April 2012 - Glasgow-haskell-users

Re: default instance for IsString
by John Lato 25 Apr '12

25 Apr '12

> From: Yitzchak Gale <gale(a)sefer.org> > > Erik Hesselink wrote: >> I don't think IsString should be dismissed so easily. > > I'm just saying I don't want to be forced to use it. > If others like it, I'm not dismissing it. > >> we have a couple of newtypes over Text that do different kinds of >> normalization. An IsString instance for these is useful and total. > > True. Perhaps you'd be able to get IsBuiltinString instances > for those too, using newtype deriving, if only the method > names of IsBuiltinString are hidden and the class name is > exported. > > If that doesn't work, I'm fine with using a quasiquoter for > those instead. Or even just the usual newtype unwrapping > and wrapping. And again, if you provide IsString and others > want to use it, that's fine. I don't see how it would be possible to use a hidden IsBuiltinString as you describe without bringing Text into base (or alternatively not providing Text support). Perhaps unfortunately, I think that makes this solution a non-starter. I think a neater solution would be some sort of modular String typing, as I'm pretty sure somebody else on this list already mentioned. Perhaps a pragma like "DefaultString Data.Text.Text", which would mean that string literals would be treated as the provided monomorphic type, on a per-module basis? John Lato

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Records in Haskell: Explicit Classy Records
by Matthew Farkas-Dyck 23 Apr '12

23 Apr '12

Hello GHC users. I made another proposal for records in Haskell, meant to solve just the namespace problem, and no more. http://hackage.haskell.org/trac/ghc/wiki/Records/ExplicitClassyRecords In this system, record selectors are overloaded in explicitly-user-declared type classes. Thus one can control the scope as of any other type class. Cheers, strake

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Re: default instance for IsString
by Greg Weber 22 Apr '12

22 Apr '12

so how can I update the documentation? I asked some of the most experienced Haskell users at the Hackathon about this, and looked through any documentation I could find and there was nothing indicating I could do what you sent in your last message. On Sun, Apr 22, 2012 at 8:15 AM, Markus Läll <markus.l2ll(a)gmail.com> wrote: > The core of it is in the GHC docs' overloaded strings section [1]. > > It could be clearer though -- reading about defaulting in the reports, > in the type defaulting section of GHC docs and in [1] can be a bit > confusing. > > [1] http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensio… > > On Sun, Apr 22, 2012 at 4:54 PM, Greg Weber <greg(a)gregweber.info> wrote: >> Thanks Markus, I think you have saved the day! >> Even after googling for this extension and searching in the manual I >> am still coming up pretty blank. >> Is there somewhere I missed where this is documented or somewhere I >> can contribute documentation? >> >> On Sun, Apr 22, 2012 at 4:47 AM, Markus Läll <markus.l2ll(a)gmail.com> wrote: >>> ExtendedDefaultRules > > > > -- > Markus Läll

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Finding/fixing stack overflow.
by David Brown 21 Apr '12

21 Apr '12

I've isolated the below small piece of code that is giving me a stack overflow. I'm kind of at a loss as has to fix, or even find what is happening here. (The real program is reading the data from a file, and doing something more complex with it). I'm not even sure how to work around this issue. It fails for me when given an argument of 1000000. Any ideas? I'm running 7.0.4, but I've tried this with ghc 6.12.3, and 7.4.1. Thanks, David ---------------------------------------------------------------------- import Control.Applicative import Control.Monad import Data.Binary import Data.Binary.Put import Data.Binary.Get import Data.List import Data.Word import System.Environment putValues :: [Int] -> Put putValues vals = mapM_ (putWord32le . fromIntegral) vals getValues :: Int -> Get [Int] getValues count = replicateM count (fromIntegral <$> getWord32le) goofySum :: Int -> Int goofySum count = let block = runPut $ putValues [1 .. count] in foldl' max 0 $ runGet (getValues count) block main :: IO () main = do args <- getArgs let count = case args of [c] -> read c _ -> error "Invalid usage" putStrLn $ show $ goofySum count ----------------------------------------------------------------------

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Build failure with HEAD
by Austin Seipp 20 Apr '12

