- Haskell-Cafe - Haskell.org

RE: can't figure out a type
by Brian Bloniarz 12 Feb '09

12 Feb '09

Hi John, > In the class I wrote, c has kind * (e.g. [a]), but then I don't see > how to write a suitable map function. For that, I would want c to > have kind * -> *. Unfortunately then I don't know to write the > others. > > Would I have to do something with c having kind (* -> *) ? > > class Chunkable2 c el where > cLength :: c el -> Int > cHead :: c el -> Maybe el > cMap :: (el -> el') -> c el -> c el' When c is (* -> *), why do you need el to be a parameter of the typeclass at all? I.e., would this work for your purposes: > class Chunkable3 c where > cLength :: c el -> Int > cHead :: c el -> Maybe el > cMap :: (a -> b) -> c a -> c b > instance Chunkable3 [] where > cLength = length > cMap = map > cHead = ... Using Functor as a superclass of Chunkable3 works too. Thanks, -Brian _________________________________________________________________ More than messages–check out the rest of the Windows Live™. http://www.microsoft.com/windows/windowslive/

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More DPH Questions
by James Swaine 12 Feb '09

12 Feb '09

I apologize - I forgot to include this in my previous posting. I also couldn't seem to find an implementation of the foldP function, which is included in several of Roman's papers on DPH and ndp in general. Is this not implemented yet? Thanks, James Swaine

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Haskell Users in the West Michigan Area?
by John Van Enk 12 Feb '09

12 Feb '09

I'm wondering if there are people in the West Michigan, USA area who'd be interested in forming WMHUG (West Michigan Haskell Users Group). Any one on here from that area? /jve

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DPH Prelude
by James Swaine 12 Feb '09

12 Feb '09

I was wondering if anyone could point me to a more in-depth explanation of why we are (currently) restricted to using a special-purpose standard Prelude when writing vectorised code with DPH. We're prototyping using several data-parallel languages for a research project here at Northwestern University, and wanted to get a better idea how we might go about adding vectorisation support for addtional types/operations. There are certain simple operations which don't appear to be supported for primitive types yet (e.g. the ^ and ** operators for an Int type). These are the kinds of things we'd like to look into as a first step. Much thanks, James Swaine

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GHC development
by Andrew Coppin 12 Feb '09

12 Feb '09

OK, so I have a small question. I was just wondering what the current state of development with GHC is. So, I had a look at the developer wiki. Unfortunately, as best as I can tell, most of the status pages haven't been updated in many months. (Most of them still talk about what will or won't be in 6.10.1, which has been *released* for a while now.) What's the best way to find out what the "real" state of affairs is? What are the developers really working on? What's on hold? How far have people got with things? Etc. (Yes, I know. I'm nosey...)

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How outdated is Hugs?
by Rafael Gustavo da Cunha Pereira Pinto 11 Feb '09

11 Feb '09

Hi, I see Hugs last source code snapshot dates back to 2006. Was there any updates? Regards -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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Re: [Gtk2hs-users] Gtk2HS 0.10.0 Released
by Peter Verswyvelen 11 Feb '09

11 Feb '09

Nice. The demos are now included only in the source distri right? On Wed, Feb 11, 2009 at 1:40 PM, Axel Simon <Axel.Simon(a)ens.fr> wrote: > On Wed, 2009-02-11 at 13:36 +0100, Peter Verswyvelen wrote: > > Great job! I have been using the RC1 version for a while and it works > > well. > > > > > > - Full switch to the new model-view implementation using a Haskell > > model > > > > > > This is very interesting. Do demos exist to show this? > > > > Yes, all the demos under demo/treeList have been converted to use the > new ModelView (the directory name is stale, it should probably be > renamed to modelView since TreeList was the old interface). > > There's also a combo box demo in demo/menu. > > Cheers, > Axel. > > > On Wed, Feb 11, 2009 at 5:32 AM, Peter Gavin <pgavin(a)gmail.com> wrote: > > Hi everyone, > > > > I'd like to release the announcement of Gtk2HS 0.10.0. A lot > > of new > > stuff has gone into this release, including: > > > > - Support for GHC 6.10 > > - Bindings to GIO and GtkSourceView-2.0 > > - Full switch to the new model-view implementation using a > > Haskell model > > - Support for many more model-based widgets such as IconView > > and an > > updated binding for ComboBox > > - Full Drag-and-Drop support > > - Better support for Attributes in Pango > > - Replaced Event for EventM monad, thereby improving > > efficiency and > > convenience > > - Functions for interaction between Cairo and Pixbuf drawing > > - Lots of bug fixes, code cleanups, and portability > > improvements > > > > With this release, the bindings to GnomeVFS and > > GtkSourceView-1.0 have > > been deprecated. The TreeList modules have been deprecated > > from the > > Gtk+ bindings. > > > > Source and Win32 binaries are available at: > > > > > > > https://sourceforge.net/project/showfiles.php?group_id=49207&package_id=424… > > > > Thanks to everyone who submitted bug fixes and features this > > time around! > > > > Thanks, > > Peter Gavin > > Gtk2HS Release Manager > > > > > > > ------------------------------------------------------------------------------ > > Create and Deploy Rich Internet Apps outside the browser with > > Adobe(R)AIR(TM) > > software. With Adobe AIR, Ajax developers can use existing > > skills and code to > > build responsive, highly engaging applications that combine > > the power of local > > resources and data with the reach of the web. Download the > > Adobe AIR SDK and > > Ajax docs to start building applications > > today-http://p.sf.net/sfu/adobe-com > > _______________________________________________ > > Gtk2hs-users mailing list > > Gtk2hs-users(a)lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/gtk2hs-users > > > > > > > ------------------------------------------------------------------------------ > > Create and Deploy Rich Internet Apps outside the browser with > Adobe(R)AIR(TM) > > software. With Adobe AIR, Ajax developers can use existing skills and > code to > > build responsive, highly engaging applications that combine the power of > local > > resources and data with the reach of the web. Download the Adobe AIR SDK > and > > Ajax docs to start building applications today- > http://p.sf.net/sfu/adobe-com > > _______________________________________________ Gtk2hs-users mailing list > Gtk2hs-users(a)lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/gtk2hs-users > >

