January 2009 - Glasgow-haskell-users

how can I run ghci monad actions from ghci command line
by Peter Hercek 28 Jan '09

28 Jan '09

Is it possible to run ghci monad actions from ghci command line somehow? For example: I would like to check whether "it" variable is of type Bool and whether it is True using normal Haskell code (i.e. not using ghci commands starting with colon like :type :print). Peter.

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permissions api in directory package is useless (and possibly harmful)
by Duncan Coutts 28 Jan '09

28 Jan '09

All, We need to think about a new better permissions api for the System.Directory module. The current api and also the implementation are at best useless and possibly harmful. I've been trying to debug various permission problems related to installing files with Cabal on Unix and Windows. Half the problems seem to stem from the permissions api and the copying of permissions in copyFile. data Permissions = Permissions { readable :: Bool, writable :: Bool, executable :: Bool, searchable :: Bool } getPermissions :: FilePath -> IO Permissions setPermissions :: FilePath -> Permissions -> IO () These are clearly designed for the unix permissions model, however they do not map onto it very usefully. get/setPermissions only get the user permissions, not the group or other. So for example if I have a file: -rw-rw-rw- 1 duncan users 0 2009-01-28 12:34 foo then setPermissions "foo" (Permissions True False False False) only removes write permissions from me, not from everyone else: -r--rw-rw- 1 duncan users 0 2009-01-28 12:34 foo which cannot be what we wanted. It's also pretty useless for installing files globally. For that we want to say that a file is readable by everyone and only writable by the owner (usually root). It might even be ok to say only readable by everyone and writable by nobody, but we cannot even do that. Combine that with copyFile copying permissions and we can easily lock people out or install world-writable files depending on the umask of the user building the software. On windows getPermissions tells us almost nothing. In particular if it says the file is readable or writable, this is no guarantee that we can read or write the file. getPermissions does not look at permissions. It only consults the old DOS read-only attribute (and the file extension to tell us if a file is executable). Similarly, on windows setPermissions also only sets the read-only attribute. The read-only attribute should really be avoided. It has rather unhelpful semantics. For example moving a file over a read-only file fails, where as if windows permissions (ACLs) are used then it works as expected (same as on POSIX). The read-only attribute is only there for compatibility with heritage software, we should really never set it. There is also an implementation of copyPermissions, it's not actually exposed in System.Directory but it is used by copyFile. It calls stat to get the permissions and chmod to set them. The implementation is the same between unix and windows, on windows it uses the stat and chmod from the msvcrt library. On Unix copyPermissions does work and is probably the right thing for copyFile to do. It's the default behaviour of /bin/cp for example. However it does mean there is no easy efficient way to make a copy of a file where the destination file gets the default permissions, given the umask of the current user. This means that copyFile is not appropriate for installing files, since the user building a package is not necessarily the same as the one installing it and their umasks can and often are different. The unix install program does not copy permissions, instead it sets them explicitly. We have no portable way of doing the same. But we can at least use the System.Posix.Files.setFileMode to do it. One nice thing about copyFile is that it replaces the destination file atomically. However if we have to set the permissions in a second step then we loose this nice property. Ideally we would create the temporary file in the destination directory (exclusively and with permissions such that no other user could write to our file), we'd copy the data over, set the new destination permissions and atomically replace the destination file. The copyPermissions function on windows is useless. It does not copy permissions. The way that msvcrt implements stat and chmod means that the only "permissions" that are copied is the old DOS read-only attribute. Perhaps copying the read-only attribute is the right thing for copyFile to do, it's what the Win32 CopyFile function does, however it's never what I want to do for installing software package files. So, can we craft a useful api for permissions? We should consider what the defaults for copyfile etc should be with respect to permissions. Where those defaults are not helpful can we think of a way to let us do what we want (eg get default or specific permissions instead of copying permissions). Or is it impossible and we just have to use system-specific functions whenever we need something non-standard. In which case we need to make those functions better, they appear to be mostly lacking for the Win32 case. In either case I'm sure the existing permissions api needs to be deprecated with big flashing warnings. Duncan Oh, and while I'm thinking about it: it's not currently possible to open new/exclusive files with specific permissions and the open temp file functions on windows do not ensure the file is writable only by the current user.

