November 2010 - Haskell-Cafe

Re: [Haskell-cafe] Confused about missing data constructor
by Antoine Latter 24 Nov '10

24 Nov '10

I meant that 'pop' and 'push' should have been written with 'modify', 'get', and 'set' instead of the raw constructor, not as a drop-in replacement. I can show you examples later if this isn't clear, unless I'm not understanding your code above. Antoine On Nov 24, 2010 10:46 AM, "Adam Miezianko" <adam(a)theorylounge.org> wrote: > On Wed, 24 Nov 2010 06:17:47 -0800, Antoine Latter <aslatter(a)gmail.com> > wrote: > >> On Tue, Nov 23, 2010 at 6:58 PM, Adam Miezianko <adam(a)theorylounge.org> >> wrote: >>> I'm working through Learn You a Haskell for Great Good [1] and getting >>> a compiler error while playing around with some of the code. I have >>> this: >>> >>> pop :: State Stack Int >>> >>> But when I try to load it into ghci I get the following errors: >>> >>> /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State' >>> >>> Now, I'm not exactly sure how to read the documentation for >>> Control.Monad.State [2] but it seems that newtype State s a = State >>> {...} defines a constructor, or am I wrong on that point too? So, >>> what am I missing here? In case it matters, I am using mtl-2.0.1.0 >>> and ghci 6.12.3. >>> >>> [1] http://learnyouahaskell.com/for-a-few-monads-more >>> [2] >>> http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html >>> >> >> It looks like your documentation doesn't match the library you're using. >> >> The documentation for mtl-2.x is here: >> http://hackage.haskell.org/package/mtl > > Thanks! > > My documentation didn't have any version identification, so discovering > that there was a breaking interface change is no surprise now. Next time > hoogle doesn't return me what I want I'll first suspect it's been removed > before questioning the accuracy of the search results. > > On a somewhat related note, I also found the link you provide, but I found > hackage.haskell.org to be intermittently timing out for me this last week. > >> However you might be better served using the 'modify' function. > > Looking at the type signature of modify, I'm unsure how to use it. It > does not look like I can just drop it in as a replacement for State > (StateT, state). Could someone provide an example? The tutorials seem to > mostly use the State constructor that's now gone. > > -- > Adam Miezianko

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Another Quasi-Quotation question
by jean-christophe mincke 24 Nov '10

24 Nov '10

Hello, I am still playing with template-haskell... I wonder, is there any reason why a quasiquoter cannot create haskell statements and declarations in addition to expressions and patterns? Or more generally create any legal Haskell syntax tree. I.e Suppose I would like to create a quasiquoter for the C language (please, imagine that it could be useful). I could write sth such as : [$c| int a = 6; int b = 7; int c = a +b; struct S { int x; int y} |] It could be nice to generate the appropriate haskell code: a = 6 b= 7 c= a + b data S = S { x::Int; y:Int} etc That would allow to really embed any kind of language in a more or less easy way into haskell, provided that code can be translated into legal haskell. Is there anything that prevent these features. Thank you Regards J-C

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Confused about missing data constructor
by Antoine Latter 24 Nov '10

