March 2011 - Haskell-Cafe

How to contact OpenGL package maintainer (where is Sven?)
by Jason Dagit 18 Mar '11

18 Mar '11

Hello, It looks like the OpenGL packages on hackage[1,2,3,4] have not been updated in some time. No updates later than Oct 2009. I tried to email Sven directly using the email address listed on hackage but after about a week I still haven't heard from him. I sent some patches to the opengl list yesterday, and then today I noticed that Sven hasn't posted on that list since Oct 2009 when he released the current version of OpenGLRaw. None of the public darcs repos have patches newer than Oct 2009. Also, the homepage url listed in the packages is a 404: http://www.haskell.org/HOpenGL/ My concern is that Sven has disappeared for some reason. I hope he's well. He has always done top notch work in the past maintaining these libraries. Perhaps he's simply busy or lost interest? Does anyone know if he was looking for a new maintainer? Perhaps you've heard from him more recently than Oct 2009? If a new maintainer is needed, I would consider nominating myself :) Thanks, Jason [1] http://hackage.haskell.org/package/OpenGLRaw [2] http://hackage.haskell.org/package/OpenGL [3] http://hackage.haskell.org/package/GLURaw [4] http://hackage.haskell.org/package/GLUT

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ghci Vim completion within Vim - need some advice
by Marc Weber 18 Mar '11

18 Mar '11

Hi, https://github.com/MarcWeber/vim-addon-async implements a hacky way to run external processes asynchronously. An external helper application send stdout of the process to Vim which adds the bytes to the buffer. Its not perfect but quite usable. Now I'd like to show the completion of \t sent to ghci to the user. I've done so for Ruby, Python and Scala: Vim sends input to ghci, and the reply is catched and put into Vim's completion list. issue 1): ghci's tab only works if its connected to a "terminal". This can be fixed easily by using a pseudo terminal issue 2): ghci's prompt sends data to /dev/tty (the controlling terminal) rather than to stdout/err. I don't know how to fix issue 2). Any ideas? The result could look like this python REPL screenshot: http://mawercer.de/~marc/vim-addon-async-python-repl.jpg Marc Weber

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Re: [Haskell-cafe] [Agda] Defining subtraction for naturals
by wren ng thornton 18 Mar '11

18 Mar '11

On 3/17/11 4:22 PM, Martin Escardo wrote: > On 17/03/11 18:35, wren ng thornton wrote: >> (2) Use saturating subtraction, i.e. if the result would drop below zero >> then return zero; > > This is what people working with quantales do. > > Subtraction y-z, when it exists, is the solution in s to the equation > s+z =y. > > Truncated subtraction y - z is the supremum of the set of solutions s to > the inequality s+z <= y, when this supremum exists. (In your example, > when z>y, the set of solutions is empty, and the empty set has supremum > zero.) > > This amounts to saying that the truncated subtraction function (-) - z > is the right adjoint to the addition function (-) + z. > > So (2) is very natural. Thanks! That's exactly the kind of thing I was looking for. I'd been needlessly thinking of it as a hack :) -- Live well, ~wren

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X11 package bug: XClientMessageEvent long data
by Dylan Alex Simon 18 Mar '11

18 Mar '11

Does anyone know the current maintenance status of the X11 package? I emailed Spencer Janssen a number of months ago and never heard back. So, I'll put this here in case any one else runs into it or can get it to the right place. This is a proposed bug fix for a problem I ran into using xmonad client messages to send remote commands on 64LP architectures (i.e., amd64), wherein the C X11 library and Haskell's disagree about the size of client message arguments. Tue Nov 16 23:41:49 EST 2010 Dylan Simon <dylan(a)dylex.net> * change XClientMessageEvent long data The XClientMessageEvent.data.l field is actually a long, not an int, so it must be interpreted as such, even though format is set to 32 in this case. Ostensibly this is an Xlib bug, but it is unlikely to be fixed there. diff -rN -u old-src/Graphics/X11/Xlib/Extras.hsc new-src/Graphics/X11/Xlib/Extras.hsc --- old-src/Graphics/X11/Xlib/Extras.hsc 2011-03-15 22:40:39.687844812 -0400 +++ new-src/Graphics/X11/Xlib/Extras.hsc 2011-03-15 22:40:39.724522814 -0400 @@ -601,7 +601,7 @@ 16 -> do a <- peekArray 10 datPtr return $ map fromIntegral (a::[Word16]) 32 -> do a <- peekArray 5 datPtr - return $ map fromIntegral (a::[Word32]) + return $ map fromIntegral (a::[CLong]) _ -> error "X11.Extras.clientMessage: illegal value" return $ ClientMessageEvent { ev_event_type = type_

