April 2006 - Glasgow-haskell-users

unexpected ambiguity
by Niklas Sorensson 10 Apr '06

10 Apr '06

Hello, Trying to compile the attached modules gives the following error message: Main.hs:8:15: Ambiguous occurrence `lift' It could refer to either `Control.Monad.Trans.lift', ... or `Apa.lift', imported from Apa at ... Is this correct? I would have expected that an unqualified lift could only refer to the one exported from Apa in this case. /Niklas

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RE: Is there a way to optimize out some foreign calls?
by Simon Peyton-Jones 10 Apr '06

10 Apr '06

| I'm thinking in terms of some way of marking foreign functions to specify | how they can be optimized out or specifying rewrite sequences eg | | x >> api_sync >> api_pop >> api_sync >> y = x >> api_pop >> | api_sync >> y You should be able to say something like RULE api_sync >>= (\_ -> api_pop >>= rest) = api_pop >>= rest api_pop >>= (\_ -> api_sync >>= rest) = api_pop >>= rest Do-notation is desugared to (>>=) stuff. Give it a try on a small example and see if it works SImon | | I've read the manual (7.10) on rewrite rules, but I'm having difficulty | understanding how to express the lhs of the above equation as a rewrite rule | so that it matches a sequence anywhere in a do block (to deal with nested | parentheses and the use of >>= instead of >>) | | Any ideas? | | Thanks, Brian. | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users(a)haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

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darcsweb
by John Meacham 10 Apr '06

10 Apr '06

is there a darcsweb browser for the ghc repos anywhere? a la http://repetae.net/dw/darcsweb.cgi?r=jhc;a=summary John -- John Meacham - ⑆repetae.net⑆john⑈

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FFI Bug - Finalizer always called with NULL pointer?
by Brian Hulley 10 Apr '06

10 Apr '06

Hi - I've discovered that the finalizer in the code below is always called with a NULL pointer even though the pointer returned by the allocation routine is always not NULL. Is this a known bug, and if so, are there any workarounds? I'm using ghc version 6.4: data RawFont newtype Font = Font (ForeignPtr RawFont) foreign import ccall duma_createFont :: CString -> Int -> Bool -> Bool -> Bool -> IO (Ptr RawFont) foreign import ccall duma_releaseFont :: FunPtr (Ptr RawFont -> IO ()) createFont :: String -> Int -> Bool -> Bool -> Bool -> IO Font createFont name pointSize bold italic underline = block $ do f <- (withCString name $ \cname -> duma_createFont cname pointSize bold italic underline) >>= newForeignPtr duma_releaseFont -- Gets a NULL pointer!!! return $ Font f Thanks, Brian.

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ghc command line evaluation
by Pedro Miguel Duarte 06 Apr '06

06 Apr '06

I am writing a Java program with a call to a Haskell module M.hs, in order to evaluate some expression expr. A very simple idea, which I got somewhere in the net, is to create a Process object p which executes a GHC command-line instruction: Process p = Runtime.getRuntime(); p.exec( " ghc M.hs -e \"expr\" " ); This would be very simple, if it worked... My problem is that expressions i want to evaluate involve strings, and GHC command-line 'ghc' misinterprets some special symbols when it parses double quoted strings. For instance, ghc -e " reverse \"2<3\" " gives an error! Thus any one know a simple way around this? The only one obvious to me is to create a temporary Haskell module M_tmp.hs, with the expression to be evaluated, which is then executed through a command-line instruction like ghc M_tmp.hs -e main

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Re[2]: ghc releasing memory during compilation
by Bulat Ziganshin 06 Apr '06

06 Apr '06

Hello Simon, Monday, March 13, 2006, 3:40:21 PM, you wrote: >> just to check available PHYSICAL memory he answered that he don't know >> that is physical memory, only virtual memory matters SM> I think that's a mischaracterisation of what I said. Actually I said SM> that I didn't understand your comment about physical vs. virtual memory, SM> and in fact, looking back at the message, I still don't understand it :) SM> http://www.haskell.org//pipermail/glasgow-haskell-users/2005-April/008373.h… SM> I think what you're suggesting is that the runtime should detect the SM> amount of physical memory on the system and auto-tune itself to switch SM> to compacting collection when its residency reaches that amount. This SM> is certainly something we could do. Bear in mind that GHC is not SM> necessarily the only process running on the machine, though, and what SM> about running multiple GHCs? i suggest checking of AVAILABLE physical ram, that is perfectly possible in windows SM> Also, you can do this by setting your GHC_RTS environment variable. SM> Suppose you have 1G of physical mem. You want GHC to give up when it SM> reaches 1.5G, and you want to switch to compacting collection when the SM> residency reaches 300M. You could do this: SM> export GHC_RTS='-M1.5G -c20' SM> because 300M is 20% of 1.5G. Perhaps a better interface would be to SM> allow you to specify exactly the residency at which to switch to compaction. you mix up two questions: 1) switching from copying to compacting algorithm 2) return memory to OS after GC the only commonality between them is what we want to do both these things only when we are short on physical memory (1) is possible in GHC. (2) can be implemented, i think. so, the questions is how to trigger each thing. GHC has mechanism to switching GC algorithm although this mechanism is not enough flexible. i personally will prefer to have API that allow to control all GC parameters (although at this moment we also have this api unofficially - we have access to all these variables). so the main question at this time is implementation of returning memory to OS and providing some simple mechanism of enabling this (say, in percents of maximal heap size like this is done "-c"). then someone like me can roll out some API to allow programs better control the entire mechanism -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Is there a way to optimize out some foreign calls?
by Brian Hulley 04 Apr '06

