February 2010 - Haskell-Cafe

EDSL's using Filet-O-Fish of Barrelfish project
by C K Kashyap 27 Feb '10

27 Feb '10

Hi All, I'd been looking for a mechanism to generate "C" from Haskell as EDSL - to develop an OS. In the process, I ran into the Barrelfish project ( http://www.barrelfish.org) - They seem to be using Haskell based "DSL generator" called Filet-O-Fish - http://www.barrelfish.org/fof_plos09.pdf It will take me some time understand Filet-O-Fish more but if any of the Haskell experts have already looked at it, I was wondering if I could use Filet-O-Fish to generate EDSL's for arbitrary things. For example, lets say, I wanted to write a EDSL for Linux's network drivers. Could I use Filet-O-Fish and generate a EDSL that allows me to write a driver in a really highlevel manner and then generate the "C" code for it? -- Regards, Kashyap

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How to understand `|` in this code snippet ?
by zaxis 27 Feb '10

27 Feb '10

xxxMain = do timeout <- getEnv "xxx_TIMEOUT" case timeout of Just str | [(t, _)] <- reads str -> do addTimeout t (hPutStrLn stderr "*** TIMEOUT" >> _exit 1) return () _ -> return () ....... What does the `|` mean in "Just str | [(t, _)] <- reads str" ? Is it a logical `or` ? Sincerely! ----- fac n = let { f = foldr (*) 1 [1..n] } in f -- View this message in context: http://old.nabble.com/How-to-understand-%60%7C%60-in-this-code-snippet---tp… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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redirecting ghc (as a library) output
by Martin Hilbig 27 Feb '10

27 Feb '10

hi, i'm writing a Haskell View Server for CouchDB. it communicates with couchdb over stdin and stdout. it gets JSON encoded haskell code, compiles it (like on http://www.haskell.org/haskellwiki/GHC/As_a_library) gets values, runs the given code over the given values and writes the results back (also json encoded). when there is an error in the given haskell code it should reply with the error json encoded and exit. but f.e. when the given code imports a module, which cannot be found (here Reaction) ghc just spits out a panic directly to stdin and exits like this: > ViewServer: panic! (the 'impossible' happened) > (GHC version 6.12.1 for x86_64-unknown-linux): > Could not find module `Reaction': > Use -v to see a list of the files searched for. > > > Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug it puts other errors directly to stdout too, like: > Assembler messages: > Fatal error: can't create > /home/*/AlkylRadicalDecomposition.o: > Permission denied this confuses couchdb, because it expects some JSON and i cant see whats going on between them. now, how can i prevent ghc from using stdout and wrap the output in some JSON? is this even possible? with ghc 6.10 this usage of 'handle' worked for me: > main = handle (\e -> do > let e' = show (e::SomeException) > case fromException e of > Just UserInterrupt -> exitSuccess > _ -> do > let err = error2json "the impossible happened..." e' > putStrLn err > logToFile err > return []) main_loop did i got the Exception handling wrong? thanks in advance. have fun martin

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save/restore of STRef state
by Job Vranish 27 Feb '10

27 Feb '10

I was using STRefs the other day and I ran across a case where I really wanted the ability to save the state of all my references, execute some action, and then restore the state later if needed. I didn't find anything that does this on hackage so I implemented a small wrapper around STRefs which does what I want. Before I put it on hackage I thought I'd run it by haskell-cafe. It works as follows: test :: (String, Bool) test = runContext $ do a <- newRef "foo" b <- newRef False restore <- save writeRef a "bar" writeRef b True when someCondition restore x <- readRef a y <- readRef b return (x, y) If someCondition, the existing state is restored and test returns ("foo", False) otherwise nothing special happens and test returns ("bar", True) Also each reference has a unique key that can make it much easier to convert structures that use STRefs to pure stuctures. Reads, writes and creating refs are still constant time operations (with only a very very small overhead) and garbage collecting behavior should be the same as with regular STRefs. My implementation is here: http://gist.github.com/316738 What do you think? Any suggestions? Does anything like this already exist in hackage? Does this seem useful to other people besides me? :) Any glaring purity issues that I overlooked? Thanks for your input, - Job

