June 2011 - Beginners

How are complex Errors implemented
by Christopher Howard 08 Jun '11

08 Jun '11

So, I know from the source code at hackage that the IOException type is implemented with multiple fields, like so: data IOException = IOError { ioe_handle :: Maybe Handle, -- the handle used by the action flagging -- the error. ioe_type :: IOErrorType, -- what it was. ioe_location :: String, -- location. ioe_description :: String, -- error type specific information. ioe_errno :: Maybe CInt, -- errno leading to this error, if any. ioe_filename :: Maybe FilePath -- filename the error is related to. } deriving Typeable However, the hackage docs for Control.Monad.Error indicates that there is an Error IOException instance. But the Error class requires the implementation of at least one of two functions, noMsg and strMsg, which create an error with either a String or with nothing. Question: in the implementation of Error IOException, how does noMsg or strMsg create new IOExceptions when they don't take all the required information? (I couldn't find the implementation in the source code by clicking the "source" buttons on the page... maybe somebody could post that.) Reason I ask: Lets say I want to create my own error type which is a member of Error, but has multiple record fields. How would I implement noMsg and strMsg? -- frigidcode.com theologia.indicium.us

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A question about network programming
by Raphael Päbst 08 Jun '11

08 Jun '11

Hey all! This is probably a question that shows my incompetence more than anything else, but I'm stuck with some TCP programming. I have some code that looks like this: hdl <- open TCP connection message <- hGetContents hdl answer <- do stuff with message hPut hdl answer hClose hdl My problem is that the stuff that gets done with message happens, but only after I close the connection rom the client's side and then of course it is too late to get the answer out of said connection. What am I doing wrong? Many Thanks Raf

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Help with meaningful error message...
by Sean Charles 08 Jun '11

08 Jun '11

On 08/06/11 01:07, beginners-request(a)haskell.org wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org [snip] Damn this list is hot! I like the 'foldl' things and all in all I feel better this morning. Once again, thanks everybody for keeping me 'sane'. :) Sean (Sane!) > ------------------------------ > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > End of Beginners Digest, Vol 36, Issue 21 > *****************************************

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Re: [Haskell-beginners] Function definition order and performance
by Aleksandar Dimitrov 07 Jun '11

07 Jun '11

> I have not met a performance issue yet. Thanks. I'm just a beginner. > I was just wondering how they will be different. For instance, > > safeHead [] = Nothing > safeHead (x : xs) = Just x > > vesus > > safeHead (x : xs) = Just x > safeHead [] = Nothing There is no difference in this case. GHC compiles that down to the very same thing. Given this file: > safeHead [] = Nothing > safeHead (x:xs) = Just x > > safeHead' (x:xs) = Just x > safeHead' [] = Nothing You can look at the ghc core (the intermediate compiler output produced by GHC) to find out what the difference is. Core is basically like machine-generated Haskell with some syntactical oddities. Don Steward's ghc-core program on hackage displays core nicely. Here are the relevant core blocks: > safeHead' = > \ (@ a_acw) (ds_dcU :: [a_acw]) -> > case ds_dcU of _ { > [] -> Data.Maybe.Nothing @ a_acw; > : x_abJ xs_abK -> Data.Maybe.Just @ a_acw x_abJ > } > safeHead = > \ (@ a_acy) (ds_dcX :: [a_acy]) -> > case ds_dcX of _ { > [] -> Data.Maybe.Nothing @ a_acy; > : x_abx xs_aby -> Data.Maybe.Just @ a_acy x_abx > } And, well, they're exactly the same (sans a few differences in variable naming, which have no impact whatsoever.) Your pattern match gets compiled down to a case pattern match, and the arguments are re-ordered. While you *can* look at the assembly, from here on out, it doesn't make sense, because GHC uses Core as a basis for compilation, *not* your code (it does your code as a basis for Core.) For completion's sake, the assembly, which is unexcitingly identical in both cases, is below. Note that the only thing interesting here is that safeHead' got translated to safeHeadzq, which is… well. Cute, and nothing else. Really though, if you're still at RWH, chapter three, it's Ok to be interested in these kinds of subtle performance differences, but you shouldn't concentrate on them. At this point, you can trust the compiler to optimize your code enough, and you can worry about performance if your program is actually slow. Happy Haskell hacking :-) Aleks Main_safeHeadzq_closure: .quad Main_safeHeadzq_info .text .align 8 .quad 0 .quad 32 se2_info: .Lcek: movq %rbx,%rax andq $7,%rax cmpq $2,%rax jae .Lcel movl $base_DataziMaybe_Nothing_closure+1,%ebx addq $8,%rbp jmp *0(%rbp) .Lcel: addq $16,%r12 cmpq 144(%r13),%r12 ja .Lces movq $base_DataziMaybe_Just_con_info,-8(%r12) movq 6(%rbx),%rax movq %rax,0(%r12) leaq -6(%r12),%rbx addq $8,%rbp jmp *0(%rbp) .Lces: movq $16,184(%r13) .Lceq: jmp *-16(%r13) .text .align 8 .quad 4294967301 .quad 0 .quad 15 Main_safeHead_closure: .quad Main_safeHead_info .text .align 8 .quad 0 .quad 32 seJ_info: .Lcf1: movq %rbx,%rax andq $7,%rax cmpq $2,%rax jae .Lcf2 movl $base_DataziMaybe_Nothing_closure+1,%ebx addq $8,%rbp jmp *0(%rbp) .Lcf2: addq $16,%r12 cmpq 144(%r13),%r12 ja .Lcf9 movq $base_DataziMaybe_Just_con_info,-8(%r12) movq 6(%rbx),%rax movq %rax,0(%r12) leaq -6(%r12),%rbx addq $8,%rbp jmp *0(%rbp) .Lcf9: movq $16,184(%r13) .Lcf7: jmp *-16(%r13) .text .align 8 .quad 4294967301 .quad 0 .quad 15

