[Haskell-cafe] Upgrading GHC 7.6.3 global packages to latest versions
Hi, I am trying to install the latest version of some hackage packages I use often in GHC 7.6.3. However, I am having some problems with packages like [containers] and [time], which are shipped with GHC as global packages. Now when I try to install the latest version of the [time] package, I have 2 time packages, 1 in the global package db (the older version), and 1 in the user package db (the newer version). Now I would like subsequent installed packages to always use the latest version of the [time] package. But if i try to install the [plugins] package, cabal says: #In order, the following would be installed: #ghc-paths-0.1.0.9 (new package) #random-1.0.1.1 (reinstall) changes: time-1.4.1 - 1.4.0.1 #plugins-1.5.3.0 (new package) So I guess I have 2 options here: 1. unregister all the global packages so that I can install the latest versions of all the global packages. 2. force cabal to only look at latest packages in user package db (not sure if this is possible). Can anyone please point me in the right direction here. I have read about [cabal-dev], but I am not sure if it can be used to solve my problem. Regards --Rouan___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Why does Enum succ and pred functions throw exception
Hi everyone, Can anyone shed some light on why the succ and pred functions of the Enum typeclass throw exceptions if we go over the upper or lower boundary, and not return Maybe a? I was hoping to have some functions like: safeSucc :: (Enum a) = a - Maybe a Because the succ and pred functions throw exceptions I can only catch them in the IO monad. This makes it hard to deal with this situation in pure code. Regards Rouan.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] struggling with blocking shell cmd when using System.Process
Hi Everyone,
I am strugling to get the following code to work correctly and I'm hoping
someone will be able to shed some light as to why the code is behaving
as it does:
{-| runs the supplied command and args as a SHELL command -}
runShell :: String - IO (String, ExitCode)
runShell shellCmd = do
(_, Just hOut, Just hErr, hProcess) - createProcess (shell shellCmd) {
std_out = CreatePipe, std_err = CreatePipe }
exitCode - waitForProcess hProcess
outTxt - hGetContents hOut
errTxt - hGetContents hErr
return $ (outTxt ++ errTxt, exitCode)
If I try to su the runShell function with the following command, it simply
blocks forever:
git --no-pager blame your git file here
but if I do:
git status
The runShell function works as expected. Can someone shed some light on why
the 'git blame' command blocks? Interrestingly enough, if I remove the
waitForProcess action, the 'git blame' command starts working as expected, but
I no longer have my blocking behaviour, waiting for the command to finish and
return the ExitCode.
Regards
Rouan
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[Haskell-cafe] struggling with blocking shell cmd when using System.Process - part 2
Hi Everyone, I forgot to say, I am running GHC 7.2.2 on Windows7 x64. Regards.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] is the evaluation order deterministic when using applicative with IO
Hi everyone.
I was wondering if I can make assumptions about the evaluation order of
the following code:
isTrue :: Int - IO Bool
isTrue val = pure (||) * boolTest1 val * boolTest2 val
{- boolTest1 is an inexpensive, quick check -}
boolTest1 :: Int - IO Bool
boolTest1 val = undefined
{- boolTest2 is a very expensive check -}
boolTest2 :: Int - IO Bool
boolTest2 val = undefined
When using Applicative in the isTrue function, I would like to make use of
the short-circuit behaviour of || and rely on the fact that the boolTest1
will be executed first. The reason I am asking is because the boolTest
functions
are in the IO monad, instead of just returning pure Bool values.
Regards
Rouan.
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[Haskell-cafe] Idiomatic error handling in Haskell
There are quite a few exception handling mechanisms provided by Haskell, as can be seen from this post: http://www.randomhacks.net/articles/2007/03/10/haskell-8-ways-to-report-errors I would like to know what is the preferred Haskell mechanism for handling exceptions in the IO monad? I am not concerned with mechanisms such as Maybe / Either, but would like to know about exception mechanisms inside the IO monad. The 2 I know of are: o) throwDyn o) ioError and catch I do need the exceptions to be extendable. So which is the preferred way to handle exceptions in Haskell for new libs? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Tree Semantics and efficiency
Hi everyone. I would like to confirm the following semantics. I want a tree which can be traversed both downwards and upwards. To do this i need to store the following for each node: o) a reference to the parent node (so we can traverse up the tree). This must be a reference as i dont want to copy the parent node each time) o) a list of child nodes that belong to this node. It is important to store only a reference to the parent and not a copy of the entire parent for efficiency. How would one write this in Haskell so that it has the above mentioned semantics. Or does GHC do that automatically for me so I don't need to do it explicitly? I am thinking of writing something like this (naive implementation) : Tree = TreeNode String Tree [Tree] -- TreeNode data ParentReference ChildNodes OR (implementation using IORef for parentNode reference) Tree = TreeNode String (IORef Tree) [Tree] -- TreeNode data ParentReference ChildNodes Thanks in advance Rouan. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Writing a compiler in Hakell
Hi everyone. I am designing my own programming language. I would like to know what is the best way to go about writing my compiler in haskell. What are the tools available in haskell that can help with compiler construction? I know about Happy. Is that a good tool to use? The compiler is intended for serious use and I would like it to be very efficient, maybe competing with compilers written in C. It should also be very easy to extend as the languoge grows. Are there any good books that you can recommend on compiler construction in general and specific to haskell? On another note, how is the operator + implemented in haskell? is there a primitve (say #+) that is wrapped by the haskell operator +? Maybe something like: (+) :: a - a - a v1 + v2 = #+ v1 v2 Thanks in advance Rouan. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Writing a compiler in Hakell (continued 1)
Hi everyone. Thanks for the speedy replies. Let me elaborate on my language and situation. The language I have in mind is a statically typed, hybrid (OOP + functional), strict language. It contains both functional and imperitive features (much the same as OCaml/F#) with ideas from haskell, scheme, smalltalk, mozart/oz, F#, C#. The syntax is kept as succint as possible, adding things that I think would be useful that are not defined in any of the inspirational languages. As for the target language, im not quite sure yet. I am doing a lot of work in .NET/C# at the moment, but I would eventually like to use my own programming language, instead of C#. I would also like to use my language for linux programming, so I will eventually support both Windows and Linux environments. My aim with this project is to further explore haskell and deepen my understanding of it, while at the same time creating something useful where I can explore my own ideas about programmnig languages and concepts. I don't really intend to write the next programming language used by billions of people. If others find it useful, that's awsome. I definitely intend to use my language for my own projects and to make it freely availble on the net. So it is not just for learning or fun, it is more focused on implementing a decent programming language that can be used for commercial applications (and fun). The compiler for my language should have a decent compile time, comparing to compilers written in C. The approach that I have in mind is to piggy back on existing languages/compilers until their implementation is no longer adequite or I have time to write my own code generator. So the first stage would be something like translate to C# and then use the C# compiler to obtain an executable. I want the first steps to go as quick as possible so that I can refine the language and test ideas quickly, and once the language has stabalized, concentrarte on my own full fledged compiler (if it is necessary). I think I will try Parsec as a starting point for creating a parser. Thanks again everyone. Regards Rouan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] daemon process
Hi Everyone. I have been learning Haskell the past few months and there are tons of things I still don't understand :) I would like to write a very very simple haskell daemon that disconnects itself from the terminal and sists in the background doing something. What is the easiest way to go about this. I have tried several things but can't seem to get the process disconnected from the terminal. Thanks in advance Rouan Send instant messages to your online friends http://uk.messenger.yahoo.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
