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IO Help
by Mark Wallsgrove 08 May '08

08 May '08

Hey, I am studying Haskell as a unit at University. I find the concept and design idea's of Haskell interesting but I am finding my self struggling to understand the relationship between the normal state and IO state of Haskell. This is my situation: I have created 12 functions for a program which take in two types: type Catalogue = [Track] type Playlist = [Track] The definition for track is as follows: -- Track = ArtistDetails Title Length PCount data Track = Track ArtistType String Float Int deriving (Show,Eq,Read) -- Popular = Artist | Composor Performer data ArtistType = Popular String | Classical String String deriving (Show,Eq,Read) I have managed to save the data to a file using this code: --Saving Data saveData :: String -> Catalogue -> IO() saveData fileName catalogue = writeFile fileName (show catalogue) Problem now is reading the data back into the program. When I read the data back into the program it comes as IO [Track]. This is the code I have been using to load the data: loadData :: String -> Catalogue loadData fileName = do x <- readFile fileName return (read x :: Catalogue) I think I have missed a trick some where or I am using IO wrong, I really don't know. I believe it is the latter. I have been told that my definition for the function is wrong and that I should use the lazy approach to fix the problem. But this still leaves me with IO [Track]. Could someone inform me on what I am doing wrong? Thank you in advance Smoky

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(no subject)
by u4538637＠anu.edu.au 08 May '08

08 May '08

Hi I have a bit of a dilemma.I have a list of lists, eg, [[1,2,3],[4,5,6],[7,8,9]]. Imagine they represent a grid with 0-2 on the x axis and 0-2 on the y axis, eg, (0,0) is 1, (1,0) is 2, (2,1) is 6, etc and (2,3) is 9. I want to be able to put in the list of lists, and the (x,y) coordinate, and return the value. Also, I need to be able to replace a value in the list. Eg, if I wanted to replace (2,3) with 100, then the output of the expression would be [[1,2,3],[4,5,6],[7,8,100]]. Any help would be great!

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Re: [Haskell-cafe] Random numbers / monads - beginner question
by Thomas Dinsdale-Young 08 May '08

08 May '08

Madoc wrote: Given a list of numbers, I want to modify each of those numbers by adding a random offset. However, each such modified number shall stay within certain bounds, given by the integers minValue and maxValue. After that, I want to continue computation with the resulting list of type [Int]. Personally, I'd do something like this, isolate the IO code outside the algorithm to keep the algorithm pure: modify' :: Int -> Int -> Int modify' offset a = normalize (a + offset) generateInfiniteListOfRandomNumbers :: IO [Int] -- implementation left as an exercise main = do randomNumbers <- generateInfiniteListOfRandomNumbers print $ zipWith modify' randomNumbers [0, 200, 400, 600, 800, 1000] I may be wrong, but generateInfiniteListOfRandomNumbers won't terminate and I think it has to before the next IO action occurs. (Laziness is great, but I don't think you can really do lazy IO like that.) Instead of map :: (a -> b) -> [a] -> [b], I think you are looking for mapM :: Monad m => (a -> m b) -> [a] -> m [b]. * * * *Hope this helps, Thomas >

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Re: [Haskell-cafe] Induction (help!)
by PR Stanley 08 May '08

