February 2008 - Haskell-Cafe

"Computational Category Theory" in Haskell
by Dan Weston 25 Feb '08

25 Feb '08

Has anyone already ported to Haskell the ML code in Rydeheard and Burstall, "Computational Category Theory"? Dan

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Graphical graph reduction
by dainichi＠gmail.com 25 Feb '08

25 Feb '08

Hi Haskell-Cafe, I'm relatively new to Haskell, but have a background with SML. One of the things that amaze me about Haskell is lazy graph reduction, e.g. how the graph unfolds during the evaluation of, say, let fibs = 1 : 1 : zipWith (+) fibs (tail fibs) in take 10 fibs Lazy lists can be simulated in SML too, but unless I do something clever with references, I end up taking exponential time to compute the n'th Fibonacci number. Now to the point: Wouldn't it be great if I had a visual tool that visually showed me the graph while the above evaluation unfolded? I could use it to show some of my co-workers to whom laziness is a mystery, what it's all about. Does anybody know if such a tool exists? I'd be grateful for pointers if it does. I very much doubt that I'm the first person who has thoughts like this, but then again, who knows. People who really know Haskell might think this is too trivial a task to really be worth spending time on. If nothing similar exists, I was thinking about creating such a tool (i.e. an interpreter with additional graph-displaying features) for a very, very small subset/dialect of Haskell. I would probably be lazy (no pun intended) and start right away with abstract syntax trees to avoid lexing and parsing and such. My language of implementation would be SML, using references as the edges of the graph. Any ideas/comments would be welcome. Kai

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Using template haskell to create pseudo columns
by Justin Bailey 25 Feb '08

25 Feb '08

Adding pseudo columns to a projection is cumbersome, to say the list. For example, to add a field named "hidden" to a query I have to write: data Hide = Hide instance FieldTag Hide where fieldName _ = "hide" hideField = mkAttr Hide I wrote a little Template haskell that reduces this to: $(mkField "hide") If you want to customize the name of the column in the generated SQL, another function is used: $(mkFieldWithName "NumPullingParts" "num_pulling_parts") The code is below. Some questions: * Does the code look reasonable and useful? * Would the haskelldb maintainers want to introduce a dependency on template haskell to haskelldb for this kind of code? * Otherwise, would an "extensions" package make sense? If the reaction is positive, I'll work on extending this for a patch. My heartfelt thanks to Bulat for his excellent template haskell tutorials. I really wish that library had better documentation! Justin mkField :: String -> Q [Dec] mkField [] = error "Can't generate field from an empty string." mkField f = let fieldType = if isUpper (head f) then f else (toUpper (head f)) : tail f colName = if isLower (head f) then f else (toLower (head f)) : tail f in mkFieldWithName fieldType colName -- ^ Creates a compile time field declaration using the given -- arguments for the type and column name of the field. The fieldType argument -- is used to produce the "<field name>Field" function which can be used to -- add the field to a projection. -- -- Note that an error will occur if fieldType is not a proper type/constructor name. mkFieldWithName :: String -> String -> Q [Dec] mkFieldWithName [] _ = error "Can't create a field with an empty type." mkFieldWithName _ [] = error "Can't create a field with an empty column name." mkFieldWithName fieldType colName = let ty = mkName fieldType fieldName = toLower (head fieldType) : tail fieldType fieldD = [DataD [] ty [] [NormalC ty []] []] fieldI = [InstanceD [] (AppT (ConT (mkName "Database.HaskellDB.HDBRec.FieldTag")) (ConT ty)) [FunD (mkName "fieldName") [Clause [WildP] (NormalB (LitE (StringL colName))) []]]] fieldF = [ValD (VarP (mkName (fieldName ++ "Field"))) (NormalB (AppE (VarE (mkName "Database.HaskellDB.DBLayout.mkAttr")) (ConE ty))) []] in return (fieldD ++ fieldI ++ fieldF)

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hoogle & gtk2hs
by Gour 25 Feb '08

25 Feb '08

Hi! Emacs haskell-mode has haskell-hoogle function to search Haskell API, but it, unfortunately, does not work with gtk2hs API which can be accessed via web at http://haskell.org/hoogle/?package=gtk Any plan to integrate Gtk2Hs so it becomes accessible via haskell-hoogle function? Sincerely, Gour -- Gour | Zagreb, Croatia | GPG key: C6E7162D ----------------------------------------------------------------

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placeholders in hsql
by Roman Cheplyaka 25 Feb '08

25 Feb '08

Is there an ability to use placeholders in SQL statement using hsql? (Actually I'm interested in SQLite.) E.g. "INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)" and then supply values that will be escaped and inserted in place of each '?'. -- Roman I. Cheplyaka :: http://ro-che.info/ ...being in love is totally punk rock...

