November 2001 - Haskell

Re: Global variables
by Ashley Yakeley 29 Nov '01

29 Nov '01

At 2001-11-29 05:31, Juan Ignacio García García wrote: >I am interested in using global variables (in GHC). In JVM-Bridge (nearly there!) I use lifted monads to store global constants, though variables are not hard either. This does mean an extra function needed to call IO functions, but in my case you'd be calling mostly my glue functions to Java code, which are in the lifted monad anyway. Lifted monads look something like this: data MyAction a = MkMyAction ((consts,vars) -> (vars,a)); instance Monad MyAction where etc. I actually have a class for monads that lift the IO monad: <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/~checkout~/jvm-bridge/sourc e/Haskell/IOLiftedMonad.hs?rev=HEAD&content-type=text/plain> -- Ashley Yakeley, Seattle WA

1 0

Re: Global variables
by C.Reinke 29 Nov '01

29 Nov '01

> Hello, I am interested in using global variables (in GHC). I need a > variable to store list of Integers to store temporary results. I > have been reading the module MVar, but I wonder if there is an > alternative way of doing it. I have already implemented my function > using an auxiliar argument where I put my lists of Integers. Will > the use of a global variable improve my function? no!-) ah, well, perhaps.. As you've already got your function, using auxiliary arguments, you probably don't really need to re-write it in a less functional style. But you will have noticed that much of your code repeatedly does the same thing - passing the auxiliary around. It's good functional programming practice to abstract away repeated code (both for reuse and to get more concise code). One way to do that will lead you into monads (your function is a state transformer, transforming the state of the auxiliary list at each step), still a functional solution. If you really, and absolutely want and must use global, mutable variables in a functional language, you might find a recent paper by John Hughes helpful, on that very topic. See his home page: http://www.cs.chalmers.se/~rjmh/ Hth, Claus

1 0

Re: Where to ask questions regarding categories and datatypes
by C.Reinke 29 Nov '01

29 Nov '01

> Peter Douglass writes: > > Hi, > > I have a number of questions regarding categories and datatypes. I know > > that many of the folk in this mailing list could answer these question, but > > I wonder if there is a more appropriate forum. (i.e. the question are not > > Haskell specific). > > > > Why don't you try the Types Forum <types(a)cis.upenn.edu> > See also http://www.cis.upenn.edu/~bcpierce/types/ Or, if your questions are more on the categories side, there is also: Categories List The category theory mailing list, moderated by Bob Rosebrugh. http://www.mta.ca/~cat-dist/ Claus

1 0

Global variables
by Juan Ignacio García García 29 Nov '01

29 Nov '01

Hello, I am interested in using global variables (in GHC). I need a variable to store list of Integers to store temporary results. I have been reading the module MVar, but I wonder if there is an alternative way of doing it. I have already implemented my function using an auxiliar argument where I put my lists of Integers. Will the use of a global variable improve my function? Thanks in advance, J. Ignacio

3 2

RE: Possible bug/omission in Numeric library?
by Simon Peyton-Jones 29 Nov '01

29 Nov '01

| However, I would guess that changing the type signature of | the current showInt function is unacceptable for Haskell'98. | Maybe we should consider adding the more general version | under a new name like showIntBase, together with | show{Dec,Oct,Hex}? This would break no existing code, and | clear up the slight anomaly in the current capability of the | Haskell'98 library. Hmm. This is another unforced change, but admittedly to the libraries, which have received much less attention and in some cases (like this) look more accidental than deliberate. So I've been using a lower "bar" for modifications to the Libraries than to the Report. As you say, we can't change the type of showInt. I suppose we could add: showIntAtBase :: Integral a => a -- base -> (a -> Char) -- digit to char -> a -- number to show. -> ShowS showOct, showHex :: Integral a => a -> ShowS Q1: But would we really want showDec too? Identical to existing showInt? I am disinclined to use K-F's extra generality for H98, nice though it is. Too different from readInt. Also, GHC's NumExts has doubleToFloat :: Double -> Float floatToDouble :: Float -> Double Q2: If we are going to run round adding functions to Numeric, should we add those too? It's hard to know where to stop... but if that conversion is what you want to do, H98 doesn't give a good way to do it. Simon