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Re: [Gtk2hs-users] Gtk2HS 0.10.0 Released
by Peter Verswyvelen 11 Feb '09

11 Feb '09

Great job! I have been using the RC1 version for a while and it works well. - Full switch to the new model-view implementation using a Haskell model This is very interesting. Do demos exist to show this? On Wed, Feb 11, 2009 at 5:32 AM, Peter Gavin <pgavin(a)gmail.com> wrote: > Hi everyone, > > I'd like to release the announcement of Gtk2HS 0.10.0. A lot of new > stuff has gone into this release, including: > > - Support for GHC 6.10 > - Bindings to GIO and GtkSourceView-2.0 > - Full switch to the new model-view implementation using a Haskell model > - Support for many more model-based widgets such as IconView and an > updated binding for ComboBox > - Full Drag-and-Drop support > - Better support for Attributes in Pango > - Replaced Event for EventM monad, thereby improving efficiency and > convenience > - Functions for interaction between Cairo and Pixbuf drawing > - Lots of bug fixes, code cleanups, and portability improvements > > With this release, the bindings to GnomeVFS and GtkSourceView-1.0 have > been deprecated. The TreeList modules have been deprecated from the > Gtk+ bindings. > > Source and Win32 binaries are available at: > > > > https://sourceforge.net/project/showfiles.php?group_id=49207&package_id=424… > > Thanks to everyone who submitted bug fixes and features this time around! > > Thanks, > Peter Gavin > Gtk2HS Release Manager > > > > ------------------------------------------------------------------------------ > Create and Deploy Rich Internet Apps outside the browser with > Adobe(R)AIR(TM) > software. With Adobe AIR, Ajax developers can use existing skills and code > to > build responsive, highly engaging applications that combine the power of > local > resources and data with the reach of the web. Download the Adobe AIR SDK > and > Ajax docs to start building applications today- > http://p.sf.net/sfu/adobe-com > _______________________________________________ > Gtk2hs-users mailing list > Gtk2hs-users(a)lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/gtk2hs-users >

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ANN: Data.Stream 0.3
by Wouter Swierstra 11 Feb '09

11 Feb '09

I've released a new version of the Data.Stream package, a modest library for manipulating infinite lists. Changes include: * Support for lazy SmallCheck; * Improved Show instance; * Stricter scans; * Various documentation fixes; * Several new functions from Data.List. Many of these features were based on patches by Bas van Dijk - thank you Bas! Haddock: http://www.cs.nott.ac.uk/~wss/repos/Stream/dist/doc/html/Stream/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Stream/ darcs: darcs get http://www.cs.nott.ac.uk/~wss/repos/Stream All the best, Wouter

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Re: [Haskell-cafe] evaluation semantics of bind
by Alberto G. Corona 11 Feb '09

11 Feb '09

forwarded: Yes! if no state is passed, the optimization makes sense and the term is not executed, like any lazy evaluation. For example, I used the debugger (that is, without optimizations) to verify it with the Maybe monad: op x= x+x print $ Just (op 1) >>= \y-> return (Just 2) does not evaluate op 1 but print $ Just (op 1) >>= \y-> return y does execute it. The trace of the first: [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main> :set stop :list *Main> :step main Stopped at test.hs:4:6-43 _result :: IO () = _ 3 4 main= print $ Just (op 1) >>= \y-> return 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 5 [test.hs:4:6-43] *Main> :step Stopped at test.hs:4:14-43 _result :: Maybe Integer = _ 3 4 main= print $ Just (op 1) >>= \y-> return 2 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 5 [test.hs:4:14-43] *Main> :step Stopped at test.hs:4:14-24 _result :: Maybe Integer = _ 3 4 main= print $ Just (op 1) >>= \y-> return 2 ^^^^^^^^^^^ 5 [test.hs:4:14-24] *Main> :step Stopped at test.hs:4:35-43 _result :: Maybe Integer = _ 3 4 main= print $ Just (op 1) >>= \y-> return 2 ^^^^^^^^^ 5 [test.hs:4:35-43] *Main> :step Just 2 But in the second case op is executed: *Main> :step main Stopped at test.hs:4:6-43 _result :: IO () = _ 3 4 main= print $ Just (op 1) >>= \y-> return y ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 5 [test.hs:4:6-43] *Main> :step Stopped at test.hs:4:14-43 _result :: Maybe Integer = _ 3 4 main= print $ Just (op 1) >>= \y-> return y ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 5 [test.hs:4:14-43] *Main> :step Stopped at test.hs:4:14-24 _result :: Maybe Integer = _ 3 4 main= print $ Just (op 1) >>= \y-> return y ^^^^^^^^^^^ 5 [test.hs:4:14-24] *Main> :step Stopped at test.hs:4:35-43 _result :: Maybe Integer = _ y :: Integer = _ 3 4 main= print $ Just (op 1) >>= \y-> return y ^^^^^^^^^ 5 [test.hs:4:35-43] *Main> :step Just Stopped at test.hs:4:20-23 _result :: Integer = _ 3 4 main= print $ Just (op 1) >>= \y-> return y ^^^^ 5 [test.hs:4:20-23] *Main> :step Stopped at test.hs:6:0-8 _result :: Integer = _ 5 6 op x= x+x ^^^^^^^^^ 7 [test.hs:6:0-8] *Main> :step Stopped at test.hs:6:6-8 _result :: Integer = _ x :: Integer = _ 5 6 op x= x+x ^^^ 7 [test.hs:6:6-8] *Main> :step Just 2 2009/2/5 Gregg Reynolds <dev(a)mobileink.com>- Ocultar texto citado - > > On Thu, Feb 5, 2009 at 9:27 AM, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: >> >> Hello Gregg, >> >> Thursday, February 5, 2009, 6:20:06 PM, you wrote: >> >> > An optimizer can see that the result of the first getChar is >> > discarded and replace the entire expression with one getChar without >> > changing the formal semantics. >> >> this is prohibited by using pseudo-value of type RealWorld which is >> passed through entire action stream. actually, order of execution is >> controlled by dependencies on this values >> >> http://haskell.org/haskellwiki/IO_inside >> > Thanks. I actually read that a few weeks ago and forgot all about it. So the gist is that type IO has special magic semantics. Formal, but hidden. Which means monad semantics are built in to the language, at least for that type. The Haskell Report doesn't seem to say anything about this, which seems odd. > > But then for non-IO monads, the optimization would be allowed, no? Of course; only the IO monad has external world behavior. > > Thanks, > > gregg > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Why does sleep not work?
by John Ky 11 Feb '09