24 Nov '10

On Tue, Nov 23, 2010 at 6:58 PM, Adam Miezianko <adam(a)theorylounge.org> wrote: > I'm working through Learn You a Haskell for Great Good [1] and getting > a compiler error while playing around with some of the code. I have > this: > > -- BEGIN state.hs > import Control.Monad.State > > type Stack = [Int] > > pop :: State Stack Int > pop = State $ \(x:xs) -> (x,xs) > > push :: Int -> State Stack () > push x = State $ \xs -> ((), x:xs) > > stackManip :: State Stack Int > stackManip = do > push 3 > pop > pop > -- END state.hs > > But when I try to load it into ghci I get the following errors: > > Prelude> :load "/home/admi/.pe/state.hs" > [1 of 1] Compiling Main ( /home/admi/.pe/state.hs, interpreted ) > > /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State' > > /home/admi/.pe/state.hs:9:9: Not in scope: data constructor `State' > Failed, modules loaded: none. > Prelude> > > Now, I'm not exactly sure how to read the documentation for > Control.Monad.State [2] but it seems that newtype State s a = State > {...} defines a constructor, or am I wrong on that point too? So, > what am I missing here? In case it matters, I am using mtl-2.0.1.0 > and ghci 6.12.3. > > [1] http://learnyouahaskell.com/for-a-few-monads-more > [2] http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html > It looks like your documentation doesn't match the library you're using. The documentation for mtl-2.x is here: http://hackage.haskell.org/package/mtl Starting in version 2.0, the mtl package no longer exports a constructor called 'State' - but there is a function 'state' which servers the same purpose. However you might be better served using the 'modify' function. I hope that helps, Antoine > -- > Adam Miezianko > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Confused about missing data constructor
by Adam Miezianko 24 Nov '10

24 Nov '10

I'm working through Learn You a Haskell for Great Good [1] and getting a compiler error while playing around with some of the code. I have this: -- BEGIN state.hs import Control.Monad.State type Stack = [Int] pop :: State Stack Int pop = State $ \(x:xs) -> (x,xs) push :: Int -> State Stack () push x = State $ \xs -> ((), x:xs) stackManip :: State Stack Int stackManip = do push 3 pop pop -- END state.hs But when I try to load it into ghci I get the following errors: Prelude> :load "/home/admi/.pe/state.hs" [1 of 1] Compiling Main ( /home/admi/.pe/state.hs, interpreted ) /home/admi/.pe/state.hs:6:6: Not in scope: data constructor `State' /home/admi/.pe/state.hs:9:9: Not in scope: data constructor `State' Failed, modules loaded: none. Prelude> Now, I'm not exactly sure how to read the documentation for Control.Monad.State [2] but it seems that newtype State s a = State {...} defines a constructor, or am I wrong on that point too? So, what am I missing here? In case it matters, I am using mtl-2.0.1.0 and ghci 6.12.3. [1] http://learnyouahaskell.com/for-a-few-monads-more [2] http://cvs.haskell.org/Hugs/pages/libraries/mtl/Control-Monad-State.html -- Adam Miezianko

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ANNOUNCE: Graphalyze-0.11.0.0 and SourceGraph-0.7.0.0
by Ivan Lazar Miljenovic 24 Nov '10

24 Nov '10

I am pleased to announce the 0.7.0.0 release of my graph-theoretic source code analysis tool SourceGraph [1], and the library it uses Graphalyze 0.11.0.0 [2]. [1]: http://hackage.haskell.org/package/SourceGraph [2]: http://hackage.haskell.org/package/Graphalyze Changes in SourceGraph (apart from those resulting from improvements to Graphalyze): * Edge widths are now log-based rather than being the number of function calls; this avoids having absurdly thick edges. * Source files that were not able to be parsed are now listed both to stderror and in the resulting report (thus helping you work out why you're missing some code) * Also builds with Cabal-1.10; whilst I do not currently have GHC 7 installed anywhere and thus can't test this, I know of no reason why SourceGraph cannot be built with GHC 7 (unless it uses a library that isn't buildable on GHC 7 that I have no control over). Changes in Graphalyze: * Legends can now contain text-only fields rather than just graphs; also, the title of each legend is now displayed first rather than what it describes. * To fix a bug spotted by Han Joosten, the reports currently use Posix-style path separators even on Windows (otherwise, HTML documents produced on Windows were not in a state to be uploaded elsewhere). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANNOUNCE: graphviz-2999.11.0.0
by Ivan Lazar Miljenovic 24 Nov '10