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Testing and module export lists
by Magnus Therning 27 Feb '10

27 Feb '10

How do people who like unit testing / property testing deal with export lists? I often find that I do want an export list to reduce clutter in the finished code, but for testing I'd like to expose everything in a module. Is there a nice way to deal with this (using the C pre-processor would not qualify as "nice" ;-)? Maybe there's a switch that causes GHC to simply ignore the export list of a module and export everything? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Generalizing nested list comprehensions
by Ishaaq Chandy 27 Feb '10

27 Feb '10

Hi all, If this question sounds a bit noob-ish, that would be because I am one - so apologies in advance! I have functions that look something like these: f1 :: [a] -> [b] f1 xs = [foo [x1, x2] | x1 <- xs, x2 <- bar x1, baz x1 /= baz x2] f2 :: [a] -> [b] f2 xs = [foo [x1, x2, x3] | x1 <- xs, x2 <- bar x1, x3 <- bar x2, baz x1 /= baz x2, baz x1 /= baz x3, baz x2 /= baz x3] f3 :: [a] -> [b] f3 xs = [foo [x1, x2, x3, x4] | x1 <- xs, x2 <- bar x1, x3 <- bar x2, x4 <- bar x3, baz x1 /= baz x2, baz x1 /= baz x3, baz x1 /= baz x4, baz x2 /= baz x3, baz x1 /= baz x4] Note that for the purposes of this discussion it does not matter what foo, bar and baz do. Now what I want to do is write a generalized function fn based on the pattern set by f1, f2 and f3 such that: fn :: Int -> [a] -> [b] and: (fn 1 xs) == f1 xs (fn 2 xs) == f2 xs (fn 3 xs) == f3 xs (fn 25 xs) == f25 xs - obviously if I were to implement f25 as nested list comprehensions it would be ridiculously tedious! Any ideas how I can implement fn? Thanks, Ishaaq -- View this message in context: http://old.nabble.com/Generalizing-nested-list-comprehensions-tp27719199p27… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Hackage redirect loop
by Daniel Fischer 26 Feb '10

26 Feb '10

Trying to access http://hackage.haskell.org/trac/hackage I land in a redirection loop no matter which page or which browser I try. It worked earlier today. Has anybody an idea what the heck might be going on? Is it just me?

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Hackage statistics
by Andrew Coppin 26 Feb '10

26 Feb '10

Is there a simple way to discover the answer to questions like "how many packages are on Hackage?" or "how many downloads per day does Hackage serve?" Or is this information not posted anywhere? (I know there was a blog article a while back with some stats in it, but what about the statistics today?)

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do we need types?
by Pasqualino "Titto" Assini 26 Feb '10

26 Feb '10

Hi, just a silly question (or maybe more than one): In Haskell we have data types (Integer,[a],...) as well as type classes (Num, Ord...). But, if we have type classes do we still need types? Why shouldn't the objects that we process be defined only by their 'interfaces' (assuming that a type class is a kind of interface)? Maybe the real question is: are type classes a more primitive concept than data types? And if so, in a language that had only type classes what would a data declaration like the following map to: data List a = Cons a (List a) | Nil And what about pattern matching? Would that still be possible, and what form would it take? And finally, would having only type classes make the type system any simpler? Thanks, titto

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Redirecting output
by Louis Wasserman 26 Feb '10

26 Feb '10

Is there any way I can temporarily reassign stdout? In particular, will this be handled well by the FFI? I'm hoping there's maybe something like PHP's output buffering thingy <http://php.net/manual/en/book.outcontrol.php>. I'm *binding* (not doing a process call) to an external library which outputs directly to stdout, but whose output I'd like to use... Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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