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Re: [Haskell-cafe] Lists and monads
by John Lato 26 Jul '10

26 Jul '10

> From: Kevin Jardine <kevinjardine(a)gmail.com> > > As a Haskell neophyte, one of the things I find confusing is the way > that the usual list functions (map, fold, ++, etc.) often cannot be > used directly with monadic lists (m [a] or [m a]) but seem to require > special purpose functions like ap, mapM etc. > > I suspect that things are not quite as difficult as they appear, > however, but cannot find any tutorials on monadic list manipulation. > It's not really a tutorial, but have you read the Typeclassopedia, http://haskell.org/sitewiki/images/8/85/TMR-Issue13.pdf ? I found it very helpful to get a good understanding of functors, applicatives, and monads, after which it became much simpler to get good usage from Control.Monad and Control.Applicative. John

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Iteratee package: combining enumerators
by Max Cantor 26 Jul '10

26 Jul '10

I have a series of files with binary encoded data in them, and want to create an enumerator iterates on the first element at the front of all the files. Something like the pseudocode: return . minimum =<< mapM (fmap (heads . lines) readFile) listOfFileNames I can use convStream to create an enumerator which runs iteratees on each tuple in a single file: (convStream decodeStrings) :: Monad m => IterateeG [] String m a -> IterateeG WrappedByteString Word8 m (IterateeG [] MyDataType m a) or, with the EnumerateeGMM tysyn: (convStream decodeStrings) :: Monad m => EnumerateeGMM WrappedByteString Word8 [] String m a My question is if there is a simple way to combine the Enumeratees to enumerate on a set of files or if I have to write an enumerator from scratch. Thank you in advance, Max P.S. John, apologies for the duped email, sent from the wrong address by mistake.

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Random this! ;-)
by michael rice 26 Jul '10

26 Jul '10

Hi All, From: http://en.wikibooks.org/wiki/Haskell/Understanding_monads/State Exercises 1. Implement a function rollNDiceIO :: Int -> IO [Int] that, given an integer, returns a list with that number of pseudo- random integers between 1 and 6. After a lot of learning what not to do, this is the best I could come up with. rollNDiceIO :: Int -> IO [Int] rollNDiceIO n = mapM (\x -> randomRIO(1,6)) (replicate n 1) I know, ugly, but at least I got it to work. What's a better way to generate this list? Michael

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Re[Haskell-cafe] ad large file and match lines to a pattern
by grzyb 26 Jul '10

26 Jul '10

Hi, I'm a beginner in haskell, I was trying to write the following code, but I still encourage some problems, can you help me with that? I need to read a large file and try to match each line to a pattern which is int,int value="string" for example: 0,1 value="string1" 1,5 value="string2" when the line matches the pattern then I need to get those 3 values (int, int, string) and pass them to my function. -- View this message in context: http://old.nabble.com/Read-large-file-and-match-lines-to-a-pattern-tp292603… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Tiger compiler in Haskell: annotating abstract syntax tree
by José Romildo Malaquias 25 Jul '10

25 Jul '10

Hello. In his book "Modern Compilder Implementation in ML", Appel presents a compiler project for the Tiger programming language where type checking and intermediate code generation are intrinsically coupled. There is a function transExp :: Absyn.Exp -> (Tree.Exp,Types.Type) that do semantic analysis, translating an expression to the Tree intermediate representation language and also do type checking, calculating the type of the expression. Maybe the compiler can be made more didatic if these phases are separate phases of compilation. The type checker would annotate the abstract syntax tree (ast) with type annotations, that could be used later by the translater to intermediate representation. In an imperative language probably each relevant ast node would have a field for the type annotation, and the type checker would assign the type of the node to this field after computing it. I am writing here to ask suggestions on how to annotate an ast with types (or any other information that would be relevant in a compiler phase) in Haskell. As an example, consider the simplified ast types: data Exp = IntExp Integer | VarExp Symbol | AssignExp Symbol Exp | IfExp Exp Exp (Maybe Exp) | CallExp Symbol [Exp] | LetExp [Dec] Exp data Dec = TypeDec Symbol Ty | FunctionDec Symbol [(Symbol,Symbol)] (Mybe Symbol) Exp | VarDec Symbol (Maybe Symbol) Exp Expressions can have type annotations, but declarations can not. Comments? Regards, Romildo -- Computer Science Department Universidade Federal de Ouro Preto, Brasil

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Yet another monad transformer or silly usage of Either?
by Eugeny N Dzhurinsky 25 Jul '10