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haskellwiki and Project Euler
by Daniel Fischer 25 Feb '08

25 Feb '08

Hi all, I try not to be too rude, although I'm rather disgusted. I know there are several sites out on the web where solutions to PE problems are given. That is of course absolutely against the sporting spirit of Project Euler, but hey, not all people are sporting. I've found http://www.haskell.org/haskellwiki/Euler_problems irritating for a while, but wasn't overly annoyed by it while it only contained code for solving a few dozen problems. Today I learnt that it now contains code for all problems. Really bad! On top of that, the code for many problems isn't even Haskell, but C, WTF! Other code was submitted without consent of the author, copied from the PE fora, which are restricted access and so, even if perhaps not legally, but in spirit, do not fall under the legitimate resources for haskellwiki: "You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource. DO NOT SUBMIT COPYRIGHTED WORK WITHOUT PERMISSION!" To make matters worse still, there was a page containing nothing but the answers. That was changed, but Cale chose to reintroduce that crap. I just removed it again. Your turn, Cale. I call on the Haskell community to vote for immediate removal of these pages from the wiki! Show that you're a sporting bunch. Daniel Fischer

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parsing c
by Evan Martin 25 Feb '08

25 Feb '08

I'd like to parse some C headers for a toy project. It seems like c2hs probably has the functionality I want, but it's not packaged up as a library. So two questions: Is there an alternative C-parsing library? Has anyone looked into librarifying c2hs's parser?

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Re: static constants -- ideas?
by Don Stewart 24 Feb '08

24 Feb '08

jay: > Don Stewart dons(a)galois.com: > >jay: > >> I also have constants that are too large to compile. I am resigned to > >> loading them from data files--other solutions seem even worse. > ... > >> Data.Binary eases the irritation somewhat. > > > >Did you try bytestring literals (and maybe parsing them in-memory with > >Data.Binary)? > > That didn't occur to me, since neither of my large constants includes > strings.... I think you're suggesting that each constant could appear in > the source as a long bytestring and be deserialized into the data > structure. If that works, it should improve the startup time, but it's > still not as nice as simply compiling it straight up. > > I'll try it. Here's an example, which stores a Data.Map in a gzip-compressed bytestring literal (a C string literal in the compiled code). The Map is reconstructed on startup. {-# LANGUAGE OverloadedStrings #-} import Data.Binary import qualified Data.Map as M import qualified Data.ByteString.Char8 as S import Data.ByteString.Lazy import Codec.Compression.GZip -- -- this is a gzip compressed literal bytestring, storing a binary-encoded Data.Map -- mytable = "\US\139\b\NUL\NUL\NUL\NUL\NUL\NUL\ETXEN\219\SO\194 \f\197\224\188\196\CAN\227\US\224\171~\NAKc\GS4ce\161`\178\191\215(\176\190\180\167\231\210\n\241\171\203\191\ti\157\217\149\249< \ENQ\214\&9>\202\162\179a\132X\233\ESC=\231\215\164\SYN\157\DC2D\226*\146\174o\t\167\DLE\209\"i_\240\193\129\199<W\250nC\CAN\212\CAN\162J\160\141C\178\133\216;\\@4\144-W\203\209x\205\140\166\RS\163\237]9f\170\143\ACK\163g\223\STX\184\&7\rH\222\FSW\130\&7D\197\NUL\164\&0U\193\186\t\186o\228\180~\NUL\a6\249\137#\SOH\NUL\NUL" main = print =<< M.lookup "ghc" m where -- build the table from the bytestring: m :: M.Map String (Maybe String) m = decode . decompress . fromChunks . return $ mytable Running it: $ ./A Just "dinosaur!" :) Important to use a bytestring, since that gets compiled to a C string literal (and not messed with by the simplifier). -- Don

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A little toy of Haskell Trivia
by Steve Lihn 24 Feb '08

24 Feb '08

I proudly announce a little toy that lists the frequency of modules being imported by other modules. Do you know Control.Monad is the most frequently imported module? I did not! Currently it only includes GHC 6.8 core library. If you have any idea how to parse through HackageDB code, please let me know. http://haskell.ecoin.net/cgi-bin/modules.pl Disclaimer: This is entirely for fun. It is by no means accurate or complete. I only spent a couple hours on a little perl script and a mysql table...

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how to catch keyboard interrupts?
by Uwe Hollerbach 24 Feb '08

24 Feb '08

Hi, all, I am continuing to mess with my little scheme interpreter, and I decided that it would be nice to be able to hit control-C in the middle of a long-running scheme computation to interrupt that and return to the lisp> prompt; hitting control-C and getting back to the shell prompt works, but is a little drastic. So I looked at System.Posix.Signals and after a bit of messing about got the following: > mysighandler = > Catch (do hPutStrLn stderr "caught a signal!" > fail "Interrupt!") > > runREPL :: IO () > runREPL = > do getProgName >>= writeHdr > env <- setupBindings [] True > runInit env > installHandler sigINT mysighandler Nothing > installHandler sigQUIT mysighandler Nothing > doREPL env This compiles just fine, the interpreter runs as usual... but the added code doesn't seem to do anything. You can probably guess already... the print statement in mysighandler is there to see if it actually caught a signal. It does: I see "caught a signal!" just fine, in fact I see dozens of them as I lean on the control-C; but now my scheme calculation doesn't get interrupted at all! I see in the System.Posix.Signals documentation that the signal handler gets invoked in a new thread; is this the source of the problem? If so, what should I do to fix it? I'm afraid that sort of stuff is still beyond my haskell-fu... many thanks! Uwe

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