20 Apr '12

Hi, Building GHC HEAD (git HEAD SHA1 = 88f476b98709731d997ab57612cce4753cb65a0a) this morning, I've encountered 2 build failures that seem to be a result of some recent changes in the past day or two. If I build a clean repository with `make -j13` on my 12 core machine, I get: ------------------------------------------------------------------------------ compiler/hsSyn/Convert.lhs:178:30: `td_fvs' is not a (visible) field of constructor `TyData' compiler/hsSyn/Convert.lhs:190:31: `td_fvs' is not a (visible) field of constructor `TyData' make[1]: *** [compiler/stage2/build/Convert.o] Error 1 make[1]: *** Waiting for unfinished jobs.... --------------------------------------------------------------------------- However, reinvoking `make` with no parallelism and attempting to continue the build (there's no race condition obviously, I was just curious,) I get another (possibly related) build failure as well: --------------------------------------------------------------------------- compiler/deSugar/DsMeta.hs:326:18: Constructor `FamInstD' should have 2 arguments, but has been given 1 In the pattern: FamInstD fi_decl In the pattern: L loc (FamInstD fi_decl) In an equation for `repInstD': repInstD (L loc (FamInstD fi_decl)) = do { dec <- repFamInstD fi_decl; return (loc, dec) } make[1]: *** [compiler/stage2/build/DsMeta.o] Error 1 --------------------------------------------------------------------------- I would assume someone forgot to push something perhaps, but I figured naturally someone should be aware so they can fix it. :) -- Regards, Austin

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Another profiling question.
by Jurriaan Hage 18 Apr '12

18 Apr '12

Dear all, from the RTS option -s I get : INIT time 0.00s ( 0.00s elapsed) MUT time 329.99s (940.55s elapsed) GC time 745.91s (751.51s elapsed) RP time 765.76s (767.76s elapsed) PROF time 359.95s (362.12s elapsed) EXIT time 0.00s ( 0.00s elapsed) I can guess what most components mean, but do not know what RP stands for. I also could not find this information in the GHC documentation that is online. Can anyone tell me? Jur

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Invariants for GHC.Event ensureIOManagerIsRunning
by Edward Z. Yang 18 Apr '12

18 Apr '12

Hello all, I recently ran into a rather reproduceable bug where I would get this error from the event manager: /dev/null: hClose: user error (Pattern match failure in do expression at libraries/base/System/Event/Thread.hs:83:3-10) The program was doing some rather strange things: - It was running the Haskell RTS inside another system (Urweb) which was making use of pthreads, sockets, etc. - The Haskell portion was linked against the threaded RTS, and doing communication with a process. and is rather complicated (two compilers are involved). But the gist of the matter is that if I added a quick call to ensureIOManagerIsRunning after hs_init, the error went away. So, if the IO manager is not eagerly loaded at the call to hs_init, how do we decided when it should be loaded? It seems probably that we missed a case. Edward P.S. I tried reproducing on a simple test case but couldn't manage it.

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faking universal quantification in constraints
by Nicolas Frisby 17 Apr '12

17 Apr '12

I'm simulating skolem variables in order to fake universal quantification in constraints via unsafeCoerce. http://hpaste.org/67121 I'm not familiar with various categories of types from the run-time's perspective, but I'd be surprised if there were NOT a way to use this code to create run-time errors. Is there a way to make it safer? Perhaps by making Skolem act more like GHC's Any type? Or perhaps like the -> type? I'd like to learn about the varieties of types from the run-time's perspective. I know Dimitrios Vytiniotis is trying to implement these legitimately in GHC, but I don't know much about that project's status, nor any documentation indicating how to try it out in GHC HEAD (e.g. what's the syntax?). Is there a page on the GHC wiki or something to check that sort of thing? I wonder how far this Forall_S trick can take me in the interim towards Vytiniotis' objective functionality when paired with a (totally safe) class for implication. > class Implies (ante :: Constraint) (conc :: Constraint) where > impliesD :: Dict ante -> Dict conc Thanks, Nick

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What do the following numbers mean?
by Jurriaan Hage 10 Apr '12

