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Motivation for having indexed access in Data.Map?
by Christoph Breitkopf 07 Jan '12

07 Jan '12

Hello, I wonder why Data.Map provides the indexed access functions: http://hackage.haskell.org/packages/archive/containers/latest/doc/html/Data… These functions seem rather out-of-place to me in the map api. The only use case I could think of so far would be to find the median, or in general n-th smallest key, but that does not seem sufficient reason (also, I think there are faster methods for that). Anything else? Regards, Chris

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Problems with libgmp on Amazon Linux
by Nicu Ionita 07 Jan '12

07 Jan '12

Hello, I saw [1] there is a lot of activity in the Haskell community regarding Amazon Web Services (AWS). As they allow an easy start [2], I decided to giv it a try and followed the procedure published by JP Moresmau [3] to install the Haskell Plattform on Amazon Linux (a minimal linux distribution maintained by Amazon), but with the new Haskell Platform, 2011.4.0.0 (and ghc 7.0.4). It seemed to work pretty well, with some problems when configuring HP, because although libgmp was already installed, it was not recognized. I installed the newer version 5.0.2 (from sources) [4], did some tricks (cp /usr/lib/lib/libgmp* /usr/lib), then configure worked and HP was built and installed. But then later I wanted to install the aws package (cabal install aws) and then the vector package could not be installed - again because of libgmp! Here are the last messages, followed by a ls of /usr/lib, where it's clear that the libraries are there: [ 4 of 30] Compiling Data.Vector.Internal.Check ( Data/Vector/Internal/Check.hs, dist/build/Data/Vector/Internal/Check.o ) [ 5 of 30] Compiling Data.Vector.Fusion.Stream.Monadic ( Data/Vector/Fusion/Stream/Monadic.hs, dist/build/Data/Vector/Fusion/Stream/Monadic.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... <command line>: can't load .so/.DLL for: gmp (libgmp.so: cannot open shared object file: No such file or directory) cabal: Error: some packages failed to install: vector-0.9.1 failed during the building phase. The exception was: ExitFailure 1 [ec2-user@ip-10-234-49-92 ~]$ ls -l /usr/lib/libgmp* -rw-r--r-- 1 root root 1175336 Jan 4 22:21 /usr/lib/libgmp.a -rwxr-xr-x 1 root root 915 Jan 4 22:21 /usr/lib/libgmp.la -rwxr-xr-x 1 root root 484673 Jan 4 22:21 /usr/lib/libgmp.so -rwxr-xr-x 1 root root 484673 Jan 4 22:21 /usr/lib/libgmp.so.10 -rwxr-xr-x 1 root root 484673 Jan 4 22:21 /usr/lib/libgmp.so.10.0.2 [ec2-user@ip-10-234-49-92 ~]$ I'm just asking here hoping that someone had already solved this problem. Or perhaps someone can explain how cabal ensures that a library is there. The error message - as it is - is meaningless, as the file exists and has read permissions for everybody. Thank you, Nicu [1] http://www.reddit.com/r/haskell/comments/o1u2g/deploying_a_haskell_app_to_a… [2] https://aws.amazon.com/free/ [3] http://jpmoresmau.blogspot.com/2011/04/install-ghc7-and-yesod-on-amazon-lin… [4] ftp://ftp.gnu.org/gnu/gmp/

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Package for QuickCheck instances
by Antoine Latter 07 Jan '12

07 Jan '12

Hi Haskell, I was writing some tests that involved a large number of quickcheck properties which don't ship with the library itself, so I thought I would package them all together and put the orphan instances on Hackage. Here's what I have so far: https://github.com/aslatter/qc-instances/tree/93a87fa06b5575ddcc12c2f9eb0f4… The policy would be to allow anything into to package which is a part of the Haskell Platform or is a GHC build library. Has anyone already done this? I didn't find anything from grepping the Hackage package list. Has anyone already talked about doing this sort of thing? Are their any non-obvious pitfalls (aside from the package eventually becoming a cesspool of CPP conditions)? Thanks, Antoine

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Re: [Haskell-cafe] Experiments in Haskell Packaging
by Chris Dornan 07 Jan '12

