March 2009 - Haskell-Cafe

RE: Hashing over equivalence classes
by Galchin, Vasili 14 Mar '09

14 Mar '09

Hi Roman, So are you really talking about an equivalence relation on the function's domain? The reason I ask is that it is well known that "f(a)=f(b)" establishes an equivalence relation on f's co-domain! Vasili

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Does anybody dislike implicit params as much as I do?
by Thomas Hartman 14 Mar '09

14 Mar '09

http://blog.patch-tag.com/2009/03/09/implicitparams-are-evil-thoughts-on-ad… I understand there are arguments for using IPs, but after this experience, the ImplicitParams extension is a "code smell" for me.

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Style Guide for Haskell Code
by Manlio Perillo 14 Mar '09

14 Mar '09

After a quick search with Google, it seems that there is not yet an "official" document for Style Guide for Haskell Code. I was only able to found: http://www.cs.caltech.edu/courses/cs11/material/haskell/misc/haskell_style_… http://urchin.earth.li/~ian/style/haskell.html http://github.com/tibbe/haskell-style-guide/tree/master and, of course, the GHC style guide. Any plans for a common document? Thanks Manlio Perillo

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Proof of induction case of simple foldl identity.
by R J 14 Mar '09

14 Mar '09

Can someone provide the induction-case proof of the following identity: foldl (-) ((-) x y) ys = (foldl (-) x ys) - y If foldl is defined as usual: foldl :: (b -> a -> b) -> b -> [a] -> b foldl f e [] = e foldl f e (x : xs) = myFoldl f (f e x) xs The first two cases, _|_ and [], are trivial. Here's my best attempt at the (y : ys) case (the left and right sides reduce to expressions that are obviously equal, but I don't know how to show it): Case (y : ys). Left-side reduction: foldl (-) ((-) x y) (y : ys) = {second equation of "foldl"} foldl (-) ((-) ((-) x y) y) ys = {notation} foldl (-) ((-) (x - y) y) ys = {notation} foldl (-) ((x - y) - y) ys = {arithmetic} foldl (-) (x - 2 * y) ys Right-side reduction: (foldl (-) x (y : ys)) - y = {second equation of "foldl"} (foldl (-) ((-) x y) ys) - y = {induction hypothesis: foldl (-) ((-) x y) ys = (foldl (-) x ys) - y} ((foldl (-) x ys) - y) - y = {arithmetic} (foldl (-) x ys) - 2 * y Thanks as always. _________________________________________________________________ Express your personality in color! Preview and select themes for Hotmail®. http://www.windowslive-hotmail.com/LearnMore/personalize.aspx?ocid=TXT_MSGT…

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Re: [Haskell-cafe] Natural Numbers: Best implementation?
by Henning Thielemann 14 Mar '09

14 Mar '09

On Fri, 13 Mar 2009, Mark Spezzano wrote: > 1. Don’t bother. Just use Integer. > > 2. Use the type > > data Natural = Zero | Succ !Natural > > 3. Use the following definition taken from the Gentle Introduction to Haskell 98 > > newtype Natural = MakeNatural Integer This option looks like non-negative package.

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What's new on Hackage this week? March 14, 2009
by Don Stewart 14 Mar '09

14 Mar '09

http://archhaskell.wordpress.com/2009/03/14/arch-haskell-news-mar-14-2009/ A regular update of Haskell in Arch Linux Arch now has 974 Haskell packages in AUR. That’s 12 new packages this week, and lots of updates as well. See the blog for the full list of updates. -- Don

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Microsoft PhD Scholarship at Strathclyde
by Conor McBride 14 Mar '09

14 Mar '09

Apologies for crossposting. Please forward this message to individuals or lists who may be interested. In addition to the recently advertised PhD position at Strathclyde on "Reusability and Dependent Types", I am delighted to advertise the following PhD opportunity. {--------------------------------------------------------- -- Haskell Types with Numeric Constraints ---------------- ---------------------------------------------------------} We are grateful to Microsoft Research for their sponsorship of this project, which includes an internship, and with it the chance to make a real difference to world of principled but practical programming. The project investigates the practical and theoretical impact of extending Haskell's type system with numeric expressions (representing sizes, or ranges, or costs, for example) and constraints capturing richer safety properties than are currently managed by static typing. It has three strands: (1) to investigate type inference with numeric constraints, (2) to investigate new programming structures, patterns, and techniques which exploit numeric indexing, and (3) to study the performance benefits derivable from richer guarantees. A bright student could bring significant benefits to developers using Haskell, a language with increasing industrial traction — not least at Microsoft. Work on the Glasgow Haskell Compiler, at Strathclyde! {---------------------------------------------------------} The position is fully funded, covering stipend, fees (at the home/EU rate), equipment, and travel, starting in October 2009. The closing date for applications is 15th April 2009. For further details, see: http://personal.cis.strath.ac.uk/~conor/phds/ or email me (conor(a)cis.strath.ac.uk) I look forward to hearing from you. Yours &c Conor McBride

