February 2009 - Beginners

Circular Linked Lists
by Matthew J. Williams 04 Feb '10

04 Feb '10

Dear All How would one mimic, in Haskell, a C++ circular linked list i.e., where the last element precedes (points to) the first? Sincerely Matthew J. Williams

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clarification on IO
by Michael Easter 02 Mar '09

02 Mar '09

Folks, I've been reading RWH and this http://www.haskell.org/haskellwiki/IO_inside#Haskell_is_a_pure_language I think I understand monads. I think I understand how IO is much different from, e.g. Maybe, State, etc. However there are some turns of phrase with respect to IO that have me baffled. I am giving a presentation on monads next week, and hope you can help. (I'm not quite as lost as this might imply!) Q1: The web page mentions that normal Haskell functions cannot cause side-effects, yet later talks about side-effects with putStrLn. I assume the key point here that IO actions are, by definition, _not_ normal functions? Q2: Is it true to say that *any* monadic action *could *cause side-effects, depending on the design of that monad? i.e. Does one generalize from the IO monad to (possibly) an arbitrary monad? *Musing* This must be true as using State must surely be considered a side-effect. Q3: The web page mentions IO as being a baton, or token, that is used to thread/order the actions. Is true that this is merely one simple perspective, with respect to order of evaluation? This is hard to articulate, but it seems to me that "in the IO monad" there is a large subsystem of (inaccessible) state, machinery, etc. Is it really a token? Q4: Is the following idea accurate: a Haskell program is partitioned into 2 spaces. One is a sequence of IO actions; the other is a space of pure functions and 'normal' Haskell operations. The execution of a program begins with the main :: IO () action and, effectively, crosses from one space to the other. In the pure space, the math-like functions can be highly optimized but only insofar as they do not disrupt the implied order of the IO actions. Because of the type system, the program recognizes when it enters "back" into the IO space and follows different, less optimized rules. My concern is that the above is *not* accurate, but I don't know why. thanks so much for your help Michael Easter -- ---------------------- Michael Easter http://codetojoy.blogspot.com: Putting the thrill back in blog http://youtube.com/ocitv -> Fun people doing serious software engineering

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Non-recursive binding expression
by j.romildo＠gmail.com 01 Mar '09

01 Mar '09

Hello. Is there any non-recursive binding expression in Haskell? Something that would allow me to write, for instance, test = let' xs = [] in let' xs = 3 : xs in let' xs = 8 : xs in let' xs = 7 : xs in xs Here let' is an hypothetical construction similar to let, except that it would be non-recursive. In the example, the value of test would be [7,8,3]. So, is there something in this line in Haskell? Regards, José Romildo

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Catching Exceptions in Haskell
by Bjoern Brandenburg 28 Feb '09

28 Feb '09

Hi all, I'm struggling to figure out how to catch an exception in Haskell. Eventually, what I want to do is to use the Control.Macho library (from Hackage) to parse binaries on Mac OS X. Naturally, I would like my program to gracefully handle corrupted files or files that aren't binaries. So I need to catch exceptions if the parsing with Data.Macho fails... This is what I've got so far: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=1877#a1877 Digression: Note that if I leave out the `foo e` call, then I get scratch.hs:21:9: Ambiguous type variable `e' in the constraint: `Exception e' arising from a use of `try' at scratch.hs:21:9-11 Probable fix: add a type signature that fixes these type variable(s) I find this somewhat counter-intuitive; this is also not discussed in the book "Real-World Haskell" (at least I haven't found it), which is what I'm using to learn Haskell. What is the best way to get around this? Back to my problem. The program produces the following output: $ runghc scratch.hs Testing Pure... Caught a bad exception: abort via error Testing Macho... No exception occured, the result is: scratch.hs: too few bytes. Failed reading at byte position 4 This counter-intuitive in at least two ways: 1) How can it fail at position 4 in an empty string? That looks like a sloppy error message too me; why doesn't it report the actually failing position? 2) Why can I catch an exception raised with an error in my own code, but not the one raised by Data.Binary.Get? Looking at http://hackage.haskell.org/packages/archive/binary/0.5/doc/html/src/Data-Bi… , an exception within Data.Binary.Get is also raised with error. What is the difference? Why am I seeing what I'm seeing, and how should I be doing it in the first place? Thanks in advance, Bjoern

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Defeating type inference
by Philip Scott 28 Feb '09

