- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Heavy lift-ing
by michael rice 24 Jul '10

24 Jul '10

Thanks all, Wild, at least up to the "optional" part, which I haven't dug into yet. So the (+) for the Maybe monad and the (+) for the List monad are one in the same, the magic springs from the monads. Why is it called "lift"-ing? Michael --- On Fri, 7/23/10, Jürgen Doser <jurgen.doser(a)gmail.com> wrote: From: Jürgen Doser <jurgen.doser(a)gmail.com> Subject: Re: [Haskell-cafe] Heavy lift-ing To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Friday, July 23, 2010, 4:50 PM El vie, 23-07-2010 a las 15:05 -0400, Nick Bowler escribió: > On 11:43 Fri 23 Jul , michael rice wrote: > [...] > > But how does one add [0,1] and [0,2] to get [0,2,1,3]? > > liftM2 (+) [0,1] [0,2] gives the list > > [0+0, 0+2, 1+0, 1+2] which one could have found out by asking ghci: Prelude Control.Monad> let f a b = show a ++ " + " ++ show b Prelude Control.Monad> liftM2 f [0,1] [0,2] ["0 + 0","0 + 2","1 + 0","1 + 2"] or simpler: Prelude Control.Monad> liftM2 (,) [0,1] [2,3] [(0,2),(0,3),(1,2),(1,3)] i.e., the in the list monad, you pair each element of one list with each element of the other(s). > (recall that (>>=) in the list monad is concatMap). (>>=) = flip concatMap, to be precise. Or, concatMap = (=<<) Now let's have some fun with equational reasoning to see what liftM2 does exactly: (Only read this if you really want to!) liftM2 f a b = { definition of liftM2 } do {x <- a; y <- b; return (f x y)} = { simplified translation of do-notation } a >>= \x -> (b >>= \y -> return (f x y)) = { change (>>=) to (=<<) and flip arguments } (\x -> ((\y -> return (f x y)) =<< b)) =<< a = { specialized to the list monad } (\x -> ((\y -> [f x y])) `concatMap` b)) `concatMap` a = { change concatMap to prefix application } concatMap (\x -> concatMap (\y -> [x+y]) b) a and indeed: Prelude> concatMap (\x -> concatMap (\y -> [x+y]) [0,2]) [0,1] [0,2,1,3] with some effort, I think one can understand what happens here. It should also be clear how this is generalized to liftM3, liftM4, etc. Oh, btw, what about liftM1? Obviously, this should be the following: liftM1 f a = { definition } do { x <- a ; return f a } = { same changes as above } concatMap (\x -> [f x]) a = { definition of concatMap } concat (map (\x -> [f x]) a = { concating singletons can be simplified } map (\x -> f x) a = { eta-reduction } map f a i.e., liftM1 = map, which is indeed just fmap for lists, as already pointed out. You can use this to simplify the last line of the concatMap derivation above: concatMap (\x -> concatMap (\y -> [x+y]) b) a = { see above } concatMap (\x -> map (\y -> x+y) b) a = { use operator section } concatMap (\x -> map (x+) b) a which is about as clear as possible a definition for liftM2 (+) Jürgen

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HaTeX build failure
by Daniel Díaz 23 Jul '10

23 Jul '10

Hi, I uploaded a package, named HaTeX, to Hackage, but it gets a build failure: cabal: Couldn't read cabal file "./HaTeX/1.0.0/HaTeX.cabal It makes reference to 1.0.0 version, even when building version 1.0.1. I get the same error when I use cabal install. However, using configure>build>register I don't have any problem. I would like this package works well, but I don't know how to solve this problem. In fact, I don't know what's the problem. Here a link to the HaTeX package: http://hackage.haskell.org/package/HaTeX Any idea?

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cabal, Setup.lhs example
by Tom Hawkins 23 Jul '10

23 Jul '10

I have a script I'm using to generate some Haskell code for a library. How do I specify this flow in the cabal setup file? Would someone point me to a relevant library I can reference as an example? -Tom

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src/Text/XML/HaXml/Lex.hs:(156, 0)-(160, 22): Non-exhaustive patterns in function white
by Alexander Kotelnikov 23 Jul '10

23 Jul '10

Hello. I decided to run a couple of test XML-RPC applications using haxr. Everything worked fine while responces from my (actually, other people's) server were something relatively small (a number, a line), but with larger responces (starting around couple of kilobytes) my applications began to fail with the error I put into subject. Sounds like XML parsing problem. Though server gives a proper responce (I used tcpdump to see what is returned, looks like a valid XML with methodResponce). Any idea what is wrong and how this can be fixed? Thanks, Alexander

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Page rank and GHC docs directory organization
by Ryan Newton 23 Jul '10

23 Jul '10

GHC docs seem to have the problem that newer versions only gradually overtake older ones in page rank, resulting in the effect that if one uses Google to find library documentation, they may accidentally look at an old version. For example, if I google "Data.Data Haskell" the first link brings me to: http://www.haskell.org/ghc/docs/6.10.2/html/libraries/base/Data-Data.html Oops, version 6.10.2! I'm no web expert, but I think the problem is that the "latest" directory isn't used consistently by others and/or the fact that latest/ redirects to a concrete version number. Thus if you go to: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Data.html It redirects immediately to 6.12.2: http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base/Data-Data.html So is the 6.12.2 target accruing pagerank rather than the latest one? Even if someone links the /latest/ URL? If that's the problem, would it fix things just to make latest/ a full directory structure in its own right (a clone rather than redirect)? Cheers, -Ryan

