September 2010 - Haskell-Cafe

Infinite types should be optionally allowed
by Job Vranish 22 Feb '11

22 Feb '11

There are good reasons against allowing infinite types by default (mostly, that a lot of things type check that are normally not what we want). An old haskell cafe conversation on the topic is here: http://www.nabble.com/There%27s-nothing-wrong-with-infinite-types!-td771373… However, I think infinite types should be allowed, but only with an explicit type signature. In other words, don't allow infinite types to be inferred, but if they are specified, let them pass. I think it would be very hard to shoot yourself in the foot this way. Newtype is the standard solution to situations where you really need an infinite type, but in some cases this can be a big annoyance. Using newtype sacrifices data type abstraction and very useful type classes like Functor. You can use multiparameter type classes and functional dependencies to recover some of the lost abstraction, but then type checking becomes harder to reason about and the code gets way more ugly (If you doubt, let me know, I have some examples). Allowing infinite types would fix this. I'm imagining a syntax something like this: someFunctionThatCreatesInfiniteType :: a -> b | b = [(a, b)] Thoughts? Opinions? Am I missing anything obvious? - Job

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trouble with HDBC-mysql on Mac OS X
by Mark Lentczner 21 Nov '10

21 Nov '10

I'm having trouble with HDBC-mysql on Mac OS X (10.6, Snow Leopard). It compiles and builds fine, but when loaded (into ghci), the dynamic loader complains about not being able to find libmygcc.dylib. The issue is, MySQL's libmygcc is a static lib, not a dynamic one, so of course it won't find it. I've checked the build sequence that HDBC-mysql uses, and it calls upon mysql_config to generate the lib options. I've checked those and they are correct, expressly listing libmygcc statically (!). I've tried building against both the 64bit and 32bit versions of MySQL libs. (Suspect that only the 32bit should work.) Anyone have HDBC-mysql running on 10.6? Any ideas for me to try would be appreciated. - Mark Mark Lentczner http://www.ozonehouse.com/mark/ IRC: mtnviewmark

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Why isn't there a cheaper "split-in-two" operation for Data.Set?
by Ryan Newton 20 Oct '10

20 Oct '10

Would there be anything wrong with a Data.Set simply chopping off half its (balanced) tree and returning two approximately balanced partitions -- i.e. Data.Set.split without the pivot argument? Even though it can't quite be constant-time (still need rebalancing) it could still be cheaper than Data.Set.split, right? In the context of Set.hs, I'm thinking along the lines of something like the following: cleave :: Set a -> (Set a, Set a) cleave Tip = (Tip, Tip) cleave (Bin _ x l r) | size l > size r = (l, insertMin x r) | otherwise = (insertMax x l, r) For one thing, this seems like the proposed function would be useful for consuming a set in parallel, similar to Guy Steele's arguments in recent talks (see Organizing Functional Code for Parallel Execution; or, foldl and foldr Considered Slightly Harmful<http://research.sun.com/projects/plrg/Publications/ICFPAugust2009Steele.pdf>). That is, one splits a tree and uses "par" to process the left and right half in parallel, enabling Alternatively, it would be simple to provide a parallel version of mapMonotonic that does this internally, i.e.: mapMonotonic _ Tip = Tip mapMonotonic f (Bin sz x l r) = Bin sz (f x) (mapMonotonic f l) (mapMonotonic f r) Becomes something like: parMapMonotonic f (Bin sz x l r) = par left $ seq right $ Bin sz (f x) left right where left = mapMonotonic f l right = mapMonotonic f r Cheers, -Ryan

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IORef memory leak
by Jim Snow 15 Oct '10

15 Oct '10

I'm having some trouble with excessive memory use in a program that uses a lot of IORefs. I was able to write a much simpler program which exhibits the same sort of behavior. It appears that "modifyIORef" and "writeIORef" leak memory; perhaps they keep a reference to the old value. I tried both ghc-6.8.3 and ghc-6.10.1. Is this a known limitation, or is this a ghc bug, or am I using IORefs in the wrong way? -jim module Main where import Data.IORef import Control.Monad -- Leaks memory leakcheck1 ior = do go 1000000000 where go 0 = return () go n = do modifyIORef ior (+1) go (n-1) -- Leaks memory leakcheck2 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior writeIORef ior (x+1) go (n-1) -- Runs in constant memory leakcheck3 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior go (n-1) main :: IO () main = do ior <- newIORef 0 leakcheck2 ior compiled with: ghc -O2 --make Leak.hs -o Leak

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Web development work
by Michael Snoyman 10 Oct '10

