestring-based wrappers; * Left-context recognition is not table-based any more; * Presence of debug code. Bug reports, comments, and especially patches are welcome :) Thanks, -- JP

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Adding a field to a data record
by Henry Laxen 29 Jul '09

29 Jul '09

Dear Group, It seems to me this should be easy, but I can't quite figure out how to do it without a lot of typing. Here is the question: Suppose you have a data type like: Data Foo = Foo { a :: Int, b :: Int, ... many other fields ... y :: Int } deriving (Eq, Read, Show, Typeable, Data) Now I would like to add a field z :: Int to the end of Foo. If I have a ton of data out on disk, which I wrote with, say writeFile "a.data" (show foo) -- where foo is a [Foo] say 1000 long, I would like to get a new "a.data" file which has a new z::Int field. So far the only way I can think of is to make a new Data Foo1, which includes the z::Int, read in a.data as a list of Foo, write a function like: fooTofoo1 :: Foo -> Foo1 fooTofoo1 xx = Foo1 {a = a xx, ... y = y xx, z = 1} then write the file back out, and perhaps use emacs to query-replace all the Foo1's back to Foo's, add the z::Int field back into Foo, and read it back. Please tell me there is a better way. Thanks in advance. Best wishes, Henry Laxen PS: I have read syb1, and syb2 a couple of times now, but so far haven't been able to connect it with this kind of problem.

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ANN: The Monad.Reader - Issue 14
by Wouter Swierstra 29 Jul '09

29 Jul '09

Dear all, I am pleased to announce that a new issue of The Monad.Reader is now available: http://themonadreader.wordpress.com/ Issue 14 consists of the following three articles: * Fun with Morse Code by Heinrich Apfelmus * Hieroglyph 2: Purely Functional Information Graphics Revisited by Jefferson Heard * Lloyd Allison’s Corecursive Queues: Why Continuations Matter by Leon P Smith Please note that I've moved the Monad.Reader to a new Wordpress blog. You may want to update your bookmarks. If you’d like to write something for the next issue of The Monad.Reader, please get in touch. I haven’t fixed the deadline for the next issue just yet, but expect a deadline late 2009. Wouter

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