July 2010 - Beginners

playing around with code from the tutorials for Yesod - and this came up.
by Michael Litchard 26 Jul '10

26 Jul '10

This code I am using can be seen from this url. http://docs.yesodweb.com/yesod/tutorial/ajax.html I downloaded it from the site, tried to compile and got this error. ajax.lhs:55:29: Not in scope: `style_css' ajax.lhs:55:29: Not in scope: `script_js' Here is line 55 > hamletToContent [$hamlet| I'm lost when it comes to figuring out why this won't compile? Changes in interface since the tutorial was written maybe? Any help would be greatly appreciated.

1 1

use of First or Last
by Johann Bach 26 Jul '10

26 Jul '10

I am reading "typeclassopedia" and got curious about how the monoid instances First or Last could be applied to a real problem. Any suggestions for improvement would be welcome. So I imagined this problem: We have a list of executives in a company, and their rank. Some executives have assistants, some don't. import Data.List import Data.Monoid import Data.Function data Executive = Executive { name :: String , rank :: Int , assistant :: Maybe String } deriving (Show) We want to bring an urgent matter to an executive. We'd like to contact the highest ranking executive, but we also don't want to disturb any executive directly, so we want to contact the highest ranking executive with an assistant. This function finds the name of that assistant: findContact :: [Executive] -> Maybe String findContact = getLast . mconcat . map (Last . assistant) . sortBy (compare `on` rank) data1 = [ Executive "Mary Poppins" 1 Nothing , Executive "Fred Flinstone" 2 (Just "John Deere") , Executive "Ann Curry" 3 (Just "Terry Crews") ] > findContact data1 Just "Terry Crews"

2 1

Sub-second precision
by Alex Rozenshteyn 25 Jul '10

25 Jul '10

I'm trying to write a simple timer application (mostly to learn gui programming), and I'd like it to display 1/100ths of a second. I know I can fake this with threadDelay, but I was wondering if there's a way to get current time to more precision than one second. -- Alex R

2 2

Invoke an external command
by Hong Yang 24 Jul '10

24 Jul '10

Hi, I am trying to pass on arguments and standard input to an external command, and get back its standard output real time. I used "readProcess" from "System.Process" module. It is working fine except that the output is returned strictly, i.e., the output was dumped to the screen all at once when the command was finished. I also tried HSH module and it acted the same as readProcess: runIO $ setenv [("TERM", standard_input)] $ "echo $TERM" -|- (command ++ " " ++ (intercalate " " argsListOfStrings)). Can anyone help me? Thanks, Hong

1 0

uses for Functor or Applicative?
by Johann Bach 24 Jul '10

24 Jul '10

I'm wondering if the following problem might reveal a use for Functor or Applicative, or just generally something cooler than I'm doing now. I have a music document as type MusDoc. It has [Part]. Part has [Note]. I have defined them using field names because I am deliberating allowing for this to get more complicated later. The question is about mapping functions over the individual parts. I often find myself wanting to alter or filter all the parts. In the following example I define filterDoc with a couple of helper functions. What I would like to know: is there a cooler way to do this using instances of Functor or something? Defining operators? import Map(Map) import qualified Map as M type PartId = String -- identification of a part in a music document data PartInfo = .. -- ancillary information about a part -- the notes, markings, etc in a part -- It is conceivable other fields could be added later. data Part = Part { partNotes :: [Note] } -- Main music document, consisting of parts. data MusDoc = MusDoc { docParts :: Map PartId (PartInfo,Part) } -- Helper function to map over parts mapParts :: (Part -> Part) -> MusDoc -> MusDoc mapParts g (MusDoc parts) = MusDoc $ M.map (second g) parts -- Filter notes that meet a predicate filterDoc :: (Note -> Bool) -> MusDoc -> MusDoc filterDoc pred doc = mapParts (filterPart pred) doc -- Helper function to filterDoc. filterPart :: (Note -> Bool) -> Part -> Part filterPart pred (Part notes) = Part (filter pred notes)

4 4

pcap example?
by Patrick LeBoutillier 23 Jul '10

23 Jul '10

Hi all, I've noticed a pcap package on hackagem but by looking at the hackage docs I have no idea on how to use it. Can anyone provide a simple example? Thanks, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

2 1

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

3 2

type puzzlement
by prad 21 Jul '10

21 Jul '10

i'm looking at a factoring function which i've defined as: factorList = map factors where factors p = [ f | f <- [1..fsq p], p `mod` f == 0 ] fsq :: (RealFrac a, Integral b, Floating a) => a -> b fsq = floor . sqrt it goes into ghci just fine, but then when i give it something to go i get errors: *Main> :r Ok, modules loaded: Main. *Main> factorList [4,8,16,32] <interactive>:1:0: Ambiguous type variable `t' in the constraints: `RealFrac t' arising from a use of `it' at <interactive>:1:0-21 `Integral t' arising from a use of `it' at <interactive>:1:0-21 `Floating t' arising from a use of `it' at <interactive>:1:0-21 Probable fix: add a type signature that fixes these type variable(s) so i'm not sure where the unhappiness lies because there is no line number. i examine fsq more closely, because that looks like the most complicated part of the whole thing and try to define it separately: *Main> let fsq = floor . sqrt *Main> :t fsq fsq :: (RealFrac a, Integral b, Floating a) => a -> b ok this seems weird to me because i would have thought things are far simpler with fsq :: Int -> Int however, i guess because we have a composition here, the compiler wants the fact that sqrt :: (Floating a) => a -> a floor :: (RealFrac a, Integral b) => a -> b incorporated in the type? but even if i put the fsq :: (RealFrac a, Integral b, Floating a) => a -> b in to the factorList, i get errors, though the fsq works fine on its own. hence, i have a situation where the parts seem to work, but the whole doesn't because there seems to be some problem with type? -- In friendship, prad ... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's

2 3

cabal installs parsec-2.1 not parsec-3.1? Same with QuickCheck-1.2 not 2.1?
by Gerald Gutierrez 21 Jul '10

21 Jul '10

Why is it that when I run "cabal install parsec" or "cabal upgrade parsec", it does not install the latest 3.1 version but instead sticks with the current 2.1 version? The same goes for quickcheck, where it will stay at 1.2 and not upgrade to 2.1. To install the latest versions, I need to explicitly specify the version. I thought /upgrade/ is supposed to upgrade to the latest version. Is there reasoning behind this?

3 2

parsec question
by Michael Mossey 19 Jul '10

19 Jul '10

data Command = Play Int [Int] | Jump Int -- I want to parse a string that will have any of the following forms -- and turn it into Command -- "p" - Produces "Play 0 []" -- "p v55" - Produces "Play 55 []" -- "p c123" - Produces "Play 0 [1,2,3]" -- "p v13 c12" - Produces "Play 13 [1,2]" -- In other words the p command can have two kinds of arguments, "v" -- and "c", and there are defaults for the case that no argument is -- supplied. -- So it's going to look something like play :: Parser Command play = do char 'p' vResult <- .. maybe a v, otherwise supply default value 0 cResult <- .. maybe a c .. (could the v and c be put in either order?) return $ Play vResult cResult

5 11