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seg-fault
by Serge D. Mechveliani 24 Jan '09

24 Jan '09

Yesterday, I promised to provide a bug report with Segmentation fault on missing some method implementation. I saved this source copy. Hm ... today I still failed to reproduce this seg-fault situation! I am sorry. Regards, ----------------- Serge Mechveliani mechvel(a)botik.ru

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seg-fault in 6.10.1
by Serge D. Mechveliani 23 Jan '09

23 Jan '09

Dear GHC team, I `make' my (large) project in ghc-6.10.1, Linux Debian, i386-unknown, run the executable, and obtain Segmentation fault. Then, I noted that in a few places the compiler warned about skipping some class member implementations in some instances. I added these implementations, and now it works correct. Are you interested in the bug report on the above part of Segmentation fault ? ----------------- Serge Mechveliani mechvel(a)botik.ru

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Changes in scoped type variable behavior?
by Austin Seipp 23 Jan '09

23 Jan '09

Hi, I've been reading Oleg Kiselyov's "Implicit Configurations" paper and while getting the modulus math code to work I've hit a bit of a problem. The code is attached to this message; the problem is in the normalize function: > normalize :: (Modular s a, Integral a) => a -> M s a > normalize a = M (mod a (modulus (u :: s))) Trying this code gives: > $ ghci implicit-config.hs > GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer ... linking ... done. > Loading package base ... linking ... done. > [1 of 1] Compiling Main ( implicit-config.hs, interpreted > ) > > implicit-config.hs:34:24: > Could not deduce (Modular s a) > from the context (Modular s1 a, Integral a) > arising from a use of `modulus' at implicit-config.hs:34:24-38 > Possible fix: > add (Modular s a) to the context of > the type signature for `normalize' > In the second argument of `mod', namely `(modulus (u :: s))' > In the first argument of `M', namely `(mod a (modulus (u :: s)))' > In the expression: M (mod a (modulus (u :: s))) > Failed, modules loaded: none. > Prelude> The paper uses the following code, which annotates the return of normalize while at the same time binding the 's' type variable for use on the right hand side: > normalize :: (Modular s a, Integral a) ⇒ a → M s a > normalize a :: M s a = M (mod a (modulus (u :: s))) This code fails with: > $ ghci implicit-config.hs > GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer ... linking ... done. > Loading package base ... linking ... done. > [1 of 1] Compiling Main ( implicit-config.hs, interpreted > ) > > implicit-config.hs:34:0: Parse error in pattern > Failed, modules loaded: none. > Prelude> Even with the ScopedTypeVariables extension - am I missing something? I've also tried the actual literate haskell code from the technical report and it fails with the same error (parse error in pattern.) I'm using Mac OS X 10.6 and GHC 6.10.1. Austin

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incorrect implementation of :show bindings?
by Peter Hercek 23 Jan '09

23 Jan '09

Hi, Bindings displayed with ":show bindings" do not correspond to what can be observed by printing values explicitly for example using ":force". Is it supposed to work like that? If yes, why? Notice the value and type reported for variable "test" after it has been redefined in the example at this post end. Thanks, Peter. status:0 peter@dwarf [703] /tmp % ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> let test = 0 Prelude> :show bindings test :: Integer = _ Prelude> :force test test = 0 Prelude> let test = "zero" Prelude> :show bindings test :: Integer = 0 Prelude> :force test test = ['z','e','r','o'] Prelude> :quit Leaving GHCi. status:0 peter@dwarf [704] /tmp %

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no breakpoint location for list construction?
by Peter Hercek 21 Jan '09