11 Feb '09

Hi Haskell Cafe, I wrote very short program to sleep for 5 seconds compiled with the -threaded option in ghc on the Mac OS X 1.5. I am finding that using the sleep function doesn't sleep at all, whereas using threadDelay does: main = do putStrLn "Waiting for 5 seconds." threadDelay 5000000 -- works putStrLn "Done." main = do putStrLn "Waiting for 5 seconds." sleep 5 -- doesn't sleep at all putStrLn "Done." Anybody know what's happening? Thanks -John

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Painting logs to get a coloured tree
by Joachim Breitner 11 Feb '09

11 Feb '09

Hi, I have a problem that, it seems, I can not solve cleanly without resorting to imperative programming (via ST): Assume you have a tree (and you can think of a real tree here), defined by something like this: data Tree a = Bud | Branch a Double Tree Tree -- | ` Lenght of this branch -- ` General storage field for additional information Now, I have a nice algorithm that calulates something for each branch, but it only works on lists of branches, so I have to cut them apart first, remembering their position in space, and then work on these, well, logs. data Point = Point Double Double data Log = Log Point Point type Info = ... noInfo :: Info cutTreeApart :: Tree a -> [(Log, a)] someAlgorithm :: [(Log,a)] -> [(a, Info)] Conveniently, the algorithm allows me to tag the logs with something, to be able to keep track at least somewhat of the logs. Unfortunately, I need this information in the storage field in my Tree, as the list of logs is not sufficient for later calculations. Idea: Using ST ============== annotateTree :: Tree a -> Tree Info annotateTree tree = runSt $ do -- Put an STRef in each node treeWithPointer <- mapM const (newSTRef noInfo) tree -- Cut this tree apart let logsWithPointers = cutTreeApart treeWithPointer -- Run the algorithm let informations = someAlgorithm logsWithPointers -- Put the information back, via the ST ref mapM (\(stRef, info) -> writeSTRef stRef info) informations -- Read the ST refs mapM readIORef tree Note that I assume a instance Traversable Tree here, and mapM is Data.Traversable.mapM. Now while this works, and while ST is still somewhat pure, I’m wondering if there is no better way of expressing "This piece of information came from the point in a data structure, so something else can be put here easily". Some ideas where numbering the Nodes and then using this number as the tag on the log, but this is not much different from using STRefs, it seems. Thanks, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Additonal types for Foreign.C.Types
by Maurício 11 Feb '09

11 Feb '09

Hi, After reading an ISO draft for standard C, I found a few types that could be usefull when binding to libraries (these are from <stdint.h>): int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, uint64_t What about if they were included in the next version of GHC Foreign.C.Types module? For instance, as: CInt8, CUInt8, CInt16 etc. Some libraries (e.g. sqlite) define function parameters that are supposed to always have a given size (e.g., sqlite defines sqlite3_int64). In order to have a portable binding to those libraries, it would be nice to have types in Haskell that also offer that guarantee. There are also a few other nice types, although I'm not sure they belong to standard modules. If they did, however, they could make life easier for those people writing higher level Haskell modules after standard C functions: complex, float complex, double complex (from <complex.h>) are used in functions like ccos, csin, cexp, csqrt etc. struct lconv (from <locale.h>) struct tm (from <time.h>) From <wchar.h> and <wctype.h>: mbstate_t, wint_t, wctrans_t, wctype_t. Do you think I could open a ticket for GHC proposing that? Best, Maurício

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Re: [Haskell-cafe] ANN: convertible (first release)
by Daniel Schüssler 11 Feb '09

11 Feb '09

Hello, it is a real code snippet, but I failed to include the necessary pragma (which is: {-# OPTIONS_GHC -fglasgow-exts #-} or {-# OPTIONS_GHC -XMagicHash #-} at the beginning of the file). the "#" suffix is for unboxed types: http://www.haskell.org/ghc/docs/latest/html/users_guide/primitives.html http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#m… Greetings, Daniel On Wednesday 11 February 2009 00:19:56 Dylan Tisdall wrote: > Maybe I'm just a noob falling for someone's joke here, but I can't > make that sample, or anything else I tried with "#" on the end of > names, to compile with ghci. In the example below, I get a parse error > at the line defining d2f. Is this a real code snippet? > > > Cheers, > Dylan > > On Feb 10, 2009, at 2:46 AM, Daniel Schüssler wrote: > > Hi, > > > > On Wednesday 28 January 2009 04:30:07 John Goerzen wrote: > >> On Tue, Jan 27, 2009 at 09:41:30PM -0500, wren ng thornton wrote: > >>> I once again point out that realToFrac is *wrong* for converting > >>> from > >>> Float or Double. > >>> > >>>> realToFrac (1/0::Float) ::Double > >>> > >>> 3.402823669209385e38 > >> > >> Yes, I understand what you are saying and agree with you. But there > >> is nothing better in the standard library > > > > don't know whether you consider GHC as standard library, but if you > > do...: > > > > import GHC.Types > > import GHC.Prim > > > > -- | Double to Float > > d2f (D# d) = F# (double2Float# d) > > > > -- | Float to Double > > f2d (F# f) = D# (float2Double# f) > > > > > > ghci> f2d (1/0) > > Infinity > > it :: Double > > > > :) > > > > -- > > Daniel > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Monad explanation
by Tim Newsham 11 Feb '09

11 Feb '09

I put up a small Monad explanation (I wouldn't quite call it a tutorial): http://www.thenewsh.com/~newsham/haskell/monad.html The intent here is to is to have a short description of what a Monad is that is approachable by Haskell beginners or non- Haskell programmers who are looking for an intuitive definition but may not yet have the background or the inclination to jump into full tutorial to tackle the subject. Tim Newsham http://www.thenewsh.com/~newsham/

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evaluation semantics of bind
by Gregg Reynolds 11 Feb '09