08 May '08

So, when you apply the function to the first element in the set - e.g. Zero or Nil in the case of lists - you're actually testing to see the function works. Then in the inductive step you base everything on the assumption that p holds for some n and of course if that's true then p must hold for Succ n but you have to prove this by taking Succ from n and thus going bakc to its predecessor which is also the hypothesis p(n). So, to reiterate assumption: if hypothesis then conclusion if p(n) then p(Succ n) proof of assumption if p(Succ n) = Succ(p(n)) then we've won. If pn+1) = p(n) + p(1) then we have liftoff! I'm not going to go any further in case I'm once again on the wrong track. Cheers Paul At 22:43 07/05/2008, you wrote: >On Wed, May 7, 2008 at 9:27 PM, PR Stanley <prstanley(a)ntlworld.com> wrote: > > Hi > > One of you chaps mentioned the Nat data type > > > > data Nat = Zero | Succ Nat > > > > Let's have > > add :: Nat -> Nat -> Nat > > add Zero n = n > > add (Succ m)n = Succ (add m n) > > > > Prove > > add m Zero = m > >To prove this by induction on m, you would need to show: > >1) add Zero Zero = Zero >2) If "add m Zero = m", then "add (Succ m) Zero = Succ m" > >Number (1) is completely trivial, nothing more needs to be said. (2) >is easy, after expanding the definition. > >Here the P I used was P(x) := add m Zero = m, the thing we were trying >to prove. (1) is a base case, P(Zero). (2) is the inductive step, >"If P(m) then P(Succ m)". > >Hoping I don't sound patronizing: if you're still having trouble, then >I suspect you haven't heard what it means to prove an "if-then" >statement. Here's a silly example. > >We want to prove: If y = 10, then y - 10 = 0. > >First we *assume* the condition of the if. We can consider it true. > >Assume y = 10. >Show y - 10 = 0. >Well, y = 10, so that's equivalent to 10 - 10 = 0, which is true. > >Luke

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help in tree folding
by patrik osgnach 08 May '08

08 May '08

Hi. I'm learning haskell but i'm stuck on a generic tree folding exercise. i must write a function of this type treefoldr::(Eq a,Show a)=>(a->b->c)->c->(c->b->b)->b->Tree a->c Tree has type data (Eq a,Show a)=>Tree a=Void | Node a [Tree a] deriving (Eq,Show) as an example treefoldr (:) [] (++) [] (Node '+' [Node '*' [Node 'x' [], Node 'y' []], Node 'z' []]) must return "+∗xyz" any help? (sorry for my bad english)

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runInteractiveProcess and hGetLine on Windows
by Harri Kiiskinen 08 May '08

08 May '08

Hello all, I bumped into a "feature" which might be a bug, but to be certain, I'd like to hear your opinion. I'm running ghc 6.8.2 on Windows XP, and with ghci I do the following: Prelude System.Process System.IO> (inp,outp,err,ph) <- runInteractiveProcess "kpsewhich" ["testfile.txt"] Nothing Nothing ('kpsewhich' is a simple path searching utility used by web2c TeX system, returns the full path to the file if found in the system defined search path, but you all probably know that.) and then: Prelude System.Process System.IO> hGetLine outp which gives me: "./testfile.txt\r" as opposed to "./testfile.txt" which I get on my Linux box. Is the "\r" supposed to be at the end? I thought it is part of the line separator in Windows, and as such, should not be part of the line retrieved? Same thing happens when compiled with ghc. Best Wishes, Harri K.

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How can see old threads.
by Ambrish Bhargava 08 May '08

08 May '08

Hi, I want to know, how can see old threads (may be I can get my answers from there itself)? -- Regards, Ambrish Bhargava

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Re: [Haskell-cafe] Induction (help!)
by PR Stanley 08 May '08

