April 2009 - Beginners

Type families with kind * -> *
by Marco Túlio Gontijo e Silva 24 Apr '09

24 Apr '09

Hello, I read the type families example at http://haskell.org/haskellwiki/GHC/Indexed_types and I wanted to do something similar to the Collects example, but using a type of kind * -> *: > class StateFunctor sf where > type SFMonad sf > type SFValue sf > sfmap :: (SFValue sf -> SFValue sf) -> sf -> SFMonad sf () I wrote the instance as: > instance StateFunctor (ListStore a) where > type SFMonad (ListStore a) = IO > type SFValue (ListStore a) = a > sfmap function listStore > = listStoreGetSize listStore >>= listStoreSfmap function listStore > listStoreSfmap :: (a -> a) -> ListStore a -> Int -> IO () > listStoreSfmap _function _listStore 0 = [] > listStoreSfmap function listStore size > = listStoreGetValue listStore index > >>= listStoreSetValue listStore index . function > >> listStoreSfmap function listStore index > where > index :: Int > index = pred size I'm getting: DistroCreator/GUI/List.hs:51:47: Kind error: `SFMonad' is applied to too many type arguments In the type `SFMonad sf ()' In the type `sf -> SFMonad sf ()' In the type `(SFValue sf -> SFValue sf) -> sf -> SFMonad sf ()' Failed, modules loaded: none. How can I use type families with types * -> *? Greetings. -- marcot http://marcot.iaaeee.org/

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Help with types
by Daniel Carrera 24 Apr '09

24 Apr '09

Hi, I want to make a simple "average" function: average xs = (sum xs)/(length xs) When I try that on ghci I get the following error: No instance for (Fractional Int) arising from a use of `/' at <interactive>:1:17-36 Possible fix: add an instance declaration for (Fractional Int) In the expression: (sum xs) / (length xs) In the definition of `average': average xs = (sum xs) / (length xs) Ok, so 'sum' is of type (Num a => [a] -> a), length is of type ([a] -> Int) and (/) is of type (Fractional a => a -> a -> a). Can someone explain how I can "add an instance declaration" to my function so as to make all these types compatible? Thanks, Daniel.

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generating the set of all finite-valued functions on a finite space
by Erik Quaeghebeur 23 Apr '09

23 Apr '09

Hi, I'd like to lazily generate the set of all {-1,0,1}-valued functions on {'a','b','c'}? How should I best approach this. I was thinking about generalizing the power set definition powerset :: [a] -> [[a]] powerset = filterM (const [True, False]) but clearly don't know enough about filterM and the like to do it this way. Erik

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problem cabal install'ing hmatrix
by Erik Quaeghebeur 22 Apr '09

22 Apr '09

$ cabal install hmatrix Resolving dependencies... Configuring hmatrix-0.5.1.1... Preprocessing library hmatrix-0.5.1.1... running dist/build/Numeric/GSL/Special/Internal_hsc_make failed command was: dist/build/Numeric/GSL/Special/Internal_hsc_make >dist/build/Numeric/GSL/Special/Internal.hs cabal: Error: some packages failed to install: hmatrix-0.5.1.1 failed during the building phase. The exception was: exit: ExitFailure 1 What should I do to diagnose/resolve this problem? I'm on Gentoo/amd64 and have also posted this question in the gentoo forums. TIA for any help and pointers, Erik

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Design questions for a Pascal interpreter
by Giuliano Vilela 22 Apr '09

