July 2010 - Haskell-Cafe

datatype contexts
by Gregory Crosswhite 26 Jul '10

26 Jul '10

I agree with prior discussion on this list that adding contexts to datatype declarations seems to be more trouble than its worth, since these contexts just have to be added again to every function using the datatype. However, I have often wondered: why do function *have* to have these contexts? What would it affect in the language if we made the contexts be implicit, so that if we have data Datatype a = Context a => Datatype a then for function declarations f :: D a -> ... the context "Context a" is automatically asserted by the compiler? Cheers, Greg

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ANN: Moio, a library for compositional event-driven programming
by Patai Gergely 26 Jul '10

26 Jul '10

Hello all, I created a little library that provides first-class event sources and event stream transformers, both allowing side effects. For the time being, the code is only available on GitHub [1]. The library is called Moio, short for 'multiple-occurrence I/O', since event sources are effectively a generalisation of the IO monad that outputs a stream of values over time instead of just a single result at the end of its execution. To summarise in a short phrase, Moio could be considered as 'Rx meets Fudgets', but that's only a superficial characterisation. Check out the example application, a simple bulletin board server, which shows how to treat a callback-based GTK GUI and blocking I/O based networking uniformly using the abstractions provided. Gergely [1] http://github.com/cobbpg/moio -- http://www.fastmail.fm - Or how I learned to stop worrying and love email again

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Techniques for ensuring parser correctness?
by Jason Dagit 26 Jul '10

26 Jul '10

Hello, I find that parser correctness is often hard to verify. Therefore, I'm interested in techniques that others have used successfully, especially with Haskell. Techniques I'm aware of: * Round trip checks: Generate a datastructure, render as a string, parse back, and compare. Quickcheck can be used to automate this. * Fuzz testing: What tools exist to help me? * Formal verification: Has anyone been using this with Haskell parsers? Other than general theorem provers, say Isabelle, what tools exist? My specific need: The immediate challenge I have is that I'm modifying the parser that Darcs uses and we would like to improve the parser's test suite as well. The parser abstraction used in this case follows parsec's API. Left to my own devices I would use round trip checks, written with quickcheck, for this exercise. Because we're using a parsec style parser, I don't have a nice neat grammar handy. Thanks in advance for any advice you have! Thanks, Jason

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How to do this with associated types?
by Alexey Karakulov 26 Jul '10

26 Jul '10

Suppose I have one piece of code like this: > class Result r e | r -> e where > failure :: e -> r a > success :: a -> r a > at :: Result r String => [a] -> Int -> r a > at xs i = if i >= 0 && i < length xs > then success (xs !! i) > else failure "Wrong index" Either instance of Result is quite straightforward: > instance Result (Either e) e where > failure e = Left e > success x = Right x Maybe instance is discarding failure information: > instance Result Maybe e where > failure _ = Nothing > success x = Just x Some tests it in ghci: ghci> let xs = [0,1,2] ghci> at xs 2 :: Either String Integer Right 2 ghci> at xs 3 :: Either String Integer Left "Wrong index" ghci> at xs 2 :: Maybe Integer Just 2 ghci> at xs 3 :: Maybe Integer Nothing I'd like to replace functional dependencies with type families (associated types in my case): > class Result r where > type Failure > failure :: Failure r -> r a > success :: a -> r a Either instance is ok: > instance Result (Either e) where > type Failure (Either e) = e > failure e = Left e > success x = Right x But what to do with Maybe? > instance Result Maybe where > type Failure Maybe = forall e. e -- can't do this > failure _ = Nothing > success x = Just x

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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 25 Jul '10

25 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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