November 2010 - Haskell-Cafe

Wondering if this could be done.
by Magicloud Magiclouds 04 Dec '10

04 Dec '10

Hi, For example, I have a data A defined. Then I want to add (+) and (-) operators to it, as a sugar (compared to addA/minusA). But * or other stuff defined in class Num is meanless to A. So I just do: (+) :: A -> A -> A (+) a b = A (elem1 a + elem1 b) (elem2 a + elem2 b) -- I got errors here, for the (+) is ambiguous. So, just wondering, does this way work in Haskell? -- 竹密岂妨流水过山高哪阻野云飞

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regex libraries: matching operators (=~) and (=~~) are not methods
by José Romildo Malaquias 04 Dec '10

04 Dec '10

Hello. When learning how to use the many regular expression libraries for Haskell, I noticed that the interface API from the regex-base package introduces several high level operations that are abstracted from the implementations (backends). This is done by means of classes. For instance, there are methods for compiling an external representation (like a string or a bytestring) into a regular expression. There are also methods for matching a regular expression and some text (like a string, or a bytestring). But the most high level functions for pattern matching using regular expressions, (=~) and (=~~), are not defined as methods in a class. They are independent functions defined for each backend in the corresponding package. This prevents one from writing a general function using these operators without deliberately choosing a regex backend. Why are those operators not defined as methods, like all other relevant functions? Romildo

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In what language...?
by Andrew Coppin 03 Dec '10

03 Dec '10

Yesterday I read a rather interesting paper: http://www.cl.cam.ac.uk/~mb566/papers/tacc-hs09.pdf It's fascinating stuff, and I *think* I understand the gist of what it's saying. However, the paper is utterly festooned with formulas that look so absurdly over-the-top that they might almost be a spoof of a mathematical formula rather than the real thing. A tiny fraction of the notation is explained in the paper, but the rest is simply "taken to be obvious". The paper also uses several ordinary English words in a way that suggests that they are supposed to have a more specific technical meaning - but I have no idea what. Does anybody have any idea which particular dialect of pure math this paper is speaking? (And where I can go read about it...)

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Manatee Video.
by Andy Stewart 03 Dec '10

03 Dec '10

Hi all, Many people ask "What's Manatee?" A video worth a thousand words : here is video (select 720p HD) http://www.youtube.com/watch?v=weS6zys3U8k And i think the correct answer to "What's Manatee?" should be : Depend on you how to use it. :) Other information look : http://hackage.haskell.org/package/manatee Enjoy! -- Andy

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How to cabal fetch mtl==2.0.0.0?
by Iain Alexander 02 Dec '10

02 Dec '10

<log> F:\Util\Haskell>cabal fetch mtl==2.0.0.0 Resolving dependencies... cabal: internal error: could not construct a valid install plan. The proposed (invalid) plan contained the following problems: The following packages are involved in a dependency cycle transformers-0.2.2.0, special-functors-1.0, mtl-2.0.0.0 F:\Util\Haskell>cabal -V cabal-install version 0.8.2 using version 1.8.0.2 of the Cabal library </log> Could someone please remind me of the correct incantation to make this work. (I observe that transformers-0.1.4.0 optionally (by cabal flag) depends on special-functors, and special-functors-1.0 depends on mtl. So given that mtl-2 no doubt depends on transformers, I can see the potential for confusion, but there presumably must be a way of breaking the (apparent) loop.) TIA, Iain. -- Iain Alexander ia(a)stryx.demon.co.uk

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Fwd: typehash patch for base >= 4 && < 4.4
by Andrew Miller 02 Dec '10