07 Jan '12

'Antoine Latter' <aslatter(a)gmail.com>: > All of this is in my head, but assuming I already had some sort of > Linux build-server set up, it would be nice to combine it with your > work to make it easier to have the build-server run tests against > multiple versions of GHC/HP. That's all I was getting at. Working within the Hub framework this is straightforward. If you are using public releases rather than special builds of the tools then it should be quite easy. Supposing you needed to test with Haskell Platform 2011.2.0.1 and hexpat-0.19.7: # install the platform (probably done already) sudo yum install haskell-platform-2011.2.0.1-hub # create new hub 'test-2011.2.0.1' based this HP hub init 2011.2.0.1 test-2011.2.0.1 # install hexpat-0.19.7 into it hub install-into test-2011.2.0.1 hexpat-2011.2.0.1 Repeat as necessary to create your array of test environments. Now you just have to arrange select the appropriate hub for each test. Suppose the environment variable $testhubs contains all of your test hubs ('test-2011.2.0.1', etc.), then the following shell command would prime the tools to successively use each of the test environments on each invocation of make. for h in $testhubs; do HUB=$h make; done Note that the justhub distribution isn't necessary for this -- you can do it manually. But it does make it easier. Chris

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Re: [Haskell-cafe] Solved but strange error in type inference
by Yves Parès 06 Jan '12

06 Jan '12

2012/1/6 AUGER Cédric <sedrikov(a)gmail.com> > when you write "forall a. exists b. a -> b -> a", then you allow the > caller to have access to b to produce a (didn't you write "a->b->a"?) > Yes, sorry, I always assumed independence between the type variables. Like in: f :: forall a. a -> (forall b. b -> a) being the same than: f :: forall a b. a -> b -> a I should have specified: "*if* a doesn't depend on b in the latter." Of course the latter allows that, whereas the first does not (since its what prevents STRefs from escaping runST, by forbidding the return type of runST to depend on the phantom type 's' of the ST action).

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Re: [Haskell-cafe] Simple type-class experiment turns out not so simple...
by "Philip K. F. Hölzenspies" 06 Jan '12

06 Jan '12

Dear Steve, et al., On 6 Jan 2012, at 11:00, <haskell-cafe-request(a)haskell.org> <haskell-cafe-request(a)haskell.org> wrote: > From: Steve Horne <sh006d3592(a)blueyonder.co.uk> > Date: 6 January 2012 10:51:58 GMT > To: Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de> > Cc: Haskell Cafe Mailing List <haskell-cafe(a)haskell.org> > Subject: Re: [Haskell-cafe] Simple type-class experiment turns out not so simple... > > > On 06/01/2012 10:29, Steffen Schuldenzucker wrote: >> With 'd' not being mentioned anywhere, the signature of wbtData means "forall d. n -> Maybe d". In particular, wbtData == const Nothing. >> > I'm not sure what to make of that. Even if the result of wbtData is always Nothing, surely it still has a static type? I think what Steffen was saying here is that the only implementation of wbtData that satisfies the general type "forall d. n -> Maybe d" is "const Nothing" which has precisely that static type (the forall doesn't make it a dynamic type). > Precisely that. In that case, I get... > > C:\_SVN\dev_trunk\haskell\examples>ghci -XMultiParamTypeClasses > GHCi, version 7.0.4: http://www.haskell.org/ghc/ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer-gmp ... linking ... done. > Loading package base ... linking ... done. > Loading package ffi-1.0 ... linking ... done. > Prelude> :load BinTree > [1 of 1] Compiling BinTree ( BinTree.hs, interpreted ) > > BinTree.hs:12:12: > Illegal instance declaration for `WalkableBinTree (BT x) x' > (All instance types must be of the form (T a1 ... an) > where a1 ... an are *distinct type variables*, > and each type variable appears at most once in the instance head. > Use -XFlexibleInstances if you want to disable this.) > In the instance declaration for `WalkableBinTree (BT x) x' > Failed, modules loaded: none. > Prelude> > > If I specify both extensions (-XMultiParamTypeClasses and -XFlexibleInstances) it seems to work, but needing two language extensions is a pretty strong hint that I'm doing it the wrong way. This isn't a general truth, but if you're still in early stages of learning about GHCs type-class implementation, you're probably right. > The goal is fairly obvious - to have type-classes for binary tree capabilities so that different implementations can support different subsets of those capabilities. Being able to walk a binary tree doesn't need ordering of keys, whereas searching does. A red-black tree needs somewhere to store it's colour in the node, yet the walking and searching functions don't need to know about that. The problem with the "forall d. n -> Maybe d" type is that it makes d too general (universal, in fact), while d is fixed by n. There is an alternative extension you may want to look into for your furtherance of the capabilities of GHC, being TypeFamilies. These would lead to the following alternative implementation: {-# LANGUAGE TypeFamilies #-} class WalkableBinTree n where type ElemTp n wbtChildren :: n -> Maybe (n, n) wbtData :: n -> Maybe (ElemTp n) instance WalkableBinTree (BT x) where type ElemTp (BT x) = x wbtChildren (Branch d l r) = Just (l, r) wbtChildren Empty = Nothing wbtData (Branch d l r) = Just d wbtData Empty = Nothing This says that the instance fixes the "element type" ElemTp to something specific. However, it seems perfectly reasonable for the goal you describe to demand that whatever tree-type this class is instantiated for must be parametric in its element type. This can be done without any language extension, i.e.: class WalkableBinTree n where wbtChildren :: n d -> Maybe (n d, n d) wbtData :: n d -> Maybe d instance WalkableBinTree BT where wbtChildren (Branch d l r) = Just (l, r) wbtChildren Empty = Nothing wbtData (Branch d l r) = Just d wbtData Empty = Nothing Notice that now, the class is instantiated for "BT" and *not* "(BT x)". > As far as I remember, none of the tutorials I've read have done this kind of thing - but it seemed an obvious thing to do. Obviously in the real world I should just use library containers, but this is about learning Haskell better in case a similar problem arises that isn't about binary trees. A good way to learn a lot about type classes is to study the default library. Admittedly, this can be a bit of a daunting task to just dive in. Luckily, Brent Yorgey wrote up a nice article that was transferred and updated by numerous people on the HaskellWiki. It's worth giving it a read: http://www.haskell.org/haskellwiki/Typeclassopedia Happy Haskelling! Regards, Philip