04 Apr '06

H - I'm wondering if there is any way to specify that certain sequences of foreign calls can be merged into a single foreign call. For example, in a common situation where a state of a foreign API is pushed and popped so that the previous state is restored, I'd like successive pops to avoid wasting time restoring intermediate states as in: -- push a state onto the API's state stack (without changing the state) foreign import ccall api_push :: State -> IO () -- discards the state at the top of the stack foreign import ccall api_pop :: IO () -- synchronises the API's state with the state at the top of the stack foreign import ccall api_sync :: IO () use :: State -> IO () -> IO () use s action = do api_push s api_sync action api_pop api_sync -- put the API back into the previous state main = do use State1 $ do actionIn1a use State2 $ do actionIn2 use State3 $ do actionIn3 actionIn1b If 'use' is inlined, the compiler would (presumably) produce code like: main = do api_push State1 api_sync actionIn1a api_push State2 api_sync actionIn2 api_push State3 api_sync actionIn3 api_pop api_sync -- This is a waste of time api_pop api_sync actionIn1b api_pop api_sync I'm wondering if there is any way to cause the compiler to avoid the wasteful call to api_sync above. I'm thinking in terms of some way of marking foreign functions to specify how they can be optimized out or specifying rewrite sequences eg x >> api_sync >> api_pop >> api_sync >> y = x >> api_pop >> api_sync >> y I've read the manual (7.10) on rewrite rules, but I'm having difficulty understanding how to express the lhs of the above equation as a rewrite rule so that it matches a sequence anywhere in a do block (to deal with nested parentheses and the use of >>= instead of >>) Any ideas? Thanks, Brian.

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readChan and unGetChan?
by Li, Peng 04 Apr '06

04 Apr '06

Suppose the following happens: (1) Thread A calls readChan on an empty channel and waits (2) Thread B puts something to the read-end of the channel using unGetChan When a GHC program does this, both threads are blocked! Is it the behaviour we really want for unGetChan, or should we fix the implementation for Control.Concurrent.Chan? Thanks, Peng

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Dumb guy needs help
by Davey_dude 03 Apr '06

03 Apr '06

Im new to Haskell, hugs in particular, and was hoping you could help me solve a problem. It should be pretty easy. I have to use hugs to create an expression "data Exp = Plus Exp Exp| Sub Exp Exp| Mult Exp Exp| Power Exp Exp | Number [Int]" to define a constant "ex1 :: Exp" that represents the function ((3*4) + (2*(12^2))) - 2. Exp is an expression where numbers are represented by a list of digits (540 is [5,4,0]). Got any ideas. Any help is greatly needed! -- View this message in context: http://www.nabble.com/Dumb-guy-needs-help-t1386770.html#a3724993 Sent from the Haskell - Glasgow-haskell-users forum at Nabble.com.

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Still problems building ghc 6.5 with ghc 6.4
by Michael Marte 01 Apr '06

01 Apr '06

Hello *, when building ghc 6.5 with ghc 6.4 and alex 2.0.1, the build fails as follows: make -C ghc/compiler stage=2 make[2]: Entering directory `/home/marte/fptools/ghc/compiler' ../../ghc/compiler/ghc-inplace -H16m -O -istage2/utils -istage2/basicTypes -istage2/types -istage2/hsSyn -istage2/prelude -istage2/rename -istage2/typecheck -istage2/deSugar -istage2/coreSyn -istage2/specialise -istage2/simplCore -istage2/stranal -istage2/stgSyn -istage2/simplStg -istage2/codeGen -istage2/main -istage2/profiling -istage2/parser -istage2/cprAnalysis -istage2/ndpFlatten -istage2/iface -istage2/cmm -istage2/nativeGen -istage2/ghci -Istage2 -DGHCI -package template-haskell -threaded -package readline -DUSE_READLINE -cpp -fglasgow-exts -fno-generics -Rghc-timing -I. -IcodeGen -InativeGen -Iparser -package unix -package Cabal -ignore-package lang -recomp -Rghc-timing -H16M '-#include "hschooks.h"' -ignore-package ghc -fgenerics -funbox-strict-fields -c parser/Lexer.hs -o stage2/parser/Lexer.o -ohi stage2/parser/Lexer.hi parser/Lexer.x:578:10: lexical error in string/character literal at character '\955' <<ghc: 51822676 bytes, 5 GCs, 98216/98216 avg/max bytes residency (1 samples), 16M in use, 0.00 INIT (0.00 elapsed), 0.62 MUT (0.84 elapsed), 0.11 GC (0.13 elapsed) :ghc>> make[2]: *** [stage2/parser/Lexer.o] Error 1 make[2]: Leaving directory `/home/marte/fptools/ghc/compiler' make[1]: *** [stage2] Error 2 make[1]: Leaving directory `/home/marte/fptools' make: *** [bootstrap2] Error 2 Is this problem caused by alex? Michael

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