- Beginners - Haskell.org

not getting heap profiling data from test runs
by Sean Perry 22 Jun '11

22 Jun '11

I have done the following: $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.3 $ ghc -O2 -o myapp --make main.hs -prof -auto-all -rtsopts -fforce-recomp $ ./myapp foo.csv +RTS -hb $ cat ./myapp.hp JOB "myapp foo.csv +RTS -hb" DATE "Wed Jun 22 00:34 2011" SAMPLE_UNIT "seconds" VALUE_UNIT "bytes" BEGIN_SAMPLE 0.00 END_SAMPLE 0.00 BEGIN_SAMPLE 0.00 END_SAMPLE 0.00 I get the same result from other -h options. There are a total of 4 hs files compiled for this app. I see the same thing with my rpn calculator. Leaving off the -O2 does not help. Nor does -rtsopts=all. What am I missing? This is on OSX using the Haskel platform with a cabal installed Text.CSV (which I did --reinstall -p on to be certain).

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boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]
by anthony niro 21 Jun '11

21 Jun '11

Hello, My name is Anthony i have few question about Haskell. I was reading a tutorial everything goes well. but now i have few things that i dont understang. HERE: boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x] I dont understand this line except the "if" "then" "else". What is xs? what is the | ? and why doing this " | x <- xs, odd x]" why x <- xs????? what is that and what is odd x? thx everyone for answer me.

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Unit numeric type pattern
by Christopher Howard 21 Jun '11

21 Jun '11

What is a "unit numeric type pattern", as in the error: "Only unit numeric type pattern is valid". I can cause this error by mixing numbers into type definitions. And, therefore, I know how to avoid this error. But the error indicates that there /is/ such a thing as "valid" "unit numeric type pattern", and consequently I am quite curious as to what a such a thing is and what it would look like in code. -- frigidcode.com theologia.indicium.us

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Text.Xhtml.Table problems, with a very simple table
by Tom Murphy 20 Jun '11

20 Jun '11

Hi All, I'm trying to create a table like this: ---------- | a1 | | |----| b | | a2 | | ---------- My code is this: triTable a1 a2 b = table << ((a1 </> a2) <-> (b)) But it's mashing elements together. What am I doing wrong?? Thanks for your time, Tom

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Web frameworks
by Manfred Lotz 20 Jun '11

20 Jun '11

Hi there, There are some web frameworks out there, as snap, yesod, happstack, and perhaps even more. I'd like to get a recommendation which web framework might be easiest to get into, and to use for a haskell beginner. -- Thanks, Manfred

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Still wrestling with lazy IO
by Raphael Päbst 19 Jun '11

19 Jun '11

Hey everyone! Sorry for asking more or less the same question again, but I'm still stuck wrestling with lazy IO. What I'm trying to do is the following: sending a Bytestring to a server, via TCP Do stuff with the Bytestring that results in another Bytestring Send that new Bytestring back to the original client. At the moment this all happens inside an IO handle and doesn't work. The stuff on the server side only gets executed, once the handle is closed on the client's side and therefore the response is lost somewhere. It's a very simple program otherwise and so I don't want to get too much into iteratees, iterators and that stuff, since it feels like overkill. Is there a way to get this working in the way I'm currently trying? Or can I do this in a strict way? Thanks again Raf

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Make synonym for Main?
by Christopher Howard 18 Jun '11

18 Jun '11

Say you have a function in Main that is the same name as a function you've imported, but you don't want to rename it. But then you have to prefix it with "Main" each time you use it: Main.functionName Is there some way to rename Main or make a synonym for it, kind of like the "import qualified" trick, so you could do... M.functionName -- frigidcode.com theologia.indicium.us

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Bool -> a -> Maybe a, or something similar
by Tacit Sawk 18 Jun '11

18 Jun '11

|Hello! I tried Hoogle, but it didn't seem to have what I was after. I'm looking for something like this: > maybeWhen :: Bool -> a -> Maybe a > maybeWhen cond = > if cond > then Just > else const Nothing Is there a builtin that has the above sort of behaviour? I suppose it can even be generalised: > maybeWhen :: (MonadPlus m) => Bool -> a -> m a > maybeWhen cond = > if cond > then return > else const mzero Tacit |

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share a record type
by Kees Bleijenberg 17 Jun '11

17 Jun '11

I want to learn more about Haskell and I started with my Sudoku solver class SudokuClass s where initAreas :: s -> s createEmptySudoku :: s getBoardLength :: s -> Int setBoard :: s -> Board -> s getCellToAreas :: s ->CellToAreas ... other functions with default implementations I created a Sudoku solver for 9x9 sudoku's. data Sudoku9x9 = Sudoku9x9 { board :: Board, areas:: Areas, cellToAreas :: CellToAreas } deriving (Show) Board, Areas and CellToAreas are of type Map. After that I create an instance: instance SudokuClass Sudoku9x9 where initAreas s = ..... After implementing the 9x9 solver I wanted to create a second instance SudokuTwins, that solves a different kind of sudoku, but uses the same record: data SudokuTwins = SudokuTwins { board :: Board, areas:: Areas, cellToAreas :: CellToAreas } deriving (Show) This is almost a copy of Soduku9x9 (right side of the =) How can I express the fact that all sudoku solvers use the same recordtype? Can I make a type of the record structure {board:: Board.... } and use it in the constructor function or can I incorperate the recordtype in the class or ......? Kees

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Data.Foldable's foldr'
by Federico Mastellone 17 Jun '11

17 Jun '11

Hi, Looking at the source of Data.Foldable I found something I don't understand The Foldable class declares foldl like this: foldl :: (a -> b -> a) -> a -> t b -> a Them, outside of the class, foldr' is defined like this: -- | Fold over the elements of a structure,-- associating to the right, but strictly.foldr' :: Foldable t => (a -> b -> b) -> b -> t a -> bfoldr' f z0 xs = foldl f' id xs z0 where f' k x z = k $! f x z I don't understand this definition, foldl receives 3 parameters and here it is used with 4, how is it possible? Even the function passed to foldl has 3 parameters when a function of 2 is needed. What is the precedence and associativity involved here that makes foldr' a valid expression? Thanks! -- Federico Mastellone Computer Science Engineer - ITBA ".. there are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult." Tony Hoare, 1980 ACM Turing Award Lecture.

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