11 Feb '09

I think I've just about got monads figured out, but there's one detail that still escapes me. As I understand it, a monad is a kind of programming trick the uses data dependency to force evaluation order. x >>= f means apply f to x; since the value of f x depends on the value of x, the evaluator must evaluate x before f x. However, consider: getChar >>= \x -> getChar An optimizer can see that the result of the first getChar is discarded and replace the entire expression with one getChar without changing the formal semantics. But that would change the behavior, so to get the desired behavior, there must be some principle that prevents this from happening, ensuring that x >>= f always evaluates f x. I can see that the monad laws ensure this But I haven't found anything that states this. Am I missing something? Thanks, gregg

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Re: [Haskell] Google Summer of Code 2009
by Michael Litchard 11 Feb '09

11 Feb '09

I would love a mentor to help me with a Haskell binding to libnova. This is part of a larger project I have in mind, but the libnova binding seems like the first step. I don't expect this to be picked as an official GSoC, but this seemed like a good time to look for a mentor for this project. Michael Litchard On Tue, Feb 10, 2009 at 6:26 AM, Malcolm Wallace <Malcolm.Wallace(a)cs.york.ac.uk> wrote: > Gentle Haskellers, > > The Google Summer of Code will be running again this year. Once again, > haskell.org has the opportunity to bid to become a mentoring > organisation. (Although, as always, there is no guarantee of > acceptance.) > > If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing > lists of your choice. We especially encourage students to communicate > with the wider community: if you keep your ideas private, you have a > much worse chance of acceptance than if you develop ideas in > collaboration with those who will be your "customers", end-users, or > fellow-developers. This is the open-source world! > > The timeline is that Haskell.org will apply for GSoC membership between > 9-13 March, and if we are successful, students can submit applications > between 23 March - 3 April. > > If you wish to help publicise GSoC amongst students, there are official > posters/fliers available (not specific to haskell.org) > > http://code.google.com/p/google-summer-of-code/wiki/GsocFlyers > > Regards, > Malcolm > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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ANN: Bug fix to regex-tdfa, new version 0.97.3
by ChrisK 11 Feb '09

11 Feb '09

To Haskell and Libraries and Haskell-Cafe, Whilst improving regex-tdfa I have run across new bugs. Some patterns were getting compiled wrong and others were affected by an execution bug. As this package has actual users, I wanted to make sure they get these fixes immediately. Three Cheers For QuickCheck! The new version is 0.97.3 at http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-tdfa And this version passes all the unit tests I have, including coverage for the new bugs. This is no warranty that regex-tdfa is bug free, since I made that same claim last release. For instance: I suspect 0.97.3 may be buggy if used in the optional left-associative mode. The new improved version of regex-tdfa is still a long way off. Cheers, Chris Kuklewicz

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Asking the GHC garbage collector to run
by Mads Lindstrøm 11 Feb '09

11 Feb '09

Hi all, Is it possible to ask the GHC garbage collector to run ? Something like a collectAllGarbage :: IO() call. Greetings, Mads Lindstrøm

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Re: [Haskell] Google Summer of Code 2009
by Don Stewart 11 Feb '09

11 Feb '09

Malcolm.Wallace: > Gentle Haskellers, > > The Google Summer of Code will be running again this year. Once again, > haskell.org has the opportunity to bid to become a mentoring > organisation. (Although, as always, there is no guarantee of > acceptance.) > > If you have ideas for student projects that you think would benefit the > Haskell community, now is the time to start discussing them on mailing > lists of your choice. We especially encourage students to communicate > with the wider community: if you keep your ideas private, you have a > much worse chance of acceptance than if you develop ideas in > collaboration with those who will be your "customers", end-users, or > fellow-developers. This is the open-source world! > And I'll just note that since December we've been running a proposal submission site here, where you can vote and comment on ideas, http://www.reddit.com/r/haskell_proposals/top/ A great place to suggest ideas! -- Don

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Haskell Fest
by Lyle Kopnicky 10 Feb '09

10 Feb '09

Looks like a lot of fun! http://www.haskellchamber.com/page6.html

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Darcs 2.2.0 diff on Windows
by Peter Verswyvelen 10 Feb '09

10 Feb '09

When using this inside the Windows command prompt (CMD.EXE) I get darcs: diff: runInteractiveProcess: does not exist (No such file or directory) It works fine under MSYS. Also when using --diff-command to launch a GUI diff tool, Darcs does not seem to copy any changes the GUI tool did to the "new-program". Is this intended behavior?

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Painting logs to get a coloured tree
by oleg＠okmij.org 10 Feb '09

10 Feb '09

If I understand you correctly, the problem is to annotate an already constructed tree with arbitrary pieces of new data -- hopefully without reconstructing the tree. Perhaps the approach used in the FLOLAC type-checkers would be helpful. The `tree' was an expression in lambda-calculus to type check. The new annotations are the reconstructed types, of each node in the tree (of each sub-expression). I wanted to annotate each sub-expression with its reconstructed type -- without re-defining the data type of expressions or rebuilding the tree. The expression should stay as it was, I merely want to associate, after the fact, additional pieces of data with each node. I also wanted the code to be pure and avoid STRefs let alone StableNames and any IO. Here are the files in question: http://okmij.org/ftp/Computation/FLOLAC/TEvalNC.hs http://okmij.org/ftp/Computation/FLOLAC/TEvalNR.hs TEvalNC.hs is the ordinary type checker for the simply-typed lambda calculus with constants and the fix-point. The type reconstructor (aka, non-standard, abstract evaluator) has this type teval :: TEnv -> Term -> Typ Given the type environment and a term, it returns its inferred type (or reports an error). The file TEvalNR.hs returns not only the reconstructed type of the term but also the types of all the subterms. The latter data are returned in a `virtual' typed AST -- virtual because the original AST is not modified and the inferred types are attached to AST nodes, well, virtually. Generally speaking, after a simple modification teval could be made total: it would return reconstructed types of the subterms even if the entire term is ill-typed. That modification was one of the exercises. The intention was to model OCaml -- which, given a special flag, can dump the inferred types of all sub-expressions, even if the overall type checking failed. In Emacs and vi, one can highlight an expression or variable and see its inferred type. If the type checking failed, one can still explore what the type checker did manage to infer.