28 Feb '09

Well, either that or I being an idiot. Here's a little example. Let us say you had a datatype 'Month'* data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec deriving(Show, Eq, Ord, Ix) What I would like to make is an infinite lazy list of months (called, rather imaginitively I thought, 'months') that keeps wrapping around. That is to say, it would make a list like this [Jan, Feb, ...., Nov, Dec, Jan, Feb, ....., Nov, Dec, ... ad infinitum ] I am sure you get the picture. My first stab was this: months = range (Jan, Dec) : months But of course, what you get here is a list of lists, with each element being a list of [Jan, Feb, ...., Dec] So I puzzled for a little bit about how to do this in the most Haskelly way and I thought of this months = concat (range (Jan, Dec) : months) Which should work, right** But the type checker is not pleased at all and complains: Couldn't match expected type `[Month]' against inferred type `Month' Expected type: [[Month]] Inferred type: [Month] In the expression: concat (range (Jan, Dec) : months) In the definition of `months': months = concat (range (Jan, Dec) : months) However, if you use the first definition and make a second function: months = range (Jan, Dec) : months realmonths = concat(months) It is happy and does what one might expect. I thought perhaps I was just confusing the inference engine so I tried a liberal sprinkling of :: operators to make my intentions clear, but it still wasn't having any of it. Any thoughts welcomed! - Philip * I am aware that there is plenty of code to handle dates and times already written, probably much more nicely than mine - this is just a project I have been hacking at to get to grips with things. ** though I am pretty sure this is the Wrong Way to do this. I suspect concat takes O(n) time - more elegant approaches would be welcomed!

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Lambda Functions
by Philip Scott 27 Feb '09

27 Feb '09

Okay, thanks everyone for the help with the last question; I've got another one for you if you are keen. It's short and sweet. You can easily define a lambda expression that takes a tuple - e.g. in ghci > let foo = \(x,y) -> 42 foo :: (t, t1) -> Integer > foo (5,5) 42 Yay. Now that isn't a very exciting function. Tt takes two anythings and gives you a nice meaningful number. Now let us say for some perverse reason I want to make a lambda to test for equality. That is, reimplement the functionality of (==) using, well, (==). > let foo2 = \(x,y) -> x == y foo2 :: ((), ()) -> Bool Uh-oh. Now things are getting a little odd. I am not entirely sure what that type signature means (I was expecting to see something about the types having to be the same and an instance of the Eq class but alas..) The function certainly doesn't do what I want it to: > foo2 (5,5) No instance for (Num ()) arising from the literal `5' at <interactive>:1:6 Possible fix: add an instance declaration for (Num ()) In the expression: 5 In the first argument of `foo2', namely `(5, 5)' In the expression: foo2 (5, 5) This is itself a stupid problem, but it is a distilled version of some trouble I was having writing a filter that works on a list of tuples. Any pointers or slaps in the face for being too stupid warmly welcomed. I have already tried my usual trick of smothering the thing parentheses but it appeared to be all in vain. All the best, Philip

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Module structure
by Colin Paul Adams 27 Feb '09

27 Feb '09

If I want to write part of the code for my program as a library, and so put those modules into a hierarchical module structure (with the non-library code in no module), do I have to structure the directories in a special way to get the module search to work? -- Colin Adams Preston Lancashire

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using parametrized monads and Prelude.>>
by frea 27 Feb '09

27 Feb '09

Hello, I would like to create few functions which use parametrized monads (defined as in http://computationalthoughts.blogspot.com/2009/02/comment-on-parameterized-… ) and export their functionality outside a module and use them in a do statement. For example three exported funtions : func1 num = do x <- get put (show (x + num)) func2 num = do x <- get put (((read x)::Int) + num) execWith0 actions = runState actions 0 And in an another module, which has Prelude imported : k = execWith0 ( do func1 10 func2 5) Unfortunately this doesn't compile as in the second module >> is different than in PMonad. Is there any way which I could make it work? Michal

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Overriding >>= for a Monad?
by Patrick LeBoutillier 26 Feb '09

26 Feb '09

Hi all, I'm using Control.Monad.StateT as such: data TapState = TapState { planSet :: Bool, noPlan :: Bool, skipAll :: Bool, testDied :: Bool, expectedTests :: Int, executedTests :: Int, failedTests :: Int } deriving (Show) type TAP a = StateT TapState IO a but I'd like to provide my own >>= function. Is there a way to "derive" a new type from StateT in order to implement my own >>=? Is thin done using "instance"? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Fwd: [Haskell-beginners] Array and IArray
by Rafael Gustavo da Cunha Pereira Pinto 26 Feb '09

26 Feb '09

IArray define TONS of instances of IArray class... You will surely have to define a type for your expression. For your example you could use: arr::Array Int Int On Thu, Feb 26, 2009 at 18:19, Daniel Fischer <daniel.is.fischer(a)web.de>wrote: > Am Donnerstag, 26. Februar 2009 21:59 schrieb Rafael Gustavo da Cunha > Pereira > Pinto: > > Please, post the error messages. > > > > Maybe all is needed is an explicit type declaration... > > Yup! > Are you an experienced victim of the monomorphism restriction? > > Cheers, > Daniel > > -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc. -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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