July 2010 - Haskell-Cafe

Monad transformers, design
by Tony Morris 31 Jul '10

31 Jul '10

Hello I have a question regarding monad transformers and how to design an API with a transformer. I have a narrowed code example of the question. Please see the questions in the comments below. import Data.Monoid import Control.Monad -- Suppose some data type newtype Inter a = Inter (Int -> a) -- and a monad transformer for that data type. newtype InterT m a = InterT (m (Inter a)) -- It's easy to implement this type-class instance (Monoid a) => Monoid (Inter a) where mempty = Inter (const mempty) Inter a `mappend` Inter b = Inter (a `mappend` b) -- and for the transformer too by lifting into the monad instance (Monad m, Monoid a) => Monoid (InterT m a) where mempty = InterT (return mempty) InterT a `mappend` InterT b = InterT (liftM2 mappend a b) -- But what about this type-class? class Ints a where ints :: a -> Int -> Int -- Seems easy enough instance (Integral a) => Ints (Inter a) where ints (Inter a) n = fromIntegral (a n) -- OH NO! {- instance (Monad m, Integral a) => Ints (InterT m a) where ints (InterT a) n = error "OH NO!" -} -- We could try this class Copointed f where copoint :: f a -> a -- but it seems rather impractical instance (Copointed m, Integral a) => Ints (InterT m a) where ints (InterT a) = ints (copoint a) {- So it seems that for some type-classes it is possible to implement for both the data type and the transformer, but not all type-classes. Is there a general approach to this problem? -} -- Tony Morris http://tmorris.net/

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using the orc library
by Günther Schmidt 31 Jul '10

31 Jul '10

Hello, I'd like to download 1,000 web pages with up to 6 six concurrent downloads at a time. How can I express such a thread limit within the orc EDSL? Günther

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Re: Re: Can we come out of a monad?
by Jason Catena 31 Jul '10

31 Jul '10

On Jul 30, 11:17 am, Anton van Straaten wrote: > Prelude> :m Control.Monad.State > Prelude Control.Monad.State> let addToState :: Int -> State Int (); > addToState x = do s <- get; put (s+x) > Prelude Control.Monad.State> let mAdd4 = addToState 4 > Prelude Control.Monad.State> :t mAdd4 > m :: State Int () > Prelude Control.Monad.State> let s = execState mAdd4 2 > Prelude Control.Monad.State> :t s > s :: Int > Prelude Control.Monad.State> s > 6 By this example State doesn't seem to give you anything more than a closure would, since it doesn't act like much of an accumulator (by, for example, storing 6 as its new internal value). Could you use State for something like storing the latest two values of a Fibonacci series? For example, each time you call it, it generates the next term, discards the oldest term, and stores the newly-generated term? And could you then use this Fibonacci State monad in a lazy computation, to grab for example the first twenty even Fibonacci numbers, without computing and storing the series beyond what the filter asks for? We can generate Fibonacci series double-recursively in a lazy computation. Would it be more or less efficient to use a Fibonacci State monad instead? Would the State implementation provide a larger range before it blew the stack (which tail-recursion should prevent), or became too slow for impatient people? Would Haskell memoize already-generated values in either case? Could we write a general memoizer across both the recursive and State implementations, or must we write a specific one to each case? Jason Catena

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Announce type-level-natural-number-1.0: Simple, Haskell 2010-compatible type level natural numbers
by Gregory Crosswhite 31 Jul '10

31 Jul '10

Hey everyone, I am pleased to announce the release of the "type-level-natural-number" package, which implements natural numbers at the type level. Although there are many packages on Hackage that implement type level natural numbers, this package is distinguished by its simplicity: it does not offer any operations other than predecessor, successor, and conversion to Int, and so the only Haskell extension it needs is EmptyDataDecls. Thus, this package is compatible with Haskell 2010. The envisioned use case is for annotating types with natural numbers, so that extra functionality such as addition and subtraction of natural numbers is unnecessary. Nothing is stopping someone from implementing this functionality, and in fact I may do so myself; however, since such functionality is not needed merely for using these numbers, I decided to leave it out in order to avoid having to use non-Haskell 2010 extensions (especially UndecidableInstances). In particular, a major motivation behind designing the package in this way is to make it more appealing as a standard means for representing natural number annotations in order to promote interoperability between libraries. I welcome any feedback that the community has to offer. Cheers, Gregory Crosswhite