02 Dec '10

Hi all, Typehash 1.4.0.0, the latest version of typehash in hackage, has a cabal constraint base < 4, making it difficult to use with packages that only support newer base versions. I sent a patch to fix this to the maintainer for the typehash package (Lennart Augustsson) on the 16th of November, but haven't heard anything back - it is possible that he doesn't read the e-mail address I sent it to, or is no longer interested in maintaining typehash. Does anyone who would be willing to know Lennart in person to confirm this? Failing this, does anyone want to be maintainer for the typehash package? If I don't hear any better ideas, I could simply upload a version 1.4.0.1 with my split base patch applied, and the maintainer changed to "haskell-cafe(a)haskell.org". Best wishes, Andrew -------- Original Message -------- Subject: typehash patch for base >= 4 && < 4.4 Date: Tue, 16 Nov 2010 17:55:13 +1300 From: Andrew Miller <andrew(a)amxl.com> To: lennart(a)augustsson.net diff -rbud typehash-1.4.0.0/src/Data/TypeHash.hs typehash-1.4.0.1/src/Data/TypeHash.hs --- typehash-1.4.0.0/src/Data/TypeHash.hs 2009-10-06 03:43:44.000000000 +1300 +++ typehash-1.4.0.1/src/Data/TypeHash.hs 2010-11-16 17:43:51.917333177 +1300 @@ -44,7 +44,7 @@ data Type = Name { typeName :: String } -- Abstract type, or recursive reference - | Data { typeName :: String, constrs :: [Constructor] } + | Data { typeName :: String, dconstrs :: [Constructor] } deriving (Eq, Ord, Show, Typeable, Data) type Constructor = (String, [Field]) type Field = (String, Type) -- a unique number is used for missing field names @@ -54,7 +54,7 @@ let tn = show $ fullTypeOf x in case dataTypeRep $ dataTypeOf x of AlgRep cs | tn `notElem` tns -> - Data { typeName = tn, constrs = map (gConstr (tn:tns) x) cs } + Data { typeName = tn, dconstrs = map (gConstr (tn:tns) x) cs } _ -> Name { typeName = tn } -- Use type name for truly abstract types and recursive types. gConstr :: (Data a) => [String] -> a -> Constr -> (String, [Field]) Only in typehash-1.4.0.1/src/Data: TypeHash.hs~ diff -rbud typehash-1.4.0.0/typehash.cabal typehash-1.4.0.1/typehash.cabal --- typehash-1.4.0.0/typehash.cabal 2009-10-06 03:43:44.000000000 +1300 +++ typehash-1.4.0.1/typehash.cabal 2010-11-16 17:40:55.496953710 +1300 @@ -1,5 +1,6 @@ Name: typehash -Version: 1.4.0.0 +Cabal-Version: >= 1.2 +Version: 1.4.0.1 License: BSD3 Author: Lennart Augustsson Maintainer: Lennart Augustsson @@ -10,6 +11,18 @@ Description: Produce a hash for a type that is unique for that type. The hash takes both actual type names and type structure into account. This is useful for checking the type of persisted values. -Hs-Source-Dirs: src -Build-Depends: base < 4, mtl, bytestring, pureMD5, binary -Exposed-modules: Data.TypeHash + +Flag base4 + Description: Use base-4.* + Default: True + +Library + Hs-Source-Dirs: src + Build-Depends: mtl, bytestring, pureMD5, binary + If flag(base4) + Build-Depends: + base >= 4 && < 4.4, + syb < 0.3 + else + Build-Depends: base < 4 + Exposed-modules: Data.TypeHash

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doesDirectoryExist is always returning False
by Marcelo Sousa 02 Dec '10

02 Dec '10

Hi, I'm having currently a problem with System.Directory in my mac os. System Version: Mac OS X 10.6.5 Kernel Version: Darwin 10.5.0 Prelude System.Directory> let dirTest = do {dir <- getCurrentDirectory; doesDirectoryExist dir} Prelude System.Directory> dirTest False I noticed also that I can't change the searchable field in the permissions record. Prelude System.Directory> getPermissions "." Permissions {readable = True, writable = True, executable = True, searchable = False} I tried different versions of directory and filepath packages but with no success at the moment. Any suggestions?! Regards, Marcelo Sousa