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Hackage survey
by Sylvain Le Gall 23 Jul '10

23 Jul '10

Hello dear haskellers, I am working on OASIS-DB, a tool similar to Hackage and would like to have more information on how Haskellers use Hackage. I have setup a small poll (10 questions only) with the help of John Goerzen and Don Stewart. I will be very thankful that you answer this poll: http://polldaddy.com/s/2C49D15023CB88C6 I will share the data with the Haskell community, so that we will be able to make progress on Hackage and OASIS-DB. Cheers Sylvain Le Gall

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bounded ranges
by Chad Scherrer 22 Jul '10

22 Jul '10

Hello cafe, I'm trying to do some things with bounded indices so I can carry around arrays (well, Vectors, really) without needing to refer to the bounds. For example, if I know my indices are Bool values, I can do > rangeSize (minBound, maxBound :: Bool) 2 I'd like to be able to do this in general, but... > :t rangeSize (minBound, maxBound) <interactive>:1:11: Ambiguous type variable `a' in the constraints: `Bounded a' arising from a use of `minBound' at <interactive>:1:11-18 `Ix a' arising from a use of `rangeSize' at <interactive>:1:0-29 Probable fix: add a type signature that fixes these type variable(s) I thought it might help to put it into a module and do a better job with the type, like this: bdRangeSize :: (Ix i, Bounded i) => i -> Int bdRangeSize _ = rangeSize (minBound, maxBound :: i) but I still have problems: MyArray.hs:22:36: Could not deduce (Bounded i1) from the context () arising from a use of `maxBound' at MyArray.hs:22:36-43 Possible fix: add (Bounded i1) to the context of an expression type signature In the expression: maxBound :: i In the first argument of `rangeSize', namely `(minBound, maxBound :: i)' In the expression: rangeSize (minBound, maxBound :: i) I thought maybe it's an existential types problem or something, but I don't understand why it would be coming up here. Any thoughts? Oh yes, and I'm using GHC version 6.12.1. Thanks, Chad

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gd on Win32
by Jason Felice 22 Jul '10

22 Jul '10

Howdy, The listed maintainer for gd is no longer maintaining it. I'm wondering if anyone else is, if there is a version control URL, and how I can send patches. I need to get it working for Win32. Thanks

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Regular Expression to Determinate Finite Automata translator
by Aaron Gray 22 Jul '10

22 Jul '10

Hi, I am a Haskell newbie. I have coded a Regular Expression to Determinate Finite Automata translator. Algorithm from the Dragon Book. Would someone eyeball the code and give me suggestions please. I have not done anything on character classes yet though. And the parsing is a bit of a hack. What I am not sure about is having to have multiple versions of similar datatype, each with variations in order to enumerate and generate followPos set. Is there a better way of implementing this ? Many thanks in advance, Aaron

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music-related problem
by Michael Mossey 22 Jul '10

22 Jul '10

Wondering if I could get some suggestions for coding this problem. A musical document (or "score") consists primarily of a list of measures. A measure consists primarily of lists of "items". We'll consider only one kind of item: a note. Items have a location within the measure. A note's location indicates both where it goes on the page (i.e. a visual representation of the score) and what moment in time it begins sounding (i.e. rendering the score in sound). My concern here is sound. data Doc = [Measure] data Loc = ... (represents a location within the musical document including measure number) data Measure = Measure [(Loc,Item)] -- In the Meausre, we can assume (Loc,Item) are in -- ascending order Notes also have an end, when indicates when in time they stop sounding. See the 'end' field below. Also note the 'soundedEnd' 'tieStart' and 'tieStop' fields which I will explain. data Item = Note { pitch :: Pitch , end :: Loc , soundedEnd :: Maybe Loc , tieNext :: Bool , tiePrior :: Bool } There is a concept of "tied notes". When two notes are tied together, their durations are summed and they are sounded continuously as if one note. Ties have several uses, but one important one is to make a sound that begins in one measure and ends in a later measure, by tying notes across measures. The 'tieNext' field indicates if a note is tied to the following note (that is, the next note of the same pitch). 'tiePrior' indicates if tied to immediately prior note of same pitch. A chain of notes can be tied. Notes in the middle with have both tieNext and tiePrior set. In the event a note is within a chain of ties, its 'soundedEnd' field needs to be computed as Just e where e is the end of the last note in the chain. This information is useful when rendering the document as sound. My problem is: - given a Doc in which all fields have been set EXCEPT soundedEnd (all soundedEnd's are given a default value of Nothing) - update those notes in the Doc which need to have soundedEnd set. This involves chasing down the chain of ties. I can solve a simpler problem which is -- Given a note with tieNext set, and a list of notes, find -- the end Loc of the last note in the chain. Only notes -- with the same pitch as 'firstNote' are considered when looking -- for the chain of notes. computeSoundedEnd :: Item -> [Item] -> Loc computeSoundedEnd firstNote notes = compSndEnd (pitch firstNote) notes compSndEnd :: Pitch -> [Item] -> Loc compSndEnd _ [] = error "tie chain doesn't come to completion" compSndEnd p (n:ns) = if pitch n == p then if tieNext n then if tiePrior n then compSndEnd p ns else error "illegal tie chain" else if tiePrior n then end n else error "illegal tie chain" else compSndEnd p ns The thing that is hard for me to understand is how, in a functional paradigm, to update the entire Doc by chasing down every tie and making all necessary updates. Thanks, Mike

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