08 May '08

After you grok induction over the naturals, you can start to think about structural induction, which is usually what we use in programming. They are related, and understanding one will help you understand the other (structural induction actually made more sense to me when I was learning, because I started as a programmer and then became a mathematician, so I thought in terms of data structures). Paul: I was hoping that understanding the classic mathematical concept would help me appreciate the structural computer science) variation better. I don't know what it is about induction that I'm not seeing. It's so frustrating! Deduction in spite of the complexity in some parts makes perfect sense. This, however, is a different beast! So let's say you have a tree, and we want to count the number of leaves in the tree. data Tree = Leaf Int | Branch Tree Tree countLeaves :: Tree -> Int countLeaves (Leaf _) = 1 countLeaves (Branch l r) = countLeaves l + countLeaves r We want to prove that countLeaves is correct. So P(x) means "countLeaves x == the number of leaves in x". Paul: By 'correct' presumably you mean sound. First we prove P(Leaf i), since leaves are the trees that have no subtrees. This is analogous to proving P(0) over the naturals. Paul: I'd presume 'proof' here means applying the function to one leaf to see if it returns 1. If I'm not mistaken this is establishing the base case. countLeaves (Leaf i) = 1, by definition of countLeaves. "Leaf i" has exactly one leaf, obviously. So countLeaves (Leaf i) is correct. Now to prove P(Branch l r), we get to assume that P holds for all of its subtrees, namely we get to assume P(l) and P(r). Paul: How did you arrive at this decision? Why can we assume that P holds fr all its subtrees? You can think of this as constructing an algorithm to prove P for any tree, and we have "already proved" it for l and r in our algorithm. Paul: Is this the function definition for countLeaves? This is analogous to proving P(n) => P(n+1) in the case of naturals. Paul:I thought P(n) was the induction hypothesis and P(n+1) was the proof that the formula/property holds for the subsequent element in the sequence if P(n) is true. I don't see how countLeaves l and countLeaves r are analogous to P(n) and P(n+1). So: Assume P(l) and P(r). P(l) means "countLeaves l == the number of leaves in l" P(r) means "countLeaves r == the number of leaves in r" countLeaves (Branch l r) = countLeaves l + countLeaves r, by definition. And that is the number of leaves in "Branch l r", the sum of the number of leaves in its two subtress. Therefore P(Branch l r). Now that we have those two cases, we are done; P holds for any Tree whatsoever. In general you will have to do one such proof for each case of your data structure in order to prove a property for the whole thing. At each case you get to assume the property holds for all substructures. Generally not so many steps are written down. In fact in this example, most people who do this kind of thing a lot would write "straightforward induction", and that would be the whole proof :-) The analogy between structural induction and induction over the naturals is very strong; in fact induction over the naturals is just induction over this data structure: data Nat = Zero | Succ Nat Hope this helps. Luke

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who maintains the POSIX/Unix Unistd.hs?
by Galchin, Vasili 07 May '08

07 May '08

Hello, I am trying to find the definition for the data type "TimeSpec"/"CTimeSpec". thanks, Vasili

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Figuring out if an algebraic type is enumerated through Data.Generics?
by Alfonso Acosta 07 May '08

07 May '08

Hi all, <probably_unnnecesary_background> I'm writing a Hardware-oriented DSL deep-embedded in Haskell (similar to Lava, for those familiar with it). One of the goals of the language is to support polymorphic signals (i.e. we would like to allow signals to carry values of any type, including user-defined types) The embedded compiler has different backends: simulation, VHDL, graphical representation in GraphML ... The simulation backend manages to support polymorphic signals by forcing them to be Typeable. In that way, circuit functions, no matter what signals they process, can be transformed to Dynamic and included in the AST for later simulation. The situation is more complicated for the VHDL backend. The Typeable trick works just fine for translating a limited set of predefined types (e.g. tuples) but not for user-defined types, since the embedded-compiler doesn't have access to the user-defined type declarations. One possible solution to access the type-definition of signal values would be constraining them to instances of Data. Then, we could use dataTypeOf to get access to the type representation. (Another option would be using template Haskell's reify, but I would like to avoid that by now) It would certainly be difficult map any Haskell type to VHDL, so, by now we would be content to map enumerate algebraic types (i.e. algebraic types whose all data constructors have arity zero, e.g. data Colors = Green | Blue | Red) </probably_unnnecesary_background> So, the question is. Is there a way to figure out the arity of data constructors using Data.Generics ? I'm totally new to generics, but (tell me if I'm wrong) it seems that Constr doesn't hold any information about the data-constructor arguments. Why is it so? Do you think there is a workoaround for this problem? (maybe using some other function from Data.Generics different to dataTypeOf?) Thanks in advance, Alfonso Acosta

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