November 2008 - Haskell-Cafe

Unique functor instance
by Luke Palmer 25 Nov '08

25 Nov '08

I've been wondering, is it ever possible to have two (extensionally) different Functor instances for the same type? I do mean in Haskell; i.e. (,) doesn't count. I've failed to either come up with any examples or prove that they all must be the same using the laws. Thanks, Luke

4 10

(no subject)
by apostolos flessas 25 Nov '08

25 Nov '08

hi, i am looking for someone to help me with an assignment! can anyone help me? i am looking forward to your reply. sincerely, Tolis _________________________________________________________________ BigSnapSearch.com - 24 prizes a day, every day - Search Now! http://clk.atdmt.com/UKM/go/117442309/direct/01/

2 1

re: monads with take-out options
by Greg Meredith 25 Nov '08

25 Nov '08

John, You write: > Yes, you are describing 'co-monads'. > Good catch, but actually, that's too weak. i'm requesting something that is both a monad and a co-monad. That makes it something like a bi-algebra, or a Hopf algebra. This, however, is not the full story. i'm looking for a reference to the full story. Surely, someone has already developed this theory. Best wishes, --greg From: John Meacham <john(a)repetae.net> Subject: Re: [Haskell-cafe] monads with take-out options To: haskell-cafe(a)haskell.org Message-ID: <20081124231451.GG32430(a)sliver.repetae.net> Content-Type: text/plain; charset=utf-8 On Mon, Nov 24, 2008 at 02:06:33PM -0800, Greg Meredith wrote: > Now, are there references for a theory of monads and take-out options? For > example, it seems that all sensible notions of containers have take-out. Can > we make the leap and define a container as a monad with a notion of > take-out? Has this been done? Are there reasons for not doing? Can we say > what conditions are necessary to ensure a notion of take-out? Yes, you are describing 'co-monads'. here is an example that a quick web search brought up, and there was a paper on them and their properties published a while ago http://www.eyrie.org/~zednenem/2004/hsce/Control.Comonad.html the duals in that version are extract - return duplicate - join extend - flip (>>=) (more or less) John -- John Meacham - ⑆repetae.net⑆john⑈ -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

1 0

Request for code review: slice-1.0.0, TestSupport
by Owen Smith 25 Nov '08

25 Nov '08

Hi all, Thanks for your welcome and helpful comments. I've banged out a first attempt at a Haskell library and was curious if anybody would have time or interest in looking it over it for style, design, stuff that's just wrong, or (most likely) stuff that's been done better elsewhere. I'm willing to trade some labor in kind, if there's any odd jobs that can be done by a relative newbie (e.g. maybe setting up a Windows box for OpenGL build testing with GHC 6.10??). The library, slice, is an emulation of Python's list slicing mechanism: http://www.owendsmith.com/slice-1.0.0.tar.gz It uses a triple of Maybes to specify the range, so it's unwieldly in terms of syntax, but the functionality should be there. Mostly, besides gaining experience with writing a library, I wanted to have functionality that skips through a list the way Python slices can, and didn't see a particularly clean way of doing that particular kind of list dicing in Data.List or elsewhere. (In particular, looking towards an application where I'll need to split a list into odd-positioned elements and even-positioned-elements.) I'm doubtful if this is worth publishing as a real package, but maybe someone besides myself will see something in it worth keeping. One thing I did play with was hooking my unit tests into the Cabal build system for my package; if I've read earlier posts correctly this has not been done frequently or systematically, so if that's the case, maybe this can be a contribution towards better testing support. The scripts in the TestSupport module follow a pattern I've used elsewhere in other languages: stomp through the Test directory hierarchy, find all source files that start with Test, and run them on the fly using runhaskell. This works beautifully if you happen to have runhaskell installed and on your system path, but woe to you if you don't--and I'm not sure how to properly capture that as a dependency in Cabal. I'm also wondering if there's any existing Cabal support to configure a proper location for this (I couldn't find it). And if that weren't enough--if similar mechanisms to runhaskell exist for other compilers that I should try to build into this somehow. Thanks! -- O

2 1

cabal install HaXml installs wrong version unless I specify the version number
by Thomas Hartman 24 Nov '08

24 Nov '08

I was upgrading happstutorial.com to ghc 6.10.1 when I came across a cabal install issue. Somewhere in day 1 (probably as a HAppS dependency), HaXml 1.13.1 got installed. On day 2, I wanted the latest version of HaXml (1.19.4), a dependency for for xml-parsec. cabal install HaXml gave me: already installed. cabal upgrade HaXml gave me: some installing action, but in the end I wound up with 1.13.1 again. cabal install HaXml-1.19.4 gave me what I want, the latest version. But even then, when I do cabal install xml-parsec I get Text/ParserCombinators/Parsec/XML.hs:18:7: Could not find module `Text.XML.HaXml.Posn': it is a member of package HaXml-1.19.4, which is hidden The cabal file has Build-Depends: base, parsec, HaXml >ghc-pkg latest HaXml says HaXml-1.19.4 Even after ghc-pkg hide HaXml-1.13.3 I get the same result, so I'm not even sure if the problems I'm having are a result of the having two versions of the same package installed. This is a bug right? Any advice on how to get xml-parsec installed? thomas.

