- Haskell-Cafe - Haskell.org

Re: showsPrec: cui bono?
by C.Reinke 12 Nov '02

12 Nov '02

> has anybody here used in a non-trivial way the showsPrec anti-parser? Isn't the idea to make things trivial while avoiding performance penalties? Perhaps: simple pretty-printing of abstract syntax trees? I often use it to get simple debugging output for complex internal data structures (first, use deriving; then, define showsPrec; if that's still not good enough, do some real thinking..). Anyway, this reminded me of a litte old hack of mine. Only trivial use of showsPrec, but perhaps you'll like it anyway?-) http://www.cs.ukc.ac.uk/people/staff/cr3/toolbox/haskell/R.hs As with anything else in my toolbox, no warranty for nothing.. Cheers, Claus ------------------- cut here {- Representative thingies.. A little hack to pair values with string representations of their expressions. Useful if you want to explain what map (+1) [1..4] or foldr1 (*) [1..5] do, or if you want to demonstrate the difference between foldr (+) 0 [1..4] and foldl (+) 0 [1..4] Load this module into Hugs (Hugs mode) and type in some of these examples to get an idea of what I mean. Also try map (+) [1..4] This could be extended in various directions, but I wanted to keep things simple. I'm not convinced that extra complications would be worth the effort. Claus Reinke -} default (R Integer) data R a = R {rep:: String ,val:: a } instance Show (R a) where showsPrec _ a = showString (rep a) instance Show (R a -> R b) where showsPrec _ f = showString ("\\x->"++(rep (f x))) where x = R{rep="x",val=error "variable"} instance Show (R a -> R b -> R c) where showsPrec _ f = showString ("\\x y->"++(rep (f x y))) where x = R{rep="x",val=error "variable"} y = R{rep="y",val=error "variable"} lift1 op a = R {rep="("++(rep op)++" "++(rep a)++")" ,val= ( (val op) (val a) ) } lift2 op a b = R {rep="("++(rep op)++" "++(rep a)++" "++(rep b)++")" ,val= ( (val op) (val a) (val b) ) } lift2infix op a b = R {rep="("++(rep a)++" "++(rep op)++" "++(rep b)++")" ,val= ( (val a) `iop` (val b) ) } where iop = val op instance (Num a,Show a) => Num (R a) where (+) = lift2infix R{rep="+",val=(+)} (-) = lift2infix R{rep="-",val=(-)} (*) = lift2infix R{rep="*",val=(*)} negate = lift1 R{rep="-",val=negate} fromInteger a = (\fIa->R{rep=show fIa,val=fIa}) (fromInteger a) instance (Eq a,Num a) => Eq (R a) where a == b = (val a) == (val b) instance (Ord a,Num a) => Ord (R a) where a <= b = (val a) <= (val b) instance (Enum a,Num a,Show a) => Enum (R a) where fromEnum = fromEnum.val toEnum a = R{rep=show a,val=toEnum a} enumFrom x = map toEnum [fromEnum x..] -- missing in Hugs Prelude..

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Rational sequence
by Ferenc Wagner 12 Nov '02

12 Nov '02

With GHC-5.02.2, I do $ ghci Prelude> :m Ratio Ratio> [1%2..10%2] [1 % 2,3 % 2,5 % 2,7 % 2,9 % 2,11 % 2] The question is, why is there 11%2 at the end of the list? It's inconsistent with the (good) rules for Integer, since Ratio> [1,3..10] [1,3,5,7,9] Is this intentional? Feri.

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H98 copyright notice
by Ross Paterson 08 Nov '02

08 Nov '02

The new copyright notice is inconsistent. If it is not otherwise changed, I suggest that the first clause, "The publisher intends this Report to belong to the entire Haskell community, ..." be deleted. Maybe the word "belong" could be replaced with "be available", but that doesn't seem completely accurate.

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producing and consuming lists
by Jorge Adriano 06 Nov '02

06 Nov '02

I might have been not very clear in my last mail. I decided to post again, and go straight to the point, with some small examples. Consider the following function streams. streams :: (Int->Bool, Int->Bool)->(Int, Int)->([Int],[Int]) streams (p,q) (x,y) = (xs',ys') where (xs,ys) = streams (p,q) ((x+y),(y-x)) xs' = if p x then x:xs else xs ys' = if q y then y:xs else ys - produces a pair of ('infinite') lists - produced lists are not indepentent (you need to calculate elements of one list to calculate elements of the other) - in each recursive call an element is added to the 1st/2nd list iff it satisfies a given (Int->Bool) function p/q How should one consume (part of) both lists, avoiding space leaks? A particular example of consuming both lists might be writing them to files: main :: IO() main = do let (s1,s2)=stream ... -- stream applied to some arguments (p,q) (x,y) p' = ... q' = ... writeFile "f1.txt" (show$ takeWhile p' s1) writeFile "f2.txt" (show$ takeWhile q' s2) In this example all elements of s2 required to evaluate (takeWhile p' s1) are kept in memory, until the first file is writen. Notice that writing one element from s1 and one from s2 successively might still cause space leaks to arise. Fusing the consuming functions with the producer is a possible, but IMO dirty, way out. If my question doesn't seem to make sense for any reason, please tell me, maybe I am missing something obvious here. Thanks, J.A.