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Haskell and Java interaction
by Silviu ANDRICA 10 Feb '09

10 Feb '09

Hello, I was wondering if there is a way to call Haskell code from Java. I tried using jvm-bridge(http://sourceforge.net/projects/jvm-bridge/), but I'm stuck on building it. Thank you very much, Silviu

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lazy evaluation is not complete
by Peter Padawitz 10 Feb '09

10 Feb '09

A simplied version of Example 5-16 in Manna's classical book "Mathematical Theory of Computation": foo x = if x == 0 then 0 else foo (x-1)*foo (x+1) If run with ghci, foo 5 does not terminate, i.e., Haskell does not look for all outermost redices in parallel. Why? For efficiency reasons? It's a pity because a parallel-outermost strategy would be complete.

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Re: [Haskell-cafe] ANN: convertible (first release)
by Daniel Schüssler 10 Feb '09

10 Feb '09

Hi, On Wednesday 28 January 2009 04:30:07 John Goerzen wrote: > On Tue, Jan 27, 2009 at 09:41:30PM -0500, wren ng thornton wrote: > > I once again point out that realToFrac is *wrong* for converting from > > Float or Double. > > > > > realToFrac (1/0::Float) ::Double > > > > 3.402823669209385e38 > > Yes, I understand what you are saying and agree with you. But there > is nothing better in the standard library don't know whether you consider GHC as standard library, but if you do...: import GHC.Types import GHC.Prim -- | Double to Float d2f (D# d) = F# (double2Float# d) -- | Float to Double f2d (F# f) = D# (float2Double# f) ghci> f2d (1/0) Infinity it :: Double :) -- Daniel

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Re: [Haskell-cafe] Formal semantics for Haskell?
by gregg reynolds 10 Feb '09

10 Feb '09

Is that () or _|_? ;) Tim Newsham <newsham(a)lava.net> wrote: >null

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Re: Haskell and Java interaction
by John A. De Goes 10 Feb '09

10 Feb '09

The bridging projects are all dead and unsupported. You can still do it, but you'll have to write Haskell wrappers that use C types, then write C code that calls the wrapped Haskell code, then write JNI code to call the C code from Java. http://www.haskell.org/haskellwiki/Calling_Haskell_from_C There was a promising thesis project called LambdaVM that allowed you to compile Haskell to JVM byte codes, but it suffered the same fate as all thesis projects. Regards, John A. De Goes N-BRAIN, Inc. The Evolution of Collaboration http://www.n-brain.net | 877-376-2724 x 101 On Feb 9, 2009, at 8:56 AM, Silviu ANDRICA wrote: > Hello, > I was wondering if there is a way to call Haskell code from Java. I > tried using jvm-bridge(http://sourceforge.net/projects/jvm-bridge/), > but I'm stuck on building it. > > Thank you very much, > Silviu > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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how to use a literal string with bystring useAsCString
by minh thu 10 Feb '09

10 Feb '09

Hi, I have a Haskell source file encoded in utf-8. Inside that source, I have literal strings that I'd like to pass to a C function. withCString does the job well until I tried to use the double-quote character ". I get /usr/lib/ghc-6.10.1/ghc: `@: Bad font file format (even when using (chr 34) instead). I didn't understand the reason of this behavior (since the double quote is just ascii) but tried to use useAsCString but coudn't do it. I can ByteString.Char8.pack my string but the problem remains. I tried to use IConv but it uses UTF8.ByteString and I don't know how to make the conversion so I can use useAsCString or withCString. Any idea ? Thanks, Thu

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Control.Arrow being icky
by Louis Wasserman 10 Feb '09

10 Feb '09

In GHCi, I import Control.Arrow, and Kliesli doesn't appear: Prelude Control.Arrow> :t Kliesli <interactive>:1:0: Not in scope: data constructor `Kliesli' Prelude Control.Arrow> :browse (<<^) :: (Arrow a) => a c d -> (b -> c) -> a b d (>>^) :: (Arrow a) => a b c -> (c -> d) -> a b d class (Control.Category.Category a) => Arrow a where arr :: (b -> c) -> a b c first :: a b c -> a (b, d) (c, d) second :: a b c -> a (d, b) (d, c) (***) :: a b c -> a b' c' -> a (b, b') (c, c') (&&&) :: a b c -> a b c' -> a b (c, c') class (Arrow a) => ArrowApply a where app :: a (a b c, b) c class (Arrow a) => ArrowChoice a where left :: a b c -> a (Either b d) (Either c d) right :: a b c -> a (Either d b) (Either d c) (+++) :: a b c -> a b' c' -> a (Either b b') (Either c c') (|||) :: a b d -> a c d -> a (Either b c) d class (Arrow a) => ArrowLoop a where loop :: a (b, d) (c, d) -> a b c newtype (ArrowApply a) => ArrowMonad a b = ArrowMonad (a () b) class (ArrowZero a) => ArrowPlus a where (<+>) :: a b c -> a b c -> a b c class (Arrow a) => ArrowZero a where zeroArrow :: a b c newtype Kleisli m a b = Kleisli {runKleisli :: a -> m b} (^<<) :: (Arrow a) => (c -> d) -> a b c -> a b d (^>>) :: (Arrow a) => (b -> c) -> a c d -> a b d leftApp :: (ArrowApply a) => a b c -> a (Either b d) (Either c d) returnA :: (Arrow a) => a b b (<<<) :: (Control.Category.Category cat) => cat b c -> cat a b -> cat a c (>>>) :: (Control.Category.Category cat) => cat a b -> cat b c -> cat a c Does anybody know what's going on? Louis Wasserman wasserman.louis(a)gmail.com

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Efficient string output
by Ketil Malde 10 Feb '09