4 9

Garbage collection as a dual of laziness?
by Robin Green 24 Nov '08

24 Nov '08

It occurs to me that garbage collection can be seen as some kind of dual of laziness. Laziness means deferring the creation of values until later in the future (or even never). Garbage collection means eagerly destroying data created in the past, and reclaiming the memory used by it, before some other event which would do so (in most garbage-collected languages, I think process destruction is the only other thing that frees memory, leaving aside foreign functions). If you don't have enough laziness (e.g. because of strict pattern matching on tuples) your program might do unnecesssary work (time wastage); if you don't have enough garbage collection (e.g. because a value will never be accessed again but is still referred to from something live), your program might leak memory (space wastage). Of course, space wastage can lead to time wastage, and vice-versa. Can this intuition be made any more formal? Is it merely of pedagogical use, or does anything interesting follow from it? -- Robin

4 3

SOEGraphics in GHC?
by Dmitri O.Kondratiev 24 Nov '08

24 Nov '08

Please help, to locate in GHC distribution SOEGraphics library from Paul Hudak, book "The Haskell School of Expression" (http://www.haskell.org/soe/ ) Thanks!

3 2

Ambiguous type variable woes
by Jacques Carette 24 Nov '08

24 Nov '08

I was trying to create a typeclass for an abstract Stack class, and ran into some problems. The following 'works' fine: {-# OPTIONS_GHC -XEmptyDataDecls -XFlexibleContexts -fno-monomorphism-restriction #-} module Stack where data Void class Stack s where push_ :: s a r -> b -> s b (s a r) empty :: s () Void top :: s a (s b r) -> (a, s b r) first :: s a r -> a instance Stack (,) where push_ s a = (a,s) empty = ((),undefined::Void) top = id first = fst p = flip push_ test0 = top . p 2 . p 3 $ empty -- But the following doesn't - I get an "Ambiguous type variable `s' in the contraint `Stack s' arising from the use of `first': test1 = first . p 2 . p 3 $ empty -- sure, that makes sense, it somehow needs to know what flavour of Stack to use even though (or perhaps because) the answer is independent of it. -- So I listen to the "probable fix" and add a type signature: test1 :: Stack (,) => Integer -- This does not however help at all! The only way I have found of 'fixing' this requires annotating the code itself, which I most definitely do not want to do because I specifically want the code to be polymorphic in that way. But GHC 6.8.2 does not want to let me do this. What are my options? Jacques

3 2

How to use Unicode strings?
by Dmitri O.Kondratiev 23 Nov '08

23 Nov '08

Please advise how to write Unicode string, so this example would work: main = do putStrLn "Les signes orthographiques inclus les accents (aigus, grâve, circonflexe), le tréma, l'apostrophe, la cédille, le trait d'union et la majuscule." I get the following error: hello.hs:4:68: lexical error in string/character literal (UTF-8 decoding error) Failed, modules loaded: none. Prelude> Also, how to read Unicode characters from standard input? Thanks! -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

10 11

ANN: hledger 0.2 released
by Simon Michael 23 Nov '08

23 Nov '08

hledger is a minimal haskell clone of John Wiegley's "ledger" text-based accounting tool (http://newartisans.com/software/ledger.html) hledger generates ledger-compatible register & balance reports from a plain text ledger file, and demonstrates a functional implementation of ledger. For more information, see the hledger home page: http://joyful.com/hledger INSTALLATION ------------ If you have cabal-install, do:: cabal install hledger otherwise, unpack http://hackage.haskell.org/packages/archive/hledger/0.2/hledger-0.2.tar.gz and do:: runhaskell Setup.hs configure runhaskell Setup.hs build sudo runhaskell Setup.hs install (or for the latest code, darcs get http://joyful.com/repos/hledger) NEWS ---- * fixes * fix balance report totals when filtering by account * fix balance report selection of accounts when filtering by account * fix a bug with account name eliding in balance report * if we happen to be showing a not-yet-auto-balanced entry, hide the AUTO marker * fix print command filtering by account * omit transactions with zero amount from register report * Fix bug in parsing of timelogs * rename --showsubs to --subtotal, like ledger * drop --usage flag * don't require quickcheck * features * priced amounts (eg "10h @ $50") and --basis/--cost/-B flag to show them with cost basis * easy --depth option, equivalent to c++ ledger's -d 'l<=N' * smarter y/m/d date parsing for -b and -e (any number of digits, month and day default to 1, separator can be / - or .) * -n flag for balance command * --empty/-E flag * build a library, as well as the exe * new home page url (http://joyful.com/hledger) * publish html and pdf versions of README * detect display preferences for each commodity like c++ ledger * support amounts with multiple currencies/commodities * support --real/-R flag * support -C/--cleared flag to filter by entry status (not transaction status) * support virtual and balanced virtual transactions * parse comment lines beginning with a space, as from M-; in emacs ledger-mode * allow any non-whitespace in account names, perhaps avoiding misleading missing amounts errors * clearer error message when we can't balance an entry * when we fail because of more than one missing amount in an entry, show the full entry * document the built-in test runner in --help * add a --verbose/-v flag, use it to show more test-running detail * includes 43 tests Contributors: * Simon Michael * Tim Docker

1 0