24 Nov '10

I am pleased to announce version 2999.11.0.0 of graphviz [1], my Haskell bindings to the Graphviz suite of graph visualisation tools [2]. [1]: http://hackage.haskell.org/package/graphviz [2]: http://graphviz.org/ This release is mostly backwards-compatible (unless you dealt directly with Point and LayerList values); the changes are: * Improved documentation, including examples of how to use `GraphvizParams` values (since several people indicated that they found them confusing) * Addition of the `Labellable` class (and its method `toLabel`) to make it easier to construct labels. * Backslashes (i.e. the `\` character) are now escaped/unescaped properly (bug spotted by Han Joosten). As part of this: - Dot-specific escapes such as `\N` are now also handled correctly, so the slash does not need to be escaped. - Newline (`'\n'`) characters in labels, etc. are escaped to centred-newlines in Dot code, but not unescaped. * `Point` values can now have the optional third dimension and end in a `!` to indicate that that position should be used (as input to Graphviz). * `LayerList` uses `LayerID` values, and now has a proper `shrink` implementation in the test suite. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANN: dnscache - caching DNS resolver
by Ertugrul Soeylemez 24 Nov '10

24 Nov '10

Hello everybody, (second try, since the first mail somehow didn't arrive) I've just uploaded a caching DNS resolver library, which also includes a command line utility for quick mass DNS resolution: http://hackage.haskell.org/package/dnscache Although I've tested it throroughly it's still in beta phase. Feel free to play around with it. Feedback is highly appreciated. Thank you. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Re: [Haskell-cafe] Derived type definition
by Miguel Mitrofanov 24 Nov '10

24 Nov '10

Well, you can resort to functional dependencies, I guess... {-# LANGUAGE FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses, UndecidableInstances #-} module FunDeps where data Rec a r = Rec a r data RecNil = RecNil data Wrapper a = Wrapper a class Wrapped r w | r -> w where i :: r -> w instance Wrapped RecNil RecNil where i RecNil = RecNil instance Wrapped r w => Wrapped (Rec a r) (Rec (Wrapper a) w) where i (Rec a r) = Rec (Wrapper a) (i r) type TTest = Rec Int (Rec String RecNil) type TTestWrapped = Rec (Wrapper Int) (Rec (Wrapper String) RecNil) a :: TTest a = Rec 1 (Rec "a" RecNil) f :: TTestWrapped -> (Int, String) f (Rec (Wrapper n) (Rec (Wrapper s) RecNil)) = (n, s) r = f (i a) ...but that would be just an awkward way to do the same thing, so, my advice: don't. Type families are much nicer. On the other hand, you could do your "Rec" type polimorphic in "wrapper"; assuming your real-life Wrapper is not just an identity, this would be worth considering: {-# LANGUAGE KindSignatures #-} module PolyM where data Rec a r w = Rec (w a) (r w) data RecNil (w :: * -> *) = RecNil data Wrapper a = Wrapper a -- in reality it should be something else newtype Identity a = Identity a class Wrapped r where i :: r Identity -> r Wrapper instance Wrapped RecNil where i RecNil = RecNil instance Wrapped r => Wrapped (Rec a r) where i (Rec (Identity a) r) = Rec (Wrapper a) (i r) type TTest = Rec Int (Rec String RecNil) Identity type TTestWrapped = Rec Int (Rec String RecNil) Wrapper a :: TTest a = Rec (Identity 1) (Rec (Identity "a") RecNil) f :: TTestWrapped -> (Int, String) f (Rec (Wrapper n) (Rec (Wrapper s) RecNil)) = (n, s) r = f (i a) 24.11.2010 12:24, kg пишет: > Ok, it's exactly what i hoped. > > And I would like to know (for fun) if it's possible to do it without type family extension. > I've tried ... without success. > > Thx. > > On 11/22/2010 10:46 PM, Miguel Mitrofanov wrote: >> Sure, it's possible with TypeFamilies. The following compiles OK: >> >> {-# LANGUAGE TypeFamilies #-} >> module TypeCalc where >> data Rec a r = Rec a r >> data RecNil = RecNil >> data Wrapper a = Wrapper a >> class TypeList t where >> type Wrapped t >> i :: t -> Wrapped t >> instance TypeList RecNil where >> type Wrapped RecNil = RecNil >> i RecNil = RecNil >> instance TypeList r => TypeList (Rec a r) where >> type Wrapped (Rec a r) = Rec (Wrapper a) (Wrapped r) >> i (Rec a r) = Rec (Wrapper a) (i r) >> type TTest = Rec Int (Rec String RecNil) >> type TTestWrapped = Rec (Wrapper Int) (Rec (Wrapper String) RecNil) >> a :: TTest >> a = Rec 1 (Rec "a" RecNil) >> f :: TTestWrapped -> (Int, String) >> f (Rec (Wrapper n) (Rec (Wrapper s) RecNil)) = (n, s) >> r = f (i a) -- so, "i a" is of the type TTestWrapped. >> >> >> On 22 Nov 2010, at 23:43, kg wrote: >> >>> Hi, >>> >>> I've tried to simplify as much as possible my problem. Finally, I think I can resume it like that: >>> >>> Suppose these following data types : >>> data Rec a r = Rec a r >>> data RecNil = RecNil >>> data Wrapper a = Wrapper a >>> >>> Then, we can build the following type: >>> type TTest = Rec Int (Rec String RecNil) >>> or this type: >>> type TTestWrapped = Rec (Wrapper Int) (Rec (Wrapper String) RecNil) >>> >>> Is it possible to build TTestWrapped from TTest ? >>> >>> >>> Thx in advance, >>> Antoine. >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: [Haskell-beginners] Problem "grouping" a list
by Aai 24 Nov '10