10 Oct '10

Hi all, Often times when trying to pitch Haskell to potential clients the concern is the lack of qualified developers willing to take on projects. As I'm sure many of you are familiar with, clients prefer not to be locked in to a single programmer: an errant bus can significantly reduce the value of their investment. So I'd like to get an idea of how many people out there would be interested in Haskell web development work. For the moment I'm just interested in doing this informally via email, though perhaps in the future it would be beneficial to the community to have this information centralized on a website. I think it would be useful to have some basic skills and experience information, including system administration abilities. I might also have one or two Yesod projects that I'll need to pass off in the future, though it's still unclear if this will happen. If you're interested in that, please let me know, I can give some more details privately. Michael

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EDSL for Makefile
by C K Kashyap 07 Oct '10

07 Oct '10

Hi All, I was thinking about doing an EDSL for Makefile (as an exercise) I put down my line of thought here - http://hpaste.org/40233/haskell_makefile_edsl I'd appreciate some feedback on the approach. Also, I wanted some idea on how(in the current approach) I could make the target name and the dependency available to the action writer - as shown below. r1 = Rule { target = "file1", dependsOn = ["file2"], action = do execute ("gcc -c " ++ dependencyList ++ " -o " ++ target) } -- Regards, Kashyap

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CCC MyType
by Lafras Uys 04 Oct '10

04 Oct '10

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 List, I'm trying to wrap my head around Haskell and Category Theory---very new to both and in keeping with character, I've jumped into the deep-end. The categories and category-extras packages have helped to illustrate some concepts. However, I remain confused and hope someone can point me in the right direction. The ultimate goal is to construct my own instance of a CCC. Here is the class definition for CCC from the categories package: > class ( Cartesian (<=) > , Symmetric (<=) (Product (<=)) > , Monoidal (<=) (Product (<=)) > ) => CCC (<=) where > type Exp (<=) :: * -> * -> * > apply :: (Product (<=) (Exp (<=) a b) a) <= b > curry :: ((Product (<=) a b) <= c) -> a <= Exp (<=) b c > uncurry :: (a <= (Exp (<=) b c)) -> (Product (<=>) a b <= c) I have the following: > {-# LANGUAGE TypeFamilies, > MultiParamTypeClasses, > UndecidableInstances #-} > import Control.Category.Braided > import Control.Category.Associative > import Control.Category.Cartesian > import Control.Category.Monoidal > import Control.Category.Cartesian.Closed > data MyType a b = MyType {f::(a -> b)} > data SomeType a b = SomeType {g::(a,b)} > instance (Symmetric MyType (Product MyType), > Monoidal MyType (Product MyType), > PreCartesian MyType) => CCC MyType where > type Exp MyType = SomeType It type checks in ghci and gives the appropriate warnings that apply, curry and uncurry have not been defined---I can't see how to define these. Is it actually possible with the two data types I have? I would really appreciate some help. Lafras -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkyaGWYACgkQKUpCd+bV+kqgWgCfY7B7pUttB0xfeOAN1V3NDqRL fgQAniK/EJsV9jS7XWxmxElCVD6AW0as =l0x/ -----END PGP SIGNATURE-----

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A parsec question
by Ben Franksen 03 Oct '10

03 Oct '10

I have a question about Parsec. The following program > import Control.Applicative ((*>),(<*)) > import Text.Parsec > import Text.Parsec.Char > block p = char '{' *> p <* char '}' > parser = block (many digit) > main = parseTest parser "{123a}" gives the output parse error at (line 1, column 5): unexpected "a" expecting "}" Note the last line mentions only '}'. I would rather like to see expecting "}" or digit since the parser could very well accept another digit here. (1) What is the reason for this behaviour? (2) Is there another combinator that behaves as I would like? (3) Otherwise, how do I write one myself? BTW, I am using parsec-3.1.0 and ghc-6.12.3. Cheers Ben

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I still cannot seem to get a GUI working under Windows.
by caseyh＠istar.ca 03 Oct '10

03 Oct '10

I still cannot seem to get a GUI working under Windows. For Haskell GUI's is Ubuntu easier to setup. If so, we're losing people if Haskell GUI's are so hard to get working under Windows.

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Re: [Haskell-cafe] Coding conventions for Haskell?
by Andrew Coppin 03 Oct '10

03 Oct '10

On 29/09/2010 02:18 PM, Henning Thielemann wrote: > Andrew Coppin wrote: > >> Tastes do indeed vary. To me, both of these are incorrect, and the >> correct way is >> >> data Foo a b = >> Foo a | >> Bar b | >> Foobar a b >> deriving (Eq, Ord) > > The truth is: Given the separator style of constructor definition, > there is no correct way to format those declarations. :-) The correct > way would be to allow terminator style. Well, yes, there is that. (And this isn't the only place in the syntax where it applies either. Tried editing export lists lately? Or Cabal module lists?)

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