10 Apr '12

Dear all, I ran a small example program, and this is what I got from using the -s flag: 486,550,118,368 bytes allocated in the heap 323,749,418,440 bytes copied during GC 1,842,979,344 bytes maximum residency (219 sample(s)) 204,653,688 bytes maximum slop 4451 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 924208 collections, 0 parallel, 1861.17s, 1866.05s elapsed Generation 1: 219 collections, 0 parallel, 283.44s, 284.01s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 740.61s (745.45s elapsed) GC time 2144.61s (2150.06s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2885.23s (2895.51s elapsed) %GC time 74.3% (74.3% elapsed) Alloc rate 656,953,176 bytes per MUT second Can anyone tell me what the exact difference is between 1,842,979,344 bytes maximum residency (219 sample(s)) and 4451 MB total memory in use (0 MB lost due to fragmentation) I could not find this information in the docs anywhere, but I may have missed it. best, Jur

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Code review for new primop's CMM code.
by Ryan Newton 10 Apr '12

10 Apr '12

Hi all, In preparation for students working on concurrent data structures GSOC(s), I wanted to make sure they could count on CAS for array elements as well as IORefs. The following patch represents my first attempt: https://github.com/rrnewton/ghc/commit/18ed460be111b47a759486677960093d71ee… It passes a simple test [Appendix 2 below], but I am very unsure as to whether the GC write barrier is correct. Could someone do a code-review on the following few lines of CMM: if (GET_INFO(arr) == stg_MUT_ARR_PTRS_CLEAN_info) { SET_HDR(arr, stg_MUT_ARR_PTRS_DIRTY_info, CCCS); len = StgMutArrPtrs_ptrs(arr); // The write barrier. We must write a byte into the mark table: I8[arr + SIZEOF_StgMutArrPtrs + WDS(len) + (ind >> MUT_ARR_PTRS_CARD_BITS )] = 1; } Thanks, -Ryan -- Appendix 1: First draft code CMM definition for casArray# ------------------------------------------------------------------- stg_casArrayzh /* MutableArray# s a -> Int# -> a -> a -> State# s -> (# State# s, Int#, a #) */ { W_ arr, p, ind, old, new, h, len; arr = R1; // anything else? ind = R2; old = R3; new = R4; p = arr + SIZEOF_StgMutArrPtrs + WDS(ind); (h) = foreign "C" cas(p, old, new) []; if (h != old) { // Failure, return what was there instead of 'old': RET_NP(1,h); } else { // Compare and Swap Succeeded: if (GET_INFO(arr) == stg_MUT_ARR_PTRS_CLEAN_info) { SET_HDR(arr, stg_MUT_ARR_PTRS_DIRTY_info, CCCS); len = StgMutArrPtrs_ptrs(arr); // The write barrier. We must write a byte into the mark table: I8[arr + SIZEOF_StgMutArrPtrs + WDS(len) + (ind >> MUT_ARR_PTRS_CARD_BITS )] = 1; } RET_NP(0,h); } } -- Appendix 2: Simple test file; when run it should print: ------------------------------------------------------------------- -- Perform a CAS within a MutableArray# -- 1st try should succeed: (True,33) -- 2nd should fail: (False,44) -- Printing array: -- 33 33 33 44 33 -- Done. ------------------------------------------------------------------- {-# Language MagicHash, UnboxedTuples #-} import GHC.IO import GHC.IORef import GHC.ST import GHC.STRef import GHC.Prim import GHC.Base import Data.Primitive.Array import Control.Monad ------------------------------------------------------------------------ -- -- | Write a value to the array at the given index: casArrayST :: MutableArray s a -> Int -> a -> a -> ST s (Bool, a) casArrayST (MutableArray arr#) (I# i#) old new = ST$ \s1# -> case casArray# arr# i# old new s1# of (# s2#, x#, res #) -> (# s2#, (x# ==# 0#, res) #) ------------------------------------------------------------------------ {-# NOINLINE mynum #-} mynum :: Int mynum = 33 main = do putStrLn "Perform a CAS within a MutableArray#" arr <- newArray 5 mynum res <- stToIO$ casArrayST arr 3 mynum 44 res2 <- stToIO$ casArrayST arr 3 mynum 44 putStrLn$ " 1st try should succeed: "++show res putStrLn$ "2nd should fail: "++show res2 putStrLn "Printing array:" forM_ [0..4] $ \ i -> do x <- readArray arr i putStr (" "++show x) putStrLn "" putStrLn "Done."

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