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GUI Library for Visualizing Data
by Peter Simons 06 Nov '02

06 Nov '02

Hi, the Haskell language is fairly new to me, so please pardon me if this is a frequently asked question, but I wasn't able to find an answer myself. I'm looking for a library that allows me to visualize data under X11 screen, for instance as a pie chart or as a bar chart. I seem to recall that something like that was available for the GTK toolkit, but has this library been ported to Haskell? Or any other library? I'd appreciate any pointers! -peter

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Job
by Mark Carroll 05 Nov '02

05 Nov '02

The company I'm involved with - Aetion, a tiny defense contractor in Columbus, Ohio - will likely be wanting to hire an affordable Haskell programmer before the end of the year. So, on the offchance that any of you guys are be interested, or know of someone who might be, feel free to e-mail me for more information or to supply your resume. Unfortunately, the nature of the work is such that US citizenship or a security clearance would help, and telecommuting isn't really an option. On the upside, the work should be varied and interesting - more challenging than mundane. (I wonder if I should mention this in comp.lang.functional, too.) For the next Haskell communities report thing in a few months, I'll try to say a bit about what we're up to. (-: At this stage, we're just starting, so there's little to say. -- Mark

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streaming, state, etc...
by Jorge Adriano 03 Nov '02

03 Nov '02

Hi, OK so I had completed my (search) algorithm implementation. And the 'last' function was something like: "until p f x0". But now I wanted all iterations, I wanted to analyze the behavior of the algorithm. What could I do? I thought about using Trace... but it didn't seem adequate, I wanted to manipulate and use that info... maybe monads... that would cluter my algorithm so bad... This feeling lasted for a few seconds, then I thought, Lazy Evaluation! "iterate p f x0" There, done :) I had an infinite list, and I'd just take as much as I wanted. *Analysis* and the *algorithm* itself were *completely separated*, yet it was using the information as soon as it was calculated - or maybe "it was being calculated only when it was needed". I could not do this kind of stuff in imperative languages. Very nice :) ------------ Problem 1: ------------ I've used that many times since then, but lately it doesn't really seem to serve my needs. What if I need to get two different kinds of data, say (list1, list2), and now I want to write them: writeFile file1 (analyze list1) writeFile file2 (analyze list2) What if you need to evaluate list2 to evaluate list1? Not good! I'm evaluating list2 and not consuming it, therefore space leak. I could alternate between one list and another, but what if at a certain point, evaluating the next elem in "list1" caused the evaluation of a few thousand elements of "list2"? So how do you usually deal with this kind of stuff? Concurrency?? ------------ Problem 2: ------------ Yes "iterate p f x0" looks good. What if function "f" is not a pure one? If my monad is a Lazy one, streaming can be used(*), if it is not, that won't work. So streaming could work with monads like State, (lazy) ST. Not with IO, or (lazy) ST. (*) do list<-myalgorithm writeFile file1 (analyze list) So, how do you usually do this kind of thing - extracting lists of info from an algorithm? J.A.

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dozen
by Nuno Silva 30 Oct '02

30 Oct '02

can anyone help me how to get the dozen number? using Int example dozen 1020 the expected result is: 2 --------------------------------- Do you Yahoo!? Y! Web Hosting - Let the expert host your web site

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RE: Concurrent Haskell, asynchronous exceptions and interruptibility (again)
by Simon Marlow 30 Oct '02

30 Oct '02

> For Concurrent Haskell, with respect to asynchronous > exceptions, can a > thread receive an asynchronous exception if it's evaluating > an expression > that could "potentially block on IO" when evaluated within > the "block" > function? > > Specifically, is interruptibility determined statically in > the language definition or dynamically at run time? "dynamically at runtime", according to the semantics given in our paper. GHC does actually implement this, although it implements a slightly different version of throwTo from the one described in the paper (ours blocks until the exception has been delivered). > For example, assume a thread is evaluating the following function. > > transfer :: MVar a -> IO (MVar a) > transfer from = > block $ do > to <- newEmptyMVar > x <- takeMVar from > putMVar to x > return to > > In general, since putMVar is an IO function, it can > potentially block (if > its MVar argument is already occupied with a value). However, > since "MVar > to" will obviously be vacant in this instance, the putMVar > can not block on > IO. > > Would ghc allow an asynchronous exception to be raised at the > point that "putMVar to x" is evaluated? Not unless the putMVar blocks. The proof that it never blocks depends on the behaviour of other threads though: if you have another thread doing putMVar on the same MVar, then it may block. Other threads must be "well-behaved" for it to work. > If not, then I guess transfer is exception safe. > > Would that answer scale? Could I replace the "putMVar to x" with an > arbitrarily large/complex IO function that I might write that > is guaranteed > (by me at least) to be non-blocking and still be sure that an > exception > would not be delivered during evaluation of that function? Yes. Cheers, Simon