25 Jul '10

Hello, everybody! I am trying to develop some sort of library, which supposed to sign into a WEB service, then perform some requests with it. Initially I designed methods in the following way data DServError = InvalidCredentials | InvalidRequest | ... newtype Result a = Result { getOpResult :: Either DServError a } data DSession = Session { ... } data DLoginResponse = LoginResponse { currentSession :: DSession, ... } login :: String -> String -> IO ( Result LoginResponse ) servRequest1 :: DSession -> ParamType1 -> ParamType2 -> ... -> IO ( Result DServResponse ) Now I want to be able of doing something like authenticatedReq = do loginResponse <- login "username" "password" let session = currentSession loginResponse servRequest1 session ... ... ... servRequest2 session ... ... ... ... so if login succeeds - I will be able to extract Right data from the Either response ( with explicit or implicit usage of getOpResult), if any of operations within "do" block will fail with DServError - then an error should be reported. I think the solution for this may be using Control.Exception and it's try/catch? Or may be there's some trick available for Either? I looked at EitherT, and it seems that I have to wrap every invocation into EitherT and then chain them with >>/>>= -- Eugene Dzhurinsky

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data type declaration
by Patrick Browne 25 Jul '10

25 Jul '10

Hi, I am trying to understand the data type declaration below. What is the relation between class C3 and the data type Address below? Where is such a technique used? Thanks, Pat module A where data Person = Person String Integer deriving Show data Employee = Employee String Integer deriving Show class C1 c1 where age :: c1 -> Integer instance C1 Person where age(Person "P1" 1) = 1 instance C1 Employee where age(Employee "E1" 1) = 1 class C1 c2 => C2 c2 where name :: c2 -> String instance C2 Person where name(Person "P2" 1) = "P2" instance C2 Employee where name(Employee "E2" 1) = "E2" class C2 c3 => C3 c3 a where address :: c3 -> a instance C3 Person String where -- ** What are the semantics or purpose of this construct. -- ** Is the type declared in the context of a class. data C3 c3 a => Address c3 a = Address c3 a instance C3 Person (Address String String) where This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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default function definitions
by Patrick Browne 25 Jul '10

25 Jul '10

Hi, I am studying the Haskell type class system as part of a language comparison thesis. At this point I am looking at how default function definitions are handled in classes. Functions are usually defined in instances, not classes. I appreciate that the code below may not be an appropriate way to write Haskell programs, but it does help me understand how defaults work. I am not trying to construct a running program. Any feedback on the questions below and defaults work in general would be appreciated. Pat module A where data Person = Person String Integer deriving Show data Employee = Employee String Integer deriving Show class C1 c1 where age :: c1 -> Integer -- add default impl, can this be defined only once at class level? -- Can this function be redefined in a *class* lower down the heirarchy? age(c1) = 1 -- Is it true that instances must exists before we can run function or make subclasses? instance C1 Person where instance C1 Employee where -- Is it true that C2 can inherit age, provided an instance of C1 exists class C1 c2 => C2 c2 where name :: c2 -> String name(c2) = "C2" instance C2 Person where instance C2 Employee where -- Is it true that C3 cannot override C1 or C2 existing defaults? -- Is it true that this must be done at instance level? -- class Cx c3 => C3 c3 where -- age(c3) = 3 -- This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Type problems
by michael rice 25 Jul '10

25 Jul '10

This works: Prelude System.Random> do { randomRIO (1,6) >>= (\x -> putStrLn $ "Value = " ++ show x) } Value = 5 So does this: Prelude System.Random> do { x <- randomRIO (1,6); putStrLn $ "Value = " ++ show x } Value = 2 But not this: 1 import Control.Monad 2 import System.Random 3 4 foo :: IO () 5 foo = do 6 x <- randomRIO (1,6) 7 putStrLn $ "Value = " ++ show x foo.hs:6:18: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at foo.hs:6:18 `Random t' arising from a use of `randomRIO' at foo.hs:6:7-21 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. Prelude System.Random> Or this: 1 import Control.Monad 2 import System.Random 3 4 foo :: IO () 5 foo = randomRIO (1,6) >>= (\x -> putStrLn $ "Value = " ++ show x) foo.hs:5:17: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at foo.hs:5:17 `Random t' arising from a use of `randomRIO' at foo.hs:5:6-20 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. How to fix? Michael

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Porting ELF statifier to Haskell!
by C K Kashyap 25 Jul '10

25 Jul '10

Hi, At my work we ran into a situation where we started wishing there was a way to take a dynamically linked executable and create a statically linked bundle out of it. Little bit of googling got me to statifier - http://statifier.sourceforge.net/statifier/main.html. The project seems a little old and when I tried it out on my 32bit RHEL5 box, the statically linked file did not run. So I thought it would be a good exercise to try and use Haskell's Elf module (Data.Elf) and attempt to build a statifier. Just wanted to understand if anyone's tried this before or have any advice for me. -- Regards, Kashyap

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