24 Nov '10

I don't know what this tuple is representing, but if you want to group you'll have to specify on 'what': - the tuple, - the fst or - the snd Here's a possibility with grouping on the fst import Data.List import Data.Ord import Data.Function groupAtoms :: (Float -> Bool) -> [(Float,Integer)] -> ([[(Float,Integer)]],[[(Float,Integer)]]) groupAtoms p = partition (p.sum. map fst). groupBy ((==)`on`fst). sortBy (comparing fst) Use: groupAtoms (>=1.0) myList If this is what you want: proper lists are in the fst of the result. Hallo João Paulo Pizani Flor, je schreef op 23-11-10 18:23: > Hello dear Haskellers! > > I've been a user and big fan of Haskell for a while, but only now I'm writing > my first "big" project in Haskell (some thousands of lines of code perhaps). > It's an interpreter for a programming language, the source code is music! Yes, > sheet music! :D > > OK, so my specific problem goes like this: I have a list of tuples > :t myList > [ (Float, Integer) ] > > And I want to "group" this list into a nested list > groupAtoms :: [ (Float,Integer) ] -> [ [(Float,Integer)] ] > > Of course, I want that the concatenation of the groups yield me the original > list, i.e, (foldl (++) [] . groupAtoms == id), and the criterion that defines > a group is that: > "The sum of the first elements in the tuples comprising the list must be > greater than or equal to 1.0". That is, given a list of tuples, the boolean > predicate deciding whether this list is a PROPER group (True) or TOO SMALL > (False) is: > \g -> sum (map fst g) >= 1.0 > > > Although the criterion is very clear, I've tried hard until now and couldn't > come up with a function for producing the nested list based on this grouping > criterion. I am sure that the Haskell Hierarchical Libraries have something > to help me, but only now I see I'm still a big noob :P > > Could someone please help me writing this function? > > > My best regards from Brazil, > > João Paulo Pizani Flor > joaopizani(a)gmail.com <mailto:joaopizani@gmail.com> > Federal University of Santa Catarina > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners -- Met vriendelijke groet, =@@i

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ANN: dnscache - caching DNS resolver
by Ertugrul Soeylemez 24 Nov '10

24 Nov '10

Hello everybody, I've just uploaded a caching DNS resolver library, which also includes a command line utility for quick mass DNS resolution: http://hackage.haskell.org/package/dnscache Although I've tested it throroughly it's still in beta phase. Feel free to play around with it. Feedback is highly appreciated. Thank you. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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