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Database.CouchDB broken?
by Moritz Ulrich 31 Jul '10

31 Jul '10

Hello, I'm currently learning Haskell and I want to write a small tool to collect some data in a CouchDB-Database Sadly, the Database.CouchDB module from hackage (and from git) seems broken. It looks like a bug deep in the JSON handling of the lib. Some examples can be found in this gist: http://gist.github.com/475323 ('test' is a database with two simple documents, the doc with the id '8e9112011580882422393f6291000f7d' exists) I filed an issue, but the maintainer hasn't responded in 5 days. Is there anything I missed? Thanks in advance! -- Moritz Ulrich Programmer, Student, Almost normal Guy http://www.google.com/profiles/ulrich.moritz

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Definition of List type?
by michael rice 31 Jul '10

31 Jul '10

From: Data.Maybe Description The Maybe type, and associated operations. From: Data.List Description Operations on lists. One description has the type and associated operations, the other only has the operations. Where can I find the type definition for List, and why isn't it in Data.List? Michael

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Problem with haskell types
by Anupam Jain 30 Jul '10

30 Jul '10

Hi, I am having trouble getting a small program to compile. The helpful folks at #haskell created a version of the program that does compile - http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28406#a28408 but it is not very clear to them (and to me) why the original program wouldn't type compile in the first place. Here's the program that refuses to compile - module Delme () wheredata DecisionState = A | B | C | Dd_test :: Eq b => b -> b -> DecisionState -> DecisionState -> ()d_test test testVal trueState falseState = if (test == testVal) then d trueState else d falseStated :: DecisionState -> ()d A = d_test True True B Cd B = d_test 1 2 C Dd C = d_test True False A Bd D = () I get an error like - Delme.hs:13:0: Contexts differ in length (Use -XRelaxedPolyRec to allow this) When matching the contexts of the signatures for d_test :: forall b. (Eq b) => b -> b -> DecisionState -> DecisionState -> () d :: DecisionState -> () The signature contexts in a mutually recursive group should all be identical When generalising the type(s) for d_test, d Putting in the extension does get the program to type check but the original program should have type compiled in the first place. The ironic thing we discovered is that if we remove the type declaration for 'd', the program type checks, and GHC then derives the exact same type which we removed! Can some of the smarter people in the room please shed more light on this? -- Anupam

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couchDB
by Andrew U. Frank 30 Jul '10

30 Jul '10

a few weeks ago a question was posed on this list for examples how to use couchDB. i post the 'hello world' for couchDB using the haskell interface - perhaps it helps other to start! a more extensive and complicate example is found at www.maztravel.com/haskell/mySqlToCouchDB.html I created with Futon a db names "first" (i had no luck with names containing a - or a _ , but have not investigated further). the runcouchDB' affects the couchDB at localhost. andrew ps: if there is a better place to document examples? a wiki on haskell.org would be nice and should be available for any project in hackage. ---- {-# LANGUAGE DeriveDataTypeable #-} module Main ( ) where import Database.CouchDB import Data.Data (Typeable) import Text.JSON s1 = JSString $ toJSString "Peter" m1 = makeObj [("FirstName", s1), ("FamilytName", JSString . toJSString $ "Miller")] mydb1 = db "first" -- convert db name to checked couchdb -- problem with "-" or "_" in db names in haskell?? main = do putStrLn "start couchdb tests" (doc, rev) <- runCouchDB' $ newDoc mydb1 m1 -- works return ()

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[Yi Editor]Cabal Problem
by Alessandro Stamatto 30 Jul '10

30 Jul '10

Installing Yi Editor i get the following error: ------------------------------ ----------------------------------------------- Missing dependencies on foreign libraries: * Missing C libraries: gobject-2.0, glib-2.0, intl, iconv ----------------------------------------------------------------------------- Im on windows, using Cabal in Cygwin. How should i install those missing libs?

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Fail to install SDL with Cabal
by Eitan Goldshtrom 30 Jul '10

30 Jul '10

I'm trying to install SDL through Cabal -- I don't know another way to install it. However, I'm getting this: setup.exe: The package has a './configure' script. This requires a Unix compatibility toolchain such as MinGW+MSYS or Cygwin. cabal: Error: some packages failed to install: SDL-0.5.10 failed during the configure step. The exception was: exit: ExitFailure 1 I have MinGW and MSYS, so I don't understand why I'm having this problem. Do I need to set something special up so that Cabal can access their tools? -Eitan

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