November 2009 - Haskell-Cafe

Simple hash table creation
by michael rice 17 Nov '09

17 Nov '09

I'm trying to create a hash table. Yeah, I know, don't use hash tables, but I need to create something I'm familiar with, not something I've never worked with before. What's wrong with this code? Michael ==================== import Prelude hiding (lookup) import Data.HashTable data MyHashTable = HashTable String Int dummy:: String -> Int dummy s = 7 ht = MyHashTable.new (==) dummy ==================== [michael@localhost ~]$ ghci hash1 GHCi, version 6.10.3: 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 ( hash1.hs, interpreted ) hash1.hs:9:5: Not in scope: `MyHashTable.new' Failed, modules loaded: none. Prelude>

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ghci + user prelude
by Sean McLaughlin 17 Nov '09

17 Nov '09

Hello, If there's a file called Prelude.hs in a directory, and ghci is started from that directory, ghci dies. -- Prelude.hs module A.Prelude where $ ghci GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. <command line>: module `Prelude' is not loaded Is there a way around this? I often like to have a modified Prelude file in a subdirectory of my project, and this behavior keeps me from being able to start ghci there. Thanks! Sean

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Little errors in number calculations
by Abby Henríquez Tejera 17 Nov '09

17 Nov '09

Hi. I've seen that in GHC sometimes there are little errors in some basic number calculations: *Prelude> 123.35503 * 10.0 1233.5502999999999 *Prelude> properFraction 123.35503 (123,0.3550299999999993) whereas in Hugs no such errors seem to occur (that I have found, at least): *Hugs> 123.35503 * 10.0 1233.5503 (but:) *Hugs> properFraction 123.35503 (123,0.355029999999999) I understand that error may (and will) happen in floating point, but it surprises me that they do so easily, and, above all, the difference between GHC and Hugs. Does someone know why does this difference occur? (Thanks in advance, by the way :) ).

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FW: [Haskell-cafe] Status of TypeDirectedNameResolution proposal?
by Simon Peyton-Jones 17 Nov '09

17 Nov '09

[Resending, this time with link; sorry] | What's the status of the TDNR proposal [1]? Personally I think it is a | very good idea and I'd like to see it in Haskell'/GHC rather sooner | than later. Working around the limitations of the current record | system is one of my biggest pain points in Haskell and TDNR would be a | major improvement. Thus I wonder if someone is actively working on | this proposal? It's stalled. As far as I know, there's been very little discussion about it. It's not a trivial thing to implement, and it treads on delicate territory (how "." is treated). So I'd need to be convinced there was a strong constituency who really wanted it before adding it. I've added an informal straw poll to the bottom of [1] to allow you to express an opinion. Also I'm not very happy with the "stacking operations" part, and I'd like a better idea. Simon [1] http://hackage.haskell.org/trac/haskell-prime/wiki/TypeDirectedNameResoluti…

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Re: [Haskell-cafe] ANNOUNCE: control-monad-failure and safe-failure
by Kalman Noel 17 Nov '09

17 Nov '09

Michael Snoyman schrieb: > control-monad-failure provides a basic notion of failure which does not > commit to any concrete representation. > It is just a version of the MonadError class without the annoying bits. > >> class MonadFailure e m where failure :: e -> m a Why is it called "MonadFailure" (specifically, what's the "Monad" bit doing there)?

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Howto start a bigger project
by Günther Schmidt 16 Nov '09

16 Nov '09

Hi all, I'm stuck with a problem where I need serious help from other haskellers, in particular those that participate here on this list. It's a rather big project and I will need to set it up in an organized way, something with a blog, web page or other means. I tried to solve it by myself while asking the occasional question here but that turned out to be ineefective. The problem as such is certainly of interest for just about any programmer who is using Haskell for real world programming too. In short, to get started I'd appreciate some tips how to set this up. Günther

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ANN: wcwidth-0.0.1
by Jason Dusek 16 Nov '09

16 Nov '09

A small package with bindings to a function in <wchar.h> that assigns a column width to Unicode characters. http://hackage.haskell.org/package/wcwidth-0.0.1 This is the function that tells your terminal to make Chinese characters two columns wide while making the letter 'a' but one column wide. In addition, a little utility is provided that constructs a table of widths by character and a listing of character ranges with the same width. On different systems, `wcwidth` may assign different widths to the same character for some obscure characters. -- Jason Dusek

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How much time until dependencies are rebuilt in hackage?
by Maurício CA 16 Nov '09

16 Nov '09

Suppose package B depends on A. If a new version of A is uploaded to hackage, how much later package B will be rebuilt (either to show a problem with the new version or to solve a problem with a previous version of A)? Thanks, Maurício

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C headers in cabal dependencies
by Maurício CA 16 Nov '09

16 Nov '09

Hi, I've been using 'install-includes' in a package. I sometimes make small changes to those include files, and I've seen that cabal doesn't consider then dependencies, i.e., doesn't rebuild .hsc files depending on then. I'm not sure if this is an error, as parsing hsc2hs input may not be cabal task. Anyway, I would like to suggest that files in install-includes be considered dependencies and that files under 'includes' be also included when processing '.hsc' files. Thanks. Best, Maurício

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creating documentation links failed with cabal install --haddock-options=--hyperlink-source. is this a bug?
by Thomas Hartman 16 Nov '09

16 Nov '09

cabal haddock -–hyperlink-source installs documentation with links to source code, which also be on by defualt. cabal install –haddock-options=–hyperlink source does not install hyperlinked source. cabal install –haddock-options=-–hyperlink sourceblehblehbleh doesn’t throw an error, which suggests to me that the first command might also be passing the flags with an error. Whatever the case, seems to me the examples section in cabal --help should really include examples using --with-PROG and --PROG-options. I can't find a good one online. fwiw, cabal install --haddock-option=hyperlink-source DID generate an error in the output: target `hyperlink-source' is not a module name or a source file also, blogged about this here: http://blog.patch-tag.com/2009/10/23/cabal-install-a-package-with-documenta…

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