March 2006 - Haskell-Cafe

Re: Can't uninstall GHC or prepare SOE installation on OS X
by Xavier Elizalde 11 Mar '06

11 Mar '06

Hi Wolfgang, For some reason, GHCi now keeps launching every time I launch Terminal.app, I can't even get a regular command line prompt now. I had to uninstall all the files for GHCi manually from an X11 Xterm to get a regular prompt. It still gets an error trying to launch GHCi, but then ends up with a regular prompt. I don't know why it started doing that, it wasn't doing that before.

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Re: Can't uninstall GHC or prepare SOE installation on OS X
by Wolfgang Thaller 09 Mar '06

09 Mar '06

Xavier Elizalde wrote: > Hi. I sent this post to the haskell-cafe mailing list but haven't > gotten any replies. Ah well, I watch that list in digest mode only, easy to overlook things. > Basically, I can't get the Uninstall AppleScript for GHC 6.4.1to > run properly because it encounters an error. Oops. I *thought* I had tested that to death. Anyway, for manual removal: /usr/local/lib/ghc-6.4.1 (the big part; if disk space is all you're after, stop here) In /usr/local/bin/: ghc, ghc-6.4.1, ghc-pkg, ghc-pkg-6.4.1, ghci, ghci-6.4.1, ghcprof, hasktags, hp2ps, hsc2hs, runghc, runhaskell, stat2resid in /Libraries/Receipts: Some GHC-*.pkg file, I suspect now that it's called GHC-6.4.1.pkg /Library/Frameworks/GMP.framework (do not delete this unless you do not want to ever run any GHC-compiled program again until you re- install GHC). > I have the Haskell School of Expression book and want to try out > the examples in the book that make use of X11 for graphics, and I > don't know how to use GHC 6.4.1 for the SOE book examples. I > thought it would be easiest to just uninstall it then install the > SOE binaries for OS X. The SOE binaries use hugs, and should therefore be completely independent of GHC, no uninstalling required. > But if GHC 6.4.1 works fine for the SOE book examples and is better > than the SOE install, I'd like to use that, although I don't know > how to get it to work with X11. Do you have any idea on what I > should do? Start your programs from an xterm. If you want to use GHCi, set export DYLD_LIBRARY_PATH=/usr/X11R6/lib/ before starting ghci in the xterm. > > I have The Haskell School of Expression book and the > instructions for setting up and running the SOE software seem to have > a different configuration compared to GHC 6.4.1. Namely, it's meant > to use X11 and locations for files are different. What files? I don't have the book, so all I can tell you is that GHC includes a library module by the name of Graphics.SOE. > Or does the GHC already > have GraphicsLIb as part of it? If so, shouldn't it launch an X11 > Xterm terminal rather than the standard OS X Terminal? Shouldn't it launch...? Are you referring to the AppleScript for launching GHCi? If so, then no, for two reasons: a) X11 is an optional install on Mac OS X, so many people don't have it. b) The standard Terminal is a lot more convenient to use than an xterm, and most people don't need/want to use X11 for their Haskell programs. So good luck with SOE, if there are any more problems, ask me. Cheers, Wolfgang

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Question regarding let clauses
by Martin Percossi 09 Mar '06

09 Mar '06

Hello, the following code doesn't compile <snip> module Matrix where import Control.Monad.ST import Data.Array.ST import Data.Array.Unboxed type Block s = STUArray s Int Double data MMatrix s = MMatrix Int Int (Block s) newMatrix_ :: Int -> Int -> ST s (MMatrix s) newMatrix_ m n = do b <- newArray_ (1, m*n) return (MMatrix m n b) runMatrix = do _A <- newMatrix_ 3 3 _B <- newMatrix_ 3 3 matMul _A _B return "Success" main = show $ runST runMatrix matMul :: MMatrix s -> MMatrix s -> ST s (MMatrix s) --matMul a b = do let foo = 2*5 --return a matMul a b = do { let foo = 2*5; return a } </snip> under ghc 6.4.1, yielding the error message: question.hs:25:41: parse error on input `<-' Failed, modules loaded: none. The offending is line the one containing "let foo = 2*5", which is a little test I've done of let-clauses. Now, suppose instead that for the last function, matMul, I replace the version that's commented out. No errors! Could someone enlighten me as to why? I'm a bit confused, as I thought the two forms are equivalent save for formatting... This is on the back of a email discussion that I was reading about let-clauses, in which someone declared that they where better than where clauses for monadic code. If anyone could comment on this, I'd appreciate it as well. Many thanks in advance, Martin

