- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Missing Symbol
by PR Stanley 19 Sep '07

19 Sep '07

> > Hi > > What symbol would you replace the "?" with: > > the identity function > > id x = x? > >I don't think I'd have anything where the ? symbol is. > >id x = x works just fine. Ah, that's interesting because there's a "." to the right of x in my scanned doc. Still, if no symbol is necessary, so much the better! Cheers, Paul

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Missing Symbol
by PR Stanley 19 Sep '07

19 Sep '07

Hi What symbol would you replace the "?" with: the identity function id x = x? My scanned copy of the Hutton book contains some rather interesting characters. In fact, the OCR program has placed a "." in place of most symbols - e.g. ->, <- ^, <=, \=, etc. Fortunately, I'm able to guess the correct operator in the majority of the cases. Cheers, Paul

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ANN: hstats-0.1
by Marshall Beddoe 19 Sep '07

19 Sep '07

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hstats-0.1 I've just released hstats, a statistical computing module for the Haskell language. Current functionality includes: mean, median, mode, range, standard/average deviation, variance, iqr, kurtosis, skew, covariance, and correlation. I have plans on adding more rank correlation functions, histograms & chi square tests. darcs repo: http://www.sftank.net/code/hstats

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Building "production stable" software in Haskell
by Peter Verswyvelen 19 Sep '07

19 Sep '07

The way I see it as a newcomer, Haskell shifts the typical imperical programming bugs like null pointers and buffer overruns towards "space/time leaks", causing programs that either take exponentially long to complete, stack overflow, or fill up the swap file on disc because they consume gigabytes of memory. Other bugs I found are incomplete pattern matches at runtime, but I already got an email of how to fix this using an external tool, although it would be nice if this is part of the compiler/linker itself. How well and how can a Haskell program be tested to make sure it does not cause these space/time bugs? What tools are typically used? Thanks, Peter

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yi-gtk
by clisper 19 Sep '07

19 Sep '07

who knows how to compile yi-gtk? i tried,but it told me mine miss gtk. 2007-09-18 clisper

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Type-Marking finite/infinte lists?
by Joachim Breitner 19 Sep '07

19 Sep '07

Hi, today while mowing the lawn, I thought how to statically prevent some problems with infinte lists. I was wondering if it is possible to somehow mark a list as one of finite/infinite/unknown and to mark list-processing functions as whether they can handle infinte lists. For example, it is known that a list produced by repeat or cycle is always infinte, that foldl can’t handle infinte lists (while foldr might). length should never be called on an infinte list, and map’s output is infinite iff the input list is. So the compiler might use this information to emit errors like “line 42: program will not terminate: length applied to infinite list” or warnings “line 23: termination not guaranteed: foldl applied to possible infinte list”. I guess you can’t know it for all lists (halting problem...), but for those where it is possible, it would be nice to tell. Or am I missing something? Can this be implemented using some guru type system hacking? Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: joachimbreitner(a)amessage.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Re: [Haskell] Blocked STM & GC question
by Jules Bean 18 Sep '07

18 Sep '07

Simon Marlow wrote: > Ashley Yakeley wrote: >> If I have a thread that's blocked on an STM retry or TChan read, and >> none of its TVars are referenced elsewhere, will it get stopped and >> garbage-collected? >> >> I have in mind a pump thread that eternally reads off a TChan and >> pushes the result to some function. If the TChan is forgotten >> elsewhere, will the permanently blocked thread still sit around using >> up some small amount of memory, or will it be reaped by the garbage >> collector? > > In this case, your thread should receive the BlockedIndefinitely exception: > > http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Exceptio… > > > If the system is idle for a certain amount of time (default 0.3s, change > it with the +RTS -I option) a full GC is triggered, which will detect > any threads that are blocked on unreachable objects, and arrange to send > them the BlockedIndefinitely exception. Including MVars? Your quoted text suggests 'Yes' but the docs you link to suggest 'No'. Jules

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unique id for data types
by Barney Hilken 18 Sep '07

18 Sep '07

In order to make my records system practically useable, I need a type family > type family NameCmp n m which totally orders datatypes. More precisely, it should return one of the following types: > data NameLT = NameLT > data NameEQ = NameEQ > data NameGT = NameGT for each pair of datatypes n & m, according to whether n < m, n = m, or n > m in some global ordering. This ordering needs to be independent of the context, so it can't be affected by whatever imports there are in the current module. What I want to know is: does GHC give datatypes any global id I could use to generate such an ordering? Would fully qualified names work? Secondly (assuming it's possible) how easy would it be for me to write a patch to add NameCmp to GHC? Where in the source should I start looking? Thanks, Barney.

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Observations from ListLike
by John Goerzen 18 Sep '07

18 Sep '07

Hi folks, I just announced ListLike[1] on haskell(a)haskell.org. Rather than repeat that announcement here, I'd like to make a few observations that came out of the development of this program: * I wrote extensive QuickCheck cases for this and wrapped them in HUnit for better display and future extensibility. Two bugs in the GHC base libraries were discovered as a result (one has been fixed already in HEAD, one not) * Hugs programs that use cpphs can't use ByteString * It would be really nice if QuickCheck supported I/O and some version of HUnit's TestLabel to generate hierarchical names when failures occur. * It would also be much nicer if QuickCheck defined Arbitrary instances for Word8 and Char. On Hugs, QuickCheck also lacks an instance of Maybe, but GHC has it for some reason. * Testing functions like foldl in Hugs, where QuickCheck is supplying the list and the function, cause Hugs to crash with stack errors. Testing some functions over ByteStrings cause Hugs to crash complaining of being out of ForeignPtrs. I have yet to figure out how to fix either problem; raising the heap size doesn't help. * I had some cases in String.hs[2] where Hugs did not permit me to define the default functions within the class declaration. However, I could define the functions outside, with the exact same type signature as they'd have inside, and within the class declaration, just say something like "words = myWords" to refer to the external declaration. GHC worked fine both ways. It seems that Hugs has a number of issues with code like ListLike that uses some more advanced Haskell features, even though Hugs nominally supports them. If you look through my Mercurial changelogs, you'll see "fix blah for hugs" messages all over. Which is too bad, because Hugs seems more portable (especially to embedded devices) than GHC. [1] http://software.complete.org/listlike [2] http://software.complete.org/listlike/browser/src/Data/ListLike/String.hs

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Israel Haskell Programmers
by B K 18 Sep '07

18 Sep '07

Hello, Are there any Haskell Hackers on this mailing list who live in Israel? I am interested in starting an Israel Haskell User Group.

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