21 Jan '09

Hi, Is it correct that there is no breakpoint location at list construction place: line "5 then [x]" ? Notice that :steplocal does not stop there before "[1]" is printed when debugging expression "test 1". Thanks, Peter. status:0 peter@metod [721] ~/tmp % cat b.hs hlp x = [-x] test x = if x >= 0 then [x] else hlp x status:0 peter@metod [722] ~/tmp % ghci b.hs GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Loading package old-locale-1.0.0.0 ... linking ... done. Loading package old-time-1.0.0.0 ... linking ... done. Loading package filepath-1.1.0.0 ... linking ... done. Loading package directory-1.0.0.0 ... linking ... done. Loading package ghciext-0.1 ... linking ... done. [1 of 1] Compiling Main ( b.hs, interpreted ) Ok, modules loaded: Main. *Main> :break 4 Breakpoint 0 activated at b.hs:4:5-10 *Main> test 1 Stopped at b.hs:4:5-10 _result :: [a] = _ 3 test x = 4 if x >= 0 5 then [x] [b.hs:4:5-10] *Main> :steplocal [1] *Main> test (-1) Stopped at b.hs:4:5-10 _result :: [a] = _ 3 test x = 4 if x >= 0 5 then [x] [b.hs:4:5-10] *Main> :steplocal Stopped at b.hs:6:9-13 _result :: [Integer] = _ x :: Integer = -1 5 then [x] 6 else hlp x 7 [b.hs:6:9-13] *Main> :steplocal [1] *Main> :quit Leaving GHCi. status:0 peter@metod [723] ~/tmp %

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Re: new build system build fails on Solaris/x86
by Karel Gardas 21 Jan '09

21 Jan '09

Christian Maeder wrote: > Karel Gardas wrote: >> Hello, >> >> I need to double check: I'm talking here about new build system. I've >> invoked simple `gmake'. Also your builds is fine since editline seems to >> find readline header file which seems to satisfy it while this is not in >> my case. > > Although my configure finds readline header files, this is not enough > for editline. Both configure calls fail, but in my case the failure is > correctly handled by the build system via touching editline/unbuildable OK. > >> How have you configure to allow it to find readline? > > It is just a wrong readline.h for libreadline (lying around) and I only > called ./configure I did just as usuall: bash ./boot ./configure --prefix=/tmp/ghc-new-build-system-bin gmake For boot I'm using autoconf-2.62/automake-1.10.1 and GHC 6.8.3 Thanks, Karel

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git
by Lennart Augustsson 21 Jan '09

21 Jan '09

What port is git using for getting the ghc repo via http? I'm getting this message behind our thick firewall: fatal: http://darcs.haskell.org/ghc.git/info/refs download error - Failed connect to darcs.haskell.org:1080; Operation now in progress It makes me think port 1080 is involved, and I don't think any access to that port is allowed. If the repo is not totally available on port 80 (or the https port) then it's impossible to get from behind paranoid firewalls. -- Lennart

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Questions regarding uvector 0.1.0.3
by Tyson Whitehead 19 Jan '09

19 Jan '09

I've been looking at uvector and had some questions (hopefully this is the right mailing list for this, if not, sorry, and please direct me properly). First, is there any design thing deeper than just supporting things like the unit and pair arrays for layering UArr on top of the underlying BUArr? Second, the STG code and the native and via-C assembler generated by ghc 6.8.2 with -O2 for test :: UArr Int -> Int -> Int test = indexU where indexU :: UA e => UArr e -> Int -> e indexU arr n = indexS (streamU arr) n is a loop that simultaneously increments the offset while decrementing the index until it reaches the desired position. If I understand correctly, this indexU definition was used instead of just bring indexU from UArr into scope in order to cancel any possible unstreamU in arr, and this is a big part of the whole streamU/unstreamU magic. It replaces what would other require N^2 rules to avoid a bunch of temporaries with one. I wonder if a rule code be added to change any outstanding "indexS (streamU arr) n" into an UArr indexU after streamU/unstreamU fusion? Such rules for each of these type of functions (i.e., ones that aren't of the form "= unstreamU . ...") would also not be N^2 and would eliminate temporary streams. Thanks! -Tyson PS: Nice code by the way. : )

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