10 Feb '09

Hi, I'm currently working on a program that parses a large binary file and produces various textual outputs extracted from it. Simple enough. But: since we're talking large amounts of data, I'd like to have reasonable performance. Reading the binary file is very efficient thanks to Data.Binary. However, output is a different matter. Currently, my code looks something like: summarize :: Foo -> ByteString summarize f = let f1 = accessor f f2 = expression f : in B.concat [f1,pack "\t",pack (show f2),...] which isn't particularly elegant, and builds a temporary ByteString that usually only get passed to B.putStrLn. I can suffer the inelegance were it only fast - but this ends up taking the better part of the execution time. I tried to use lazy ByteStrings, the theory being that the components that already are (strict) ByteStrings could be recycled as chunks. I also tried to push the output down into the function (summarize :: Foo -> IO ()), but both of these were actuall slower. Since I surely can't be the first person that needs to output tab-separated text, I'd be grateful if somebody could point me in the right direction. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Formal semantics for Haskell?
by Gregg Reynolds 10 Feb '09

10 Feb '09

ML has a formal definition[1]; why not Haskell? Would this be a Good Thing, or a Waste Of Time? The Report strikes me as a hybrid of formal and informal. I guess that's not a problem so far, but there aren't that many implementations; if we had dozens, how could we verify conformance? A formal semantics would be useful, but it would also be Fun to use Category Theory notation in a language definition. Such a task would be way beyond my abilities, but I have come up with an idea for a formal semantics of IO and other non-deterministic elements of the language that I think is kind of interesting. It's inspired by Category Theory and the Z specification language. See my (brief) blog article<http://syntax.wikidot.com/blog:3> . Actually, I'm in a state of rather intense euphoria about it, so a bucket of cold water realism over my head might be a Good Thing. Then I could get some sleep instead of obsessing about category theory and Haskell. :) I propose any formal definition include the following warning, modeled on Knuth's warning about MetaFont: WARNING: Haskell can be hazardous to your other interests. Once you get hooked, you will develop intense feelings about language design; semantic models will intrude on the program texts you read. And you will perpetually be thinking of improvements to the programs that you see everywhere, including those of your own design. Thanks, gregg [1] The Definition of Standard ML (Revised); a preview is on Google Books

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Error in building profiling
by Marco Túlio Gontijo e Silva 10 Feb '09

10 Feb '09

Hello, I'm trying to package ghc-paths for debian, and I got to this: $ ./setup configure --enable-library-profiling --disable-library-vanilla Configuring ghc-paths-0.1.0.5... $ ./setup build Preprocessing library ghc-paths-0.1.0.5... Building ghc-paths-0.1.0.5... /usr/bin/ar: creating dist/build/libHSghc-paths-0.1.0.5_p.a /usr/bin/ld: dist/build/GHC/Paths.o: No such file: No such file or directory I thought this could be a problem with this package, but I've tried with mtl, and I got the same result: /usr/bin/ld: dist/build/Control/Monad/Cont.o: No such file: No such file or directory I'm using ghc6 6.10.1+dfsg1-5 and binutils 2.19-1~exp1. Greetings. -- marcot http://marcot.iaaeee.org/

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Data.Map: Enumerating ordered subset of keys
by Jared Updike 10 Feb '09

10 Feb '09

I would like to enumerate a subset of keys in a Map satisfying \ key >= key1 && key <= key2 but in the expected, reasonable amount of time (e.g. < O(log(n)) + O(m) for n total keys and m keys in the subset). (Or key > key1 and/or key < key2 or some such combination). Is there an appropriate idiom or combination of library functions to accomplish this, short of digging into the code for Data.Map and writing such a function for a forked version of Data.Map? For example I could try something like a Set.split of a Set.split of Map.keysSet of my original map, but will laziness magically do what I really want? which is to walk down the tree to key1 (or the nearest key > key1) and enumerate keys in order until key2 is reached? Data.Map almost looks like what I need if I can do this. Jared.

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Re: Haskell and Automation
by Guenther Schmidt 09 Feb '09

09 Feb '09

Dear Michael, sorry too, for not telling you sooner that meanwhile I found out how to get it working, I hope you did not spend too much time trying on your end. It had to do with the context in which these components are initialized. Once you switch to LocalProcess (as opposed to AnyProcess) in coCreateInstance everything works fine and rather smoothly. The defaults in createObject do not work for every kind of object that you do want to instantiate, certainly not Excel. Of course having an old, but still fairly current manual also helps. Good luck with your projects. Günther Michael Snoyman schrieb: > Gunther, > > Sorry for not getting back to you on your previous e-mail, I've been > swamped. > > The reason I didn't put up an example was specifically because of the > error you mention. I was hoping to get a chance to use the COM > bindings in Python to see if I got the same thing. For the life of me > I can't figure out how to get started with Excel and Haskell. Sorry I > can't be more useful at this time. > > Michael > > On Fri, Feb 6, 2009 at 6:56 PM, G?uenther Schmidt <redcom(a)fedoms.com > <mailto:redcom@fedoms.com>> wrote: > > Hi Michael, > > for some reason createObject "Excel.Application" throws an > "Interface not supported" error. > > Can you help me out here? > > Günther > >

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FFI binding to CGAL?
by Peter Verswyvelen 09 Feb '09

09 Feb '09

I was wandering of someone already made a FFI binding to http://www.cgal.org or something similar that does computational geometry?

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Does readFile "/proc/mounts" hang for you?
by David Fox 09 Feb '09

09 Feb '09

I posted a bug about this (http://hackage.haskell.org/trac/ghc/ticket/2971) but its so odd I had to ask here. Using ghc 6.10.1, both readFile "/proc/mounts" and Data.ByteString.Lazy.Char8.readFile "/proc/mounts" hang on an amd64 machine running Linux. Also, Data.ByteString.readFile "/proc/mounts" returns the empty string. Is this behavior present for others? On i386?