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Haddock Problems
by Daniel Fischer 09 Mar '06

09 Mar '06

Hi all, I've just installed haddock-0.7, nice, but... > haddock -o h7doc -h -D h7doc/fusi.haddock --use-package=base Verwaltung.hs Teams.hs Stats.hs Match.hs Main.hs Liga.hs Item.hs Helpers.hs Datum.hs Warning: Helpers: could not find link destinations for: GHC.Base.Int GHC.Base.String GHC.Show.Show Data.Maybe.Maybe GHC.Base.Eq GHC.Base.Bool and so on. Same problem, if I use "-i/usr/.../base.haddock" instead of --use-package=base. This doesn't happen with haddock-0.6, all are properly linked to Prelude.html then. Now what can I do about that? BTW, in the ghc-docs (created with haddock-0.6), in most html-pages, Int &c are linked to GHC.Base, GHC.Num... which don't exist, how would I manage to have those linked to Prelude.html? And when I tried to re-make install-docs (for ghc-6.2.2, not to put the 6.4.1-docs into peril), I got /bin/sh: line 1: t: command not found make[2]: [html/haddock.css] Fehler 127 (ignoriert) /usr/bin/install -c -m 644 html/* /usr/local/share/ghc-6.2.2/html/libraries/base /usr/bin/install: Aufruf von stat für »html/*« nicht möglich: Datei oder Verzeichnis nicht gefunden make[2]: *** [install-docs] Fehler 1 make[1]: *** [install-docs] Fehler 1 make[1]: Leaving directory `/usr/local/lib/ghc-6.2.2/libraries' make: *** [install-docs] Fehler 1 linux:/usr/local/lib/ghc-6.2.2 # Apparently had a problem with SRC_HADDOCK_OPTS += -t "Haskell Hierarchical Libraries..." How can I fix all that? Thanks, Daniel -- "In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt." -- Blair P. Houghton

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Lists of Lists
by zell_ffhut 09 Mar '06

09 Mar '06

Just wondering if anyone can help me with a Haskell problem im having. How would I get the value of, lets say, the 9th object in ["4179355"," 567412"] ? (in this case, that should return 6) Any help/adivce would be great :) -- View this message in context: http://www.nabble.com/Lists-of-Lists-t1245394.html#a3297506 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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A syntax proposal: data replacements
by ihope 09 Mar '06

09 Mar '06

I'd like to propose more syntactic sugar more Haskell. (A spoonful of syntactic sugar makes the medicine go down...) Put simply, this would provide a smallish bit of pattern matching, and hopefully clarify some things. A simple example should pretty much define the whole thing: fromJust = {Just ->- id; Nothing ->- error "Nothing"} This function takes a single value. If the constructor used to construct it is Just, then the Just is replaced with id. Likewise, if it's given Nothing, it will return (error "Nothing"). Another example: consPair = {(,) ->- (:)} This takes a pair (x,y) and replaces the constructor (,) with (:), yielding (x:y). Now, a final clarification: foldr cons nil = {(:) ->- cons; [] ->- nil} This will *not* work: only the top bit of list will be replaced this way, as data replacements are not recursive. One last example: the function {Left ->- id; Right ->- error} takes an Either a String and either pulls out the a and returns that or uses the String as an error message.

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Re: [Haskell-cafe] |> vs. $ (was: request for code review)
by Shannon -jj Behrens 09 Mar '06

09 Mar '06

By the way, thanks for everyone's comments so far! They're very helpful! > Also, most haskell programs use $ instead of |> > > > -- For convenience: > > currTokType :: ParseContext -> TokenType > > currTokType ctx = ctx |> currTok |> tokenType > > this could be written as: > tokenType $ currTok $ ctx Concerning: -- |> is like a UNIX pipe. infixl 9 |> x |> f = f x I find "ctx |> currTok |> tokenType" to be more readable than "tokenType $ currTok $ ctx" because you're not reading the code in reverse. That's my primary complaint with "." and "$". That's especially the case when I'm spreading the code over multiple lines: -- Translate a C type declaration into English. translate :: String -> String translate s = s |> createParseContext |> readToFirstIdentifier |> dealWithDeclarator |> consolidateOutput I prefer |> for readability, but I understand that it can be bad to spring new constructs on programmers in the middle of a program, and I'm totally in favor of following standard Haskell idioms. Does anyone else have strong opinions one way or the other? It seems like most of my program is centered around pipes of "|>", so it's an important issue. Thanks! -jj