3 2

Confused about Random
by Ian Lynagh 29 Nov '01

29 Nov '01

With the following module: module Main where import Random data Foo = Foo StdGen main :: IO() main = do let rs = randoms (Foo (mkStdGen 39)) :: [Int] rRs = randomRs (0,9) (Foo (mkStdGen 39)) :: [Int] putStrLn $ show $ take 100 rs putStrLn $ show $ take 100 rRs instance RandomGen Foo where genRange _ = (0, 1) next (Foo g) = (val `mod` 2, Foo g') where (val, g') = random g split _ = error "Not implemented" ghc gives me [-2147476078,7482,-2147476078,-2147476078,-2147476079,-2147476078,7483,7482,7482,-2147476079,-2147476078,-2147476079,-2147476079,7482,-2147476078,-2147476078,-2147476078,-2147476079,7482,7483,7482,7483,7482,-2147476078,-2147476078,-2147476078,7483,-2147476079,7482,-2147476078,-2147476079,-2147476078,7483,-2147476079,7483,7482,-2147476079,7483,7482,7483,-2147476078,-2147476079,-2147476079,7482,-2147476078,7482,-2147476079,-2147476079,7482,-2147476078,7483,7483,-2147476079,-2147476078,7483,7483,-2147476078,-2147476079,-2147476078,-2147476079,-2147476078,7483,7483,7482,7482,7483,-2147476078,-2147476079,-2147476079,7482,7483,-2147476078,-2147476079,-2147476079,-2147476078,7482,7483,-2147476079,7482,7482,7482,7483,-2147476079,-2147476079,-2147476078,7482,7482,7482,7482,-2147476079,7482,7482,-2147476079,7483,-2147476078,7482,7483,-2147476079,-2147476079,7482] [1,2,2,1,1,2,1,2,1,1,1,2,2,2,2,1,2,1,1,1,1,2,1,1,1,1,2,1,1,2,1,2,1,2,1,1,2,1,2,2,2,1,2,2,2,1,1,2,1,2,1,2,2,2,1,1,2,1,1,2,1,1,1,2,2,2,1,1,2,2,2,1,1,1,2,2,2,1,2,2,1,2,1,1,1,1,2,1,1,2,2,1,1,1,1,1,2,1,1,2] The first list doesn't seem to cover the whole spectrum and I was expecting the second list to be composed over 0s through 9s. Is my understanding wrong? nhc complains Context for Random.Random needed in left hand pattern at 17:11. and hugs doesn't seem to know genRange exists. With it commented out it returns the same as ghc. Thanks Ian

1 0

RE: IO errors
by Simon Peyton-Jones 29 Nov '01

29 Nov '01

| Two possibilities: either createDirectory should act like | 'mkdir -p' and make the whole path, or the library report | should document isDoesNotExistError as a possible error | thrown by createDirectory. I'd vote for the latter. I'm willing to do that for H98 unless anyone can think of a reason I shouln't. Simon

1 0

RE: IO errors
by Simon Marlow 29 Nov '01

29 Nov '01

> A quick look at the source looks like both GHC and NHC will > simply pass > on errors from the OS, so for example with > > module Main where > > import IO > import Directory > > main :: IO() > main = do catch (createDirectory "this/does/not/exist/foo") > (\e -> putStrLn $ show $ map (flip ($) e) errors) > > errors :: [(IOError -> Bool)] > errors = [isDoesNotExistError, > isIllegalOperation, > isPermissionError, > isAlreadyExistsError] > > both GHC and NHC give [True,False,False,False] (I don't seem to have a > hugs Directory.hs) while the library report only allows > isIllegalOperation, isPermissionError and isAlreadyExistsError. I > haven't looked for other cases of this. Two possibilities: either createDirectory should act like 'mkdir -p' and make the whole path, or the library report should document isDoesNotExistError as a possible error thrown by createDirectory. I'd vote for the latter. No doubt there are many other cases of this, we should really do a full audit of the I/O library specs. Cheers, Simon

1 0

Postdoc opening at ORNL
by James B. White III (Trey) 28 Nov '01

28 Nov '01

I wonder if one of the Haskell-ers on this list might know someone who would be interested in this: Postdoctoral Position Computer Science Computer Science and Mathematics Division The Computer Science & Mathematics (CSM) Division at the Oak Ridge National Laboratory (ORNL) invites outstanding candidates to apply for an immediate opening in computer science. The successful applicant will participate in a highly interdisciplinary project on the development of advanced software tools for the automatic synthesis of high-performance algorithms for complex tensor-based simulation methods, focusing initially in the areas of quantum chemistry and nuclear physics. This position offers an opportunity to interact with a broad range of research activities at a world-class laboratory for computational science, including the Center for Computational Sciences (CCS), which hosts one of the largest unclassified computing capabilities in the world. Position requires a Ph.D. in computer science or closely related field, a desire to work closely computational scientists from other fields, and experience in one or more of the following areas: * compilers and optimization techniques * domain-specific languages * generic and generative programming * functional programming and monads A strong background in scientific publications is also encouraged. Initial appointment is for one year. Appointments are administered by the Oak Ridge Institute for Science in Education (ORISE): http://www.orau.gov/orise/edu/postgrad/ornlpd.htm For consideration, please submit a CV and cover letter, including a description of research interests, software development experience, and interdisciplinary or computational science experience to: David Bernholdt Computer Science and Mathematics Division Oak Ridge National Laboratory P.O. Box 2008 Oak Ridge, TN 37831-6367 or E-mail: bernholdtde(a)ornl.gov Additional information about the Computer Science and Mathematics Division and the Center for Computational Sciences can be found at: www.csm.ornl.gov ORNL, a multiprogram research facility managed by UT-Battelle, LLC, for the U.S. Department of Energy, is an equal opportunity employer committed to building and maintaining a diverse work force. -- James B. White III (Trey) Center for Computational Sciences Oak Ridge National Laboratory whitejbiii(a)ornl.gov

1 0

IO errors
by Ian Lynagh 28 Nov '01

28 Nov '01

Hi guys, Me again I'm afraid... A quick look at the source looks like both GHC and NHC will simply pass on errors from the OS, so for example with module Main where import IO import Directory main :: IO() main = do catch (createDirectory "this/does/not/exist/foo") (\e -> putStrLn $ show $ map (flip ($) e) errors) errors :: [(IOError -> Bool)] errors = [isDoesNotExistError, isIllegalOperation, isPermissionError, isAlreadyExistsError] both GHC and NHC give [True,False,False,False] (I don't seem to have a hugs Directory.hs) while the library report only allows isIllegalOperation, isPermissionError and isAlreadyExistsError. I haven't looked for other cases of this. (this is under Linux, don't know if it would behave differently under other OSes). Thanks Ian

1 0