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The Haskell re-branding exercise
by Gwern Branwen 09 Feb '09

09 Feb '09

On Sat, Feb 7, 2009 at 1:34 PM, Don Stewart <dons(a)galois.com> wrote: > Oh, we had a long discussion about the need for condorcet voting, > not a system like the reddit which is prone to abuse. > > Also, it would be good to have the images inline. Perfect, please meet better. Better, perfect. Now get along you two! Since January 1st, we could've had hundreds or thousands of votes and easily compensated for any abuse. -- gwern

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threadDelay
by Immanuel Litzroth 09 Feb '09

09 Feb '09

Am I correct in assuming this program should run 100 secs? >import Control.Concurrent >main = do > threadDelay 100000 Why do I get the folling result then? ghc -threaded Main.hs -o delay time ./delay real 0m0.104s user 0m0.001s sys 0m0.002s Thanks in advance for all your wonderful comments, Immanuel

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Switching from Mercurial to Darcs
by Peter Verswyvelen 09 Feb '09

09 Feb '09

I'm trying to switch from HG to Darcs, but I'm failing miserably in getting the same productivity. I often do bad checkins with Darcs, merely because the amount of information I get from Darcs is overwhelming and I don't have time to read the details of the large set of commands. With Mercurial, here's my basic workflow: 1) hg st lists just the filenames of changed, deleted, new files (with a char prefix to show what kind of change it is). I only want to see the filenames, not the lines/content. If I want to see changes in the content, I use hg diff using a visual differ. My brain is not capable of making sense of the textual diff. 2) modify .hgignore to make sure only these files are added that are part of the project. I want this process to be automated, I don't want to check each time which files to add, since this is error prone. 3) hg addrem this adds new files and removes deleted files from local repos. forgetting to add files is a common problem, and is really tricky since no record is made of these files, so if after a couple of versions if a developer finds out a file was missing, the history is useless since you can't reconstruct the old content of that local file anymore, and often it's impossible to give the local file to the other developers since it might be changed. I actually would like to have an option that automatically adds/deletes files on each commit, as it is easier to delete a file after it is checked in, than it is to reconstruct an old version from a local file you forgot to add. 4) hg commit -m "message" this commits my changes locally. I always do this before pulling since then I'm sure my changes are saved in the case a merge goes wrong. 5) hg pull -u this pulls in changes, and updates and merges the files, using a visual merger of my preference. 6) hg commit -m "merged" hg push only needed if I did a merge and want to push that merge. 7) back to 1) It would be nice to know how to do this using darcs, or if a better way exists using darcs, that would be handy too :)

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Video phone application
by Jamie 09 Feb '09

09 Feb '09

Hello! I like to create H.323 (primarly) & SIP based video phone application that will run on Linux, Windows & Mac platforms and of course, written in Haskell. (Writing similar application in Haskell is not "reinventing the wheel" :) I did look/try various current H.323/SIP based video phone software that runs on Windows, Mac and Linux such as Ekiga, XMeeting, Linphone, Mirial and some others. Ekiga (despite version 3.x) is unstable, keeps crashing a lot on me. XMeeting (Mac) does not work good (choppy videos, dropped packets et al). So do Linphone. The best of the lot so far is Mirial Softphone, it has nice decent features to begin with, but there are lot more features I want to add. Please let me know if there is good H.323 based videophone software that I might miss (I think I have looked at all of them). Since my video phone application mainly focus more on deaf people thus strong emphasize on video part while all other (if not all) video softphone put stronger emphasize on audio part & traditional phone setup. Also it has to be primarly H.323 because I would say about over 95% or so of all deaf people (with high speed Internet access) use Sorenson VP-200 as their "main" videophone (Sorenson gives away VP-200 video phone device for free to all deaf people in USA in exchange for using Sorenson Video Relay Service which they make profit from). Sorenson VP-200 only supports H.323, H.263 video codec and G.711/G.723.1 audio codec. Being able to work with VP-200 well is this application's first step. Skype, ooVoo, SightSpeed, QuteCom, VZOchat, iChat AV and some others are excellent video phone/chat applications but they are not H.323 complaint so they are "out of question". It would take me a lot of time (more like forever) if I have to write my own H.323 protocol stack along with video/audio codecs as well the GUI in Haskell. So in order to write this application in timely manner (say one year from now or so) I will need external library/functions for H.323/SIP protocol stack, the audio/video codecs and the GUI. I have been contacting serveral companies that offer licensing of their H.323/SIP/codecs SDK/API/Libraries for my program to be able to access to those functions. Naturally all of them ues C/C++. Yes there are some free/open source ones such as h323plus.org, x264, ffmpeg and some others, but still all of them are written in C/C++. I think I am more likely to go for commercial libraries due to stability (least bugs), higher performance, better documentation and whole set of supported features. What do I ask to make sure those libraries/SDK would work well with Haskell? As for GUI, since I would like for this application to run on three popular platforms: Windows, Mac and Linux, so I think I would have to use Gtk+, wxWidgets or Qt which mean I would use Gtk2Hs, wxHaskell or qtHaskell. Even nicer is to use FRP GUI library such as Fruit, Grapefruit, wxFruit, et al. This application would need to support localization i.e. support different languages. What do you suggest for me to use one of middle level GUI library or even the higher level FRP GUI library (with localization support)? Fast and smooth video (i.e. 30 FPS) is the key for this application. As it stands now, looks like I'll write the "main program" entirely in Haskell, then use some kind of bindings to call to those C/C++ libraries for H.323, SIP, codecs, webcam/video camera I/O, video display, GUI et al. Any better ideas for doing this? I know it would be dream come true to see acclerated vector/raster based drawing primitives with FRP GUI on top written entirely in Haskell along with H.323, codecs et al being written in Haskell too (we may have to wait quite while for that), but we live in real world so I will have to learn to deal with what we have now and the workarounds. Thanks in advance! Jamie

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GHC 6.11 missing time package?
by Lyle Kopnicky 09 Feb '09

09 Feb '09

Hi folks, I'm getting ready to release a piece of software. Unfortunately due to a bug in GHC 6.10 on Windows it does not handle Ctrl+C properly. Since the bug has been fixed (thank you Simon Marlow), I figured I'd download a 6.11 build (I grabbed the 2009-01-29 version). Unfortunately, my project won't build with it because it's missing the time-1.1 package. This is sad because I had gone through the trouble to rewrite my use of oldtime to use time, thinking it was more future-proof. Is this an oversight in the nightly build, or is this package going out of GHC? Thanks, Lyle

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haddock-2.3.0 literate comments discarded from .lhs input
by Alistair Bayley 09 Feb '09