22 Apr '09

Hello, I've been doing some research for a upcoming homework project, a Pascal interpreter written in Haskell. As a beginner to the language, I have a few design questions I'd like to share with you. I started playing with Parsec, getting a feel for parsing with monadic combinators. My assumption here is that i'm going to take the source, transform it in an AST (built with normal data constructors present in Haskell), maybe build a symbol table while doing that (using the implicity state in Parsec), do some checking (mainly type checking I think), and the evaluation itself. Here are a few questions I had so far: - Keeping the whole AST in memory for the evalution phase seems overkill. Is there a better way? - The evalution, I think, would be a set of nice pure mutually recursive functions that do some pattern matching on the program AST. I would pass the current stack and heap for those functions to use and modify. Is the State monad a good fit for this task? Wouldn't the code become "too imperative"? - And finally, the big question. Consider the following pascal source: program HelloWorld; begin writeln('Hello World'); end. To evaluate the AST, I would eventually arrive at something like: eval (FunctionCall func_name func_args) = ... Obviously, to evaluate writeln I need to be in the IO monad. Here, my whole scheme went down. Do I really have to mix my own state (stack, heap) within the IO monad along my evaluation functions? PS: I could keep a list of things to print, that I would eventually do after I traversed the whole tree, but that wouldn't be "realistic". Thank you! -- []'s Giuliano Vilela.

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typeclass error
by Amitava Shee 22 Apr '09

22 Apr '09

I have the following code -- foo.hs module Foo where class Foo a where bar :: a -> a instance Foo Int where bar i = i + 10 ------------------ It fails to compile withe following error *Foo> bar 10 <interactive>:1:4: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `10' at <interactive>:1:4-5 `Foo t' arising from a use of `bar' at <interactive>:1:0-5 Probable fix: add a type signature that fixes these type variable(s) *Foo> - Thanks Amitava Shee

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Parsec 3.0 problems
by Ian Duncan 21 Apr '09

21 Apr '09

Hello everyone, I have the simple applicative parser combinator: parseLabel = Label <$> (between spaces (char ':') (many1 alphaNum)) This gives me the error: No instance for (Stream s m Char) arising from a use of `spaces' at tree.hs:18:32-37 Possible fix: add an instance declaration for (Stream s m Char) In the first argument of `between', namely `spaces' In the second argument of `(<$>)', namely `(between spaces (char ':') (many1 alphaNum))' In the expression: Label <$> (between spaces (char ':') (many1 alphaNum)) When I test this function in ghci, I have no problems, yet whenever I try to open it in a file it fails. What am I doing wrong? For those who are curious, Label is a data constructor in the following data type: data Loc = Label String | Addr Int deriving Show Thanks! Ian Duncan

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Question on Lists
by Nathan Holden 20 Apr '09

20 Apr '09

I have been reading Chris Okasaki's PhD thesis on Purely Functional Data Structures (www.cs.cmu.edu/~rwh/theses/*okasaki*.pdf) and it discusses his idea of lazy lists (He uses Standard ML in the paper), and it raised some questions for me. To cut to the chase, my question is this: Say I have list x of length n, and I have a single piece of data y. Does x ++ [y] take more cycles, or the same as if I'd said y:[x] (which would get it on the wrong side?)

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Installing Yi With GTK Frontend
by aditya siram 19 Apr '09

19 Apr '09

Hi all, I am having issues installing Yi with the GTK frontend. I have GTK2hs installed correctly but when I do 'yi --help' I get: Usage: yi [option...] [file] --self-check run self-checks -f [frontend] --frontend=[frontend] Select frontend, which can be one of: vty -y path --config-file=path Specify a configuration file -V --version Show version information -h --help Show this help --debug Write debug information in a log file -l [num] --line=[num] Start on line number --as=[editor] Start with editor keymap, where editor is one of: emacs, vim, cua --recompile-force Force recompile of custom yi before starting --resume Resume execution of yi from previous state --recompile Recompile custom yi if required then exit --ghc-options=[flags] Flags to pass to GHC Does Yi need a specific version of Gtk2hs? Thanks ... -deech

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Re: [Haskell-beginners] Partial Loading and Debugging with GHCI
by Kalman Noel 18 Apr '09

18 Apr '09

aditya siram schrieb: > I would then have to comment out 'bad', reload the program and query output > types to my hearts content. This is fine for a small program, but when I > have functions are downriver from 'bad', it gets extremely cumbersome. Is > there a way to load a file such that functions that compile stay loaded in > the interpreter even if something else fails? You can identify the offensive functions and write, in your example, bad :: String -- bad = hello ++ number bad = undefined

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