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Unable to install packages
by Roland Senn 01 Dec '10

01 Dec '10

I learned a little bit Haskell with Hugs on Windows and now I want to develop on Linux. I installed the Haskell Platform on Unbunto 10.10. There were no problems. Then I tried to install Leksah with sudo cabal install leksah --global It failed with: Resolving dependencies... /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/Gtk2HsSetup.hs:25:0: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/Gtk2HsSetup.hs, /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/dist/setup/Gtk2HsSetup.o ) [2 of 2] Compiling Main ( /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/Setup.hs, /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/dist/setup/Main.o ) Linking /tmp/gtksourceview2-0.11.12661/gtksourceview2-0.11.1/dist/setup/setup ... Configuring gtksourceview2-0.11.1... Preprocessing library gtksourceview2-0.11.1... Building gtksourceview2-0.11.1... [ 1 of 13] Compiling Graphics.UI.Gtk.SourceView.Signals ( dist/build/Graphics/UI/Gtk/SourceView/Signals.hs, dist/build/Graphics/UI/Gtk/SourceView/Signals.o ) Graphics/UI/Gtk/SourceView/Signals.chs:51:46: Module `System.Glib.UTFString' does not export `maybePeekUTFString' cabal: Error: some packages failed to install: gtksourceview2-0.11.1 failed during the building phase. The exception was: ExitFailure 1 leksah-0.8.0.8 depends on gtksourceview2-0.11.1 which failed to install. I searched the Internet on "gtksourceview2 maybePeekUTFString" and found a patch: http://www.mail-archive.com/gtk2hs-devel@lists.sourceforge.net/msg00971/gtk… As a newby with the Haskell tools, I searched the Internet on how to install a patch. I found dpm and I tried to install it with: sudo cabal install dpm --global It failed with: configure: error: in `/tmp/terminfo-0.3.1.31936/terminfo-0.3.1.3': configure: error: curses headers could not be found, so this package cannot be built See `config.log' for more details. cabal: Error: some packages failed to install: DPM-0.2.1 depends on terminfo-0.3.1.3 which failed to install. darcs-2.5 depends on terminfo-0.3.1.3 which failed to install. haskeline-0.6.3.2 depends on terminfo-0.3.1.3 which failed to install. terminfo-0.3.1.3 failed during the configure step. The exception was: ExitFailure 1 So after installing Haskell Platform, I only found broken packages! What can I do to install Leksah?

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ANN: HaLVM 1.0: the Haskell Lightweight Virtual Machine
by Adam Wick 01 Dec '10

01 Dec '10

Galois, Inc. is pleased to announce the immediate release of the Haskell Lightweight Virtual Machine (or HaLVM), version 1.0. The HaLVM is a port of the GHC runtime system to the Xen hypervisor, allowing programmers to create Haskell programs that run directly on Xen's "bare metal." Internally, Galois has used this system in several projects with much success, and we hope y'all will have an equally great time with it. What might you do with a HaLVM? Pretty much anything you want. :) Explore designs for operating system decomposition, examine new notions of mobile computation with the HaLVM and Xen migration, or find interesting network services and lock them inside small, cheap, single-purpose VMs. The HaLVM is the result of many years of effort, by many people inside Galois. Although it is not yet totally bug-free, we have decided that broad adoption wins over perfection and thus we are releasing it for general review. As such, there will be some rough edges, and we urge you to read the documentation to understand the platforms we test on. We are releasing the HaLVM under a non-restrictive BSD3 license. You can find it here: http://halvm.org We welcome user feedback, feature requests, bug notices, patches, and feature additions; see the page above for guidelines on getting involved. Finally, we'd like to give many things to the GHC and Xen communities, without which this work would not be possible. If you have any questions or concerns, please don't hesitate to contact the HaLVM's maintainers at halvm-devel(a)community.galois.com. Have a lovely day! - Adam

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Test
by Ian Lynagh 30 Nov '10

30 Nov '10

A1

2 1