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Help with meaningful error message...
by Sean Charles 07 Jun '11

07 Jun '11

OK, I am losing enough hair and accruing follicular damage more than I can take tonight, what is wrong, what have I done that is stupid and wrong etc... called: let routes = forwardRoutes M.empty csvdata -- [Record] from Text.CSV implemented: forwardRoutes m [] = m forwardRoutes m (row:rows) = case M.lookup m (row !! 0) of Just route -> M.update m (row !! 1):route Nothing -> M.insert from [] forwardRoutes m rows error: scread.hs:101:2: parse error on input `forwardRoutes' Um yeah, thanks, that's really helping me figure it out. All I wanted to do was build a map of keys and values where each value is a list that gradually accumulates(!) every row that ends at (!!1) from the same location (!!0) of my spreadsheet. I started off zipping with 'repeat []' and stuff but I just want to know what I have done to annoy the compiler enough to want to taunt me with that message. And, on the verge of posting I think the above code won't work anyway because I am not passing the new Map from 'insert' through....... ARGH! :(

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How to interpret the definition of MonadState
by djp＠arqux.com 07 Jun '11

07 Jun '11

I am confused about the definition of MonadState. The opening line looks like this: class Monad m => MonadState s m | m -> s where It is the vertical bar | that confuses me. What does that mean? How would one read this line in "natural language", say English? Thanks. - J -

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Monad operators: multiple parameters
by Christopher Howard 07 Jun '11

07 Jun '11

In a reply to an earlier question, someone told me that "do" expressions are simply syntactic sugar that expand to expressions with the >> and >>= operators. I was trying to understand this (and the Monad class) better. I see in my own experiments that this... main = do ln1 <- getLine putStrLn ln1 translates to this: main = getLine >>= \ln1 -> putStrLn ln1 However, what does this translate to?: main = do ln1 <- getLine ln2 <- getLine putStrLn (ln1 ++ ln2) -- frigidcode.com theologia.indicium.us

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xml light problem or pebkac
by Manfred Lotz 07 Jun '11

07 Jun '11

Hi all, I'm a bit lost. I'm having a problem with xml light but as a Haskell newbie it could be of course that the problem is sitting between my keyboard and chair. I tried hard to create a minimal example. Here is a minimal xmltest.hs: <-----------------------------snip--------------------------------> module Main where import System.Environment.UTF8 import qualified System.IO.UTF8 as U import System.IO import Text.XML.Light data CTest = CTest { ctName :: String , ctLocation :: String } deriving (Show,Read,Eq,Ord) getXmlContent :: Handle -> IO CTest getXmlContent inh = do xml <- U.hGetContents inh let content = parseXMLDoc xml case content of Just c -> do let name = case findChild (unqual "name") c of Nothing -> "<unknown>" Just n' -> strContent n' let path = case findChild (unqual "location") c of Nothing -> "<unknown>" Just path' -> case findAttr (unqual "path") path' of Nothing -> "<unknown>" Just p -> p return CTest { ctName=name, ctLocation=path} _ -> fail "not expected" readXmlFile :: FilePath -> IO CTest readXmlFile f = do inh <- U.openBinaryFile f ReadMode xml <- getXmlContent inh hClose inh return xml doSomething :: Show a => a -> IO () doSomething xml = print xml main :: IO () main = do args <- getArgs xml <- readXmlFile $ head args doSomething xml <-----------------------------snap--------------------------------> Here is the test.xml I use: <-----------------------------snip--------------------------------> <?xml version='1.0' encoding='UTF-8'?> <!DOCTYPE entry SYSTEM 'test.dtd'> <entry> <name>some name</name> <description> bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla </description> <location path='/info/somepath'/> </entry> <-----------------------------snap--------------------------------> Here the output of the program: CTest {ctName = "some name", ctLocation = "<unknown>"} which is wrong. If I delete just one of the 'bla bla' lines then the output of the program is: CTest {ctName = "some name", ctLocation = "/info/somepath"} which is correct. Question: What is my error? -- Manfred

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List -> Map -> List, to add duplicates?
by Tom Murphy 07 Jun '11

07 Jun '11

Hi All, This seems like an inefficient way to do what I'm trying to do. I'd really appreciate any suggestions or comments: import qualified Data.Map as Map data Currency = Dollar | Yen | XP | Health | Street_Cred | Peso deriving (Show, Eq, Ord) -- why ord? withDups = [(30, Dollar), (-20, Street_Cred), (-2, Dollar), (30, XP), (15, Peso), (30, XP)] flipAssoc (a, b) = (b, a) noDups = Map.fromListWith (+) (map flipAssoc withDups) final = map flipAssoc $ Map.toList noDups Thanks for your time! Tom

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Confused by type constraints
by Guy 07 Jun '11

07 Jun '11

I have some code resembling class Foo f where foo :: a -> f a data Bar f a = Foo f => Bar {bar :: f a} instance Foo (Bar f) where foo a = Bar $ foo a GHC insists on Foo f => in the instance declaration. However, the definition of Bar guarantees that this will always be the case. Why do I have to state this explicitly?

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