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How large is the Haskell community ?
by Aaron Gray 18 Mar '11

18 Mar '11

I was wondering if anyone had an idea or estimate as to how large the Haskell community is ? Aaron

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Haskell report minutiae
by wren ng thornton 18 Mar '11

18 Mar '11

So I have a rules-lawyering question about how specified the overflow behavior is for the types Int and Word. * In the Haskell 2010 report it seems to be specified that (sections 18.1, 23.1): "All arithmetic is performed modulo 2^n, where n is the number of bits in the type." And this seems to apply to Int and Word as well as Int8, Word8, Int16, Word16, Int32, Word32, Int64, Word64. * However, in the Haskell 98 report ---and this verbiage is retained in the same section of H2010--- there is no such guarantee (section 6.4): "The results of exceptional conditions (such as overflow or underflow) on the fixed-precision numeric types are undefined; an implementation may choose error (_|_, semantically), a truncated value, or a special value such as infinity, indefinite, etc." * However, the FFI addendum to H98 does specify modulo behavior for the Int8, Word8, Int16, Word16, Int32, Word32, Int64, Word64 types (section 5.3): "There is, however, the additional constraint that all arithmetic on the fixed-sized types is performed modulo 2^n." But it is unclear whether this also applies to Int and Word. So when it comes to interpreting the report(s) and ensuring proper implementation, which is it? Are Int and Word arithmetic modular, or not? Did the specification change between H98 and H98+FFI or between H98+FFI and H2010? -- Live well, ~wren

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Weird warnings from recent GHC snapshot
by Michael Vanier 18 Mar '11

18 Mar '11

I run haskell on Mac OS X (Snow Leopard). After upgrading my Xcode installation to 4.0 I had a tricky time getting ghc working again; the version bundled with the Haskell Platform no longer works and I had to compile a recent snapshot (ghc-7.1.20110315) from source. This worked fine, but now when I compile my code I keep getting really weird warnings. For instance: ld: warning: could not create compact unwind for .LFB3: non-standard register 5 being saved in prolog and SpecConstr Function `$w$j{v s3hL} [lid]' has one call pattern, but the limit is 0 Use -fspec-constr-count=n to set the bound Use -dppr-debug to see specialisations SpecConstr Function `$w$j{v s3ic} [lid]' has two call patterns, but the limit is 1 Use -fspec-constr-count=n to set the bound Use -dppr-debug to see specialisations The code seems to work, but this is quite irritating. Is there anything that can be done about these warnings? Thanks, Mike

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Lazy evaluation and tail-recursion
by Yves Parès 17 Mar '11

17 Mar '11

Hello, A question recently popped into my mind: does lazy evaluation reduce the need to "proper" tail-recursion? I mean, for instance : fmap f [] = [] fmap f (x:xs) = f x : fmap f xs Here fmap is not tail-recursive, but thanks to the fact that operator (:) is lazy, I think that it may still run in constant space/time, am I right?

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Data.Time.Calendar.Day does not seem to have an instance for Read
by C K Kashyap 17 Mar '11

17 Mar '11

Hi, I was wondering if this is a defect - Prelude> import Data.Time.Calendar Prelude Data.Time.Calendar> read "2011-10-10" :: Day <interactive>:1:1: No instance for (Read Day) arising from a use of `read' Possible fix: add an instance declaration for (Read Day) In the expression: read "2011-10-10" :: Day In an equation for `it': it = read "2011-10-10" :: Day Prelude Data.Time.Calendar> The documentation says that Day has an instance for Read. Regards, Kashyap

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Data.ByteString.Lazy.ByteString vs Data.ByteString.Lazy.Internal.ByteString
by C K Kashyap 17 Mar '11

17 Mar '11

Hi, I noticed that even though I declare the type of a function in my code as Data.ByteString.Lazy.ByteString ... when I check it out in ghci using :t, it shows this - Data.ByteString.Lazy.Internal.ByteString Is this expected? Regards, Kashyap

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