09 Feb '09

I have this test case for Haddock (2.3.0): -------------------------------------------------- | Module : Test.Haddock Copyright : (c) 2009 Alistair Bayley License : BSD-style Maintainer : alistair(a)abayley.org Stability : stable Portability : portable Test case for Haddock. > module Test.Haddock > ( > -- $named_block > Fail(..) > ) > where > data Fail = Fail | Succeed $named_block This is some hadock documentation. -------------------------------------------------- This fails with: [1 of 1] Compiling Test.Fail ( Test\Fail.hs, Test\Fail.o ) Test\Fail.hs:11:26: Can't make a derived instance of `Typeable Fail' (You need -XDeriveDataTypeable to derive an instance for this class) In the data type declaration for `Fail' If I manually unlit, then Haddock is happy: -------------------------------------------------- -- | -- Module : Test.Haddock2 -- Copyright : (c) 2009 Alistair Bayley -- License : BSD-style -- Maintainer : alistair(a)abayley.org -- Stability : stable -- Portability : portable -- -- Test case for Haddock. module Test.Haddock2 ( -- $named_block Fail(..) ) where data Fail = Fail | Succeed -- $named_block -- -- This is some hadock documentation. -------------------------------------------------- so it looks as though it's discarding the literate comments. Is this intended? I was under the impression that because it used the ghc parser, it could now properly handle .lhs input. Ona related note, we have a nice unlitter in cabal that would preserve these comments before invoking haddock. Is this still used with haddock2, or does it now assume haddock2 will do the right thing? Alistair

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ANNOUNCE: X Haskell Bindings 0.1
by Antoine Latter 09 Feb '09

09 Feb '09

I'd like to announce a version bump for the X Haskell Bindings (XHB) library, to 0.1.* from 0.0.*. The goal of XHB is to provide a Haskell implementation of the X11 wire protocol, similar in spirit to the X protocol C-language Binding (XCB). On Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xhb This release focuses on making the API a bit friendlier: + 'type BOOL = Word8' has been replaced in the API by Prelude.Bool + type synonyms BYTE, CARD8, CARD16 and CARD32 for the Data.Word types have been eliminated + type synonyms INT8, INT16 and INT32 for the Data.Int types have been eliminated + Previously, all protocol replies were represented by their own distinct data type. Now, if the reply to a request only includes a single field, the request returns that field directly. In more concrete terms: > internAtom :: Connection -> InternAtom -> IO (Receipt InternAtomReply) becomes: > internAtom :: Connection -> InternAtom -> IO (Receipt ATOM) Further work to make the API more "Haskelly" is ongoing. Related projects: X C Bindings: http://xcb.freedesktop.org/ -Antoine

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how to link statically with a c lib ?
by minh thu 09 Feb '09

09 Feb '09

Hi, I'm writing bindings for the Tiny C Compiler. It seems that tcc provide a libtcc.a but no libtcc.so. In my cabal file, I have extra-libraries: dl, tcc but when using the generated haskell module, I have the following message : ⟨thu@jones samples⟩ ghc -e "main" Test.hs <interactive>: <command line>: can't load .so/.DLL for: tcc (libtcc.so: cannot open shared object file: No such file or directory) How can I generate a module linked statically against libtcc ? Thanks, Thu

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any implementation of ACIO?
by Jianzhou Zhao 09 Feb '09

09 Feb '09

Hi, http://www.haskell.org/pipermail/haskell-cafe/2004-November/007715.html mentioned an interesting (A)ffine and (C)entral IO. Are there any packages or extensions to support ACIO in Haskell? Thanks JZ

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[ANN] Working with HLint from Emacs
by Alex Ott 08 Feb '09

08 Feb '09

Hello For Emacs users it could be interesting - I wrote small module for more comfortable work with HLint from Emacs. It has same functionality as compilation-mode - navigation between errors, etc. To use it, just add following code to emacs init file: (require 'hs-lint) (defun my-haskell-mode-hook () (local-set-key "\C-cl" 'hs-lint)) (add-hook 'haskell-mode-hook 'my-haskell-mode-hook) Module is available from http://xtalk.msk.su/~ott/common/emacs/hs-lint.el -- With best wishes, Alex Ott, MBA http://alexott.blogspot.com/ http://xtalk.msk.su/~ott/ http://alexott-ru.blogspot.com/

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Re: [Haskell-cafe] Just how unsafe is unsafe
by Yitzchak Gale 08 Feb '09

08 Feb '09

Peter Verswyvelen wrote: >>> I do have asked myself the question whether >>> a "really random generating" >>> function could be regarded as "pure" somehow I wrote: >> Not really... Alberto G. Corona wrote: > What is pure randomness? .When the algorithmic complexity of the list of > random number is equal to the lenght of the list, that is, is incompressible > by any means. But this can not be probed. the distribution of values gives a > hint of whether if the list obey any algorithm, but even so, many > compressible series have a random distribution. in practical terms, it is > random has a random distribution, the receiver does not know the algorithm > AND the initial value. In this case it can be pure. Perhaps philosophically you can consider it pure, but it is unsafe to treat it as pure in your program. If you write x = genRandomR (1, 100) then the compiler may or may not replace the variable x by the function application genRandomR (1, 100) anywhere in your program. That is probably not what you want. -Yitz

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Moggi :: CT -> Hask
by Gregg Reynolds 08 Feb '09

08 Feb '09

Hi, I had a monadic revelation at about 3 am. The answer to the question "what is an IO value, really?" is "who cares?" I just posted a blog entry discussing how CT found it's way from Moggi into Haskell at http://syntax.wikidot.com/blog (hence the title; Moggi as functor). It addresses the question of what such things are and why Moggi's insight is so brilliant. Feedback welcome, but please remember this is coming from a non-mathematician who likes to write. If you find anything there that outrages your inner Russell, please correct me, but be gentle. Thanks, gregg

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Just how unsafe is unsafe
by Andrew Wagner 08 Feb '09

08 Feb '09

So we all know the age-old rule of thumb, that unsafeXXX is simply evil and anybody that uses it should be shot (except when it's ok). I understand that unsafeXXX allows impurity, which defiles our ability to reason logically about haskell programs like we would like to. My question is, to what extent is this true? Suppose we had a module, UnsafeRandoms, which had a function that would allow you to generate a different random number every time you call it. The semantics are relatively well-defined, impurity is safely sectioned off in its own impure module, which is clearly labeled as such. How much damage does this do? Can we push the lines elsewhere? Is sectioning unsafeXXX into Unsafe modules a useful idiom that we can use for other things as well?

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