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How to split this string.
by max 06 Jan '12

06 Jan '12

I want to write a function whose behavior is as follows: foo "string1\nstring2\r\nstring3\nstring4" = ["string1", "string2\r\nstring3", "string4"] Note the sequence "\r\n", which is ignored. How can I do this?

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haxr standalone server?
by Johannes Waldmann 06 Jan '12

06 Jan '12

How could I use haxr (http://www.haskell.org/haskellwiki/HaXR) to build a stateful server? It should listen on some port, and fork threads (inside Haskell land) to handle incoming calls. Any of the Haskell web frameworks can do this? I guess this is the same question as: http://www.haskell.org/pipermail/haskell-cafe/2009-December/071185.html Thanks - J.W.

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ghc default options
by Victor Miller 06 Jan '12

06 Jan '12

Is there a place where I can specify default options for ghc? I just installed ghc 7.0.4 on my Redhat system. Since gmp isn't available on our system libraries I installed gmp in the lib file in my home directory. I've set LIB_PATH and LD_LIBRARY_PATH to point to it. ghci works ok (it finds the dynamic link library in my directory), but unless I add -L$HOME/lib as an option to ghc --make it can't find it. So how can I tell ghc to look in that directory by default? Because of this I can't install the haskell platform. Victor PS. I spent a while looking at the ghc manual but couldn't find anything appropriate.

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Monadic computation must now do IO, fails when type changed to IO.
by Matthew Farkas-Dyck 06 Jan '12

06 Jan '12

Dear fellow Haskellers, I have written an SMTP server (Main.hs at http://hpaste.org/56134, full git repo at http://strake.zanity.net:1104/ymail.git) which works, mostly – it responds to every message with that it was sent properly, whether it was or not. (Try it: send an e-mail by SMTP to this_is_not_a_user(a)strake.zanity.net or somesuch.) The reason is, that it simply tells the message (envelope, data) in a Writer monad, which is later piped to the local delivery agent. As I said, this works, but of course is suboptimal. I tried to change the type of main_ to (MonadState MTPState m, MonadIO m) => [Char] -> m ([Char], [Char]), just calling LDA in main_ (when mode is MTPTextMode) rather than after, but then the program output nil. I'm not sure what the problem is – I thought IO might not be lazy enough so I tried a lazy wrapped-IO monad, in vain. How, then, can this program call the LDA, wait to know whether it worked or failed, and then respond? Thanks for any help. Cheers, M Farkas-Dyck

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