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Re: Library survey results
by John Hughes 09 Mar '06

09 Mar '06

>> 29% Parsec >> 19% wxHaskell >> 16% QuickCheck >> 16% haddock >> 12% Monadic Parser Combinators >> 11% Gtk2Hs >> 9% hs-plugins >> 8% HaXml >> 7% Data.* >> 7% Monad foundation classes >> 6% Arrows >> 6% HOpenGL >> >> The list includes all libraries named by more than 5% of respondents. >> Sure enough, wxHaskell and Gtk2Hs are more popular, but 6% naming >> HOpenGL as among the "most important" libraries is quite respectable. > > Well, I've never said that it is among the "most important" libraries, but > OTOH I really much doubt that the way the survey was done delivers > anything > near reliable results. It heavily biases early entries, and I dare to > speculate that the people taking part in the survey were probably not even > near to a representative group, but a bunch of highly motivated, > experienced > non-Joe-Programmer kind of people who are actively participating on the > mailing lists etc. It wasn't as bad as you think--over 580 replies, with less than a quarter of those from academics (there were actually more replies from people working in industry than from academics!). So I'd dispute that the survey tells us mostly about what the research community in particular wants. I'd guess the survey was pretty representative of KEEN Haskell users. It's a bit unclear who we mean by "Joe Haskell-programmer" anyway--I bet that, apart from students of course, the number of Haskell programmers who are using the language "just because their boss tells them to" can be counted on the fingers of one hand! I'd claim Joe Haskell programmer probably IS keen, highly motivated, experienced and active. In fact, the one group that is obviously underrepresented badly in my survey is just students--I got replies from just under 300, while another survey of teachers shows that at least 5-10,000 were taught Haskell last year. But maybe tools like hoogle SHOULD be aimed at the most active users? You're right that libraries mentioned earlier in the survey received more "votes" as a result, but since I have a record of all responses *in time order* I can see the difference, for each library, between pre- and post- "first mention" behaviour. HOpenGL was mentioned in the 7th response, wxHaskell in the first, and Gtk2Hs in the 17th, so they were in the "previously mentioned" category for almost the entire survey, and the effect you're talking about was not significant for a comparison between those three. Data.*, on the other hand, was first mentioned about half way through the survey, which indicates that around 12% of respondents selected it as among the "most important" libraries, when prompted to do so by seeing its name. However, the proportion who name Data.* spontaneously is below 2%--and that conclusion is statistically significant at the 99% level. 580 replies is enough to say statistically significant things (about the population the survey sampled, anyway). I feel quite inspired--when I have a spare moment, I'll analyse the results more carefully and see what one actually CAN say with a degree of certainty, taking into account when each library was first mentioned. > Furthermore, some of the percentages above are extremely > strange, e.g. how can people use huge GUI toolkits with 30% while staying > largely away from something as fundamental as Data.*? I don't find it so strange, really. Data.* implements lots of useful standard datatypes, but you can import some of them in other ways (import Maybe for example), and in many cases it's not too hard to roll your own. OK, maybe with a worse result, but you're not *forced* to use Data.* -- so if you're used to using something else, there's no 100% compelling reason to change. Rolling your own wxHaskell or Gtk2Hs is prohibitively hard. So I'm not at all surprised that GUI toolkits rated much higher--people's natural conservatism gives them a big advantage, and even Haskell users can be conservative sometimes! Perhaps the most surprising thing here is that only 30% of keen Haskellers think a GUI is important! John

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MUA written in Haskell (was: Getting GHC to print "Done" when it's finished linking?)
by Nils Anders Danielsson 09 Mar '06

09 Mar '06

On Tue, 07 Mar 2006, "Brian Hulley" <brianh(a)metamilk.com> wrote: (Moved from ghc-users.) > Brian Hulley wrote: > (time for a proper email client to be written in Haskell! ;-) ) I had the same thought yesterday, after an Emacs-Lisp session in which I was trying to get Gnus to do exactly what I wanted it to... Out of curiosity, how much work would it take to write an easily configurable, decent MUA in Haskell? I don't know too much about MUAs, but I have a feeling that we already have quite a few libraries that are needed for the job: UIs (including HTML rendering...), plugins, various protocols, encryption... -- /NAD

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haskell-prime traffic
by Ross Paterson 09 Mar '06

09 Mar '06

Just for fun, I've put up some statistics on haskell-prime traffic at http://www.soi.city.ac.uk/~ross/haskell-prime-stats/ I'm not claiming it shows anything, except what a narrow base we have.

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