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Last generator in do {...}
by George Kosmidis 30 Oct '02

30 Oct '02

I am sure there are a billion errors in this. This is the first one: Line:17 - Last generator in do {...} must be an expression Can Anyone point the errors and maybe tell me if there is a possibility for this to work? import IO import System import List import Maybe import Char import Numeric type Name=String type Room=Int type Dr=String type PatientTup=(Name,(Room,Dr)) type PatientList=[PatientTup] main=do userText<-getText ----------------------------------------------------------------------------------- ________getText::IO String ________getText=do nc<-getText ___________________return (:nc) ----------------------------------------------------------------------------------- ________PatientList::[(String,(Int,String))] ________PatientList=[("Robson, Brian",(2,"MJH")), _____________________("Hitchin, Linda",(1,"ILR")), _____________________("Reeve, Paul", (2,"ILR"))] ----------------------------------------------------------------------------------- ________getWards::Int->Int ________getWards PatientList=[Room | (Name,(Room,Dr)) <- PatientList, Room==getText] _________________do putSpc (length Room) _________________return(RoomLength) ----------------------------------------------------------------------------------- ________getPatient::String->String ________getPatient PatientList=[Name,Room,Dr | (Name,(Room,Dr)) <- PatientList, Name==getText] ___________________do putChar (Name) ______________________putChar 'is in ward' ______________________putChar (Room) ______________________putChar 'and is treated by' ______________________putChar (Dr) ______________________return (wl) ----------------------------------------------------------------------------------- ________printFreq::WordList->IO() ________printFreq wl ----------------------------------------------------------------------------------- ________NumberInWard::printFreq(getWards(getText)) ----------------------------------------------------------------------------------- ________PatientInfo::printFreq(getPatient(getText)) ----------------------------------------------------------------------------------- ________setABC::ReceptionReport -> ReceptionReport ________setABC [ ]___=[ ] ________setABC(wn:ws)=setABC[(w,wc) | (w,wc)<-ws, not(isBigger(w,wc)wn)] _________++[wn]++setABC[(w,wc) | (w,wc) <- ws, isBigger (w,wc) wn] __________________where ___________________isBigger (w1,wc1) (w2,wc2) ____________________| ord(head(w1))-ord(head(w2))>0=True ____________________| ord(head(w1))-ord(head(w2))<0=False ____________________|length w1>1&&length w2>1 _____________________=isBigger(tail(w1),wc1) (tail(w2),wc2) ____________________|length w1>1&&length w2==1=True ____________________|otherwise=false ----------------------------------------------------------------------------------- ________ReseptionList::String->String ________ReseptionList PatientList=[Name,Room | (Name,(Room,Dr)) <- PatientList] ______________________do sequence (map ReceptionReport wl) __________________________________where ______________________________________ReceptionReport::setABC((Name,Room))->IO() ______________________________________ReceptionReport(w,wc)=do putStr w _______________________________________________________________putStr (show w) _______________________________________________________________putChar 'in' _______________________________________________________________putStr (show wc) _______________________________________________________________putChar '\n' _______________________________________________________________return wl ----------------------------------------------------------------------------------- ________setNum::DoctorsReport -> DoctorsReport ________setNum [ ] = [ ] ________setNum((wn,wcn):DoctorsReport[(w,wc)|(w,wc)<-ws, _________________________ws<=wcn++[(wn,wcn)]DoctorsReport[(w,wc)|(w,wc)<-ws,wc>wcn] ----------------------------------------------------------------------------------- ________DoctorsList::String->String ________DoctorsList PatientList=[Name,Room | (Name,(Room,Dr)) <- PatientList, Dr==getText] ____________________do sequence (map DoctorsReport wl) ________________________________where ____________________________________DoctorsReport::setNum((Name,Room))->IO() ____________________________________DoctorsReport(w,wc)=do putStr w ___________________________________________________________putStr (show w) ___________________________________________________________putChar 'in' ___________________________________________________________putStr (show wc) ___________________________________________________________putChar '\n' _______________________________________________________________return wl -----------------------------------------------------------------------------------

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