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building c-callable shared objects and dlls with Haskell
by Anton Tayanovskyy 14 Mar '09

14 Mar '09

Hello, What's the current best practice to build shared objects / dlls to call Haskell from C? I think I figured out the basics, but I would like to understand the picture completely and document the current state of affairs in the Haskell wiki. In particular, I'd like to make sure that the shared libraries are usable from multiple applications, and ideally that applications can dynamically link to multiple shared libraries built from Haskell. I was trying to do that to call Pandoc from C, and the information was rather scattered. Here are two links that I used: http://www.haskell.org/haskellwiki/Calling_Haskell_from_C http://www.haskell.org/ghc/docs/latest/html/users_guide/win32-dlls.html Based on that information, I was able to compile and run small examples. However, I'm not sure I completely understand how it works in production. Specifically, for the UNIX version, I followed a recipe for loading and unloading the Haskell runtime with library constructors/destructors: #include <HsFFI.h> static void library_init(void) __attribute__((constructor)); static void library_init(void) { hs_init(..); } static void library_exit(void) __attribute__((destructor)); static void library_exit(void) { hs_exit(); } Does this work correctly? Another concern is that every library compiled this way will include its own copy of the Haskell runtime. Will there be problems when using multiple libraries? For the Windows counterpart, there is this very suspicious code I adapted from the GHC user guide: #include <windows.h> #include <Rts.h> extern void __stginit_LibPandoc(void); BOOL STDCALL DllMain( HANDLE hModule, DWORD reason, void* reserved) { if (reason == DLL_PROCESS_ATTACH) { static char* args[] = { "ghcDll", NULL }; startupHaskell(1, args, __stginit_LibPandoc); } return TRUE; } It does not seem to unload the Haskell runtime, indeed the Guide warns that it is unsafe to do so from DllMain. What are the implications of this? Thanks! --A

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broken IO support in uvector package, when using non primitive types
by Manlio Perillo 14 Mar '09

14 Mar '09

Hi. I'm sure this is a know bug, but I can't find details with Google. The offending code is here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=2362 When I execute the program I get: uio: readChunkBU: can't read What's the problem? I'm using uvector from: http://code.haskell.org/~dons/code/uvector/ Thanks Manlio Perillo

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Re: [Haskell-cafe] Using a monad to decompose a function into functions
by Marcin Kosiba 14 Mar '09

14 Mar '09

On Friday 13 March 2009, you wrote: > 2009/3/13 Marcin Kosiba <marcin.kosiba(a)gmail.com>: > > On Thursday 12 March 2009, you wrote: > >> 2009/3/12 Marcin Kosiba <marcin.kosiba(a)gmail.com>: > >> > Hi, > >> > I'm doing a bit of research into mobility models and I'm > >> > currently exploring implementation language choices for the simulator > >> > (*snip*) > >> > The simulation algorithm requires expressing > >> > the node's mobility so that it is "stateless". The mobility model > >> > algorithm's type should be something like: > >> > mobility_model :: WorldState -> NodeState -> OtherInput -> (Action, > >> > NodeState) > >> > > >> > where Action can alter WorldState and the second NodeState is an > >> > altered input NodeState. I perform a form of speculative execution on > >> > mobility_model so sometimes I need to backtrack to a previous world > >> > and node state. This is all fairly simple stuff, and was just an > >> > introduction. What I do now is store an enum in NodeState and > >> > implement mobility_model as one big case statement. Yes, this is very > >> > imperative of me, I know. What I'd like to do is to express > >> > mobility_model, so that the code would look like: > >> > > >> > mobility_model world node input = do > >> > do_calculus > >> > emit_action > >> > if something > >> > then emit_action > >> > else emit_action > >> > do_calculus > >> > emit_action > >> > mobility_model world node input > >> > >> Hi, > >> > >> It seems you can use > >> http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control- > >>Mon ad-State-Lazy.html Just have a look at the exemple : > >> > >> tick :: State Int Int > >> tick = do n <- get > >> put (n+1) > >> return n > >> > >> your code would become something like > >> mobility_model :: OtherInput -> State (WorldState,NodeState) () > >> mobility_model input = do > >> world <- gets fst > >> node <- gets snd > >> .... > >> let (world',node') = ... > >> put (world',node') > > > > ok, that solves part of the problem. what this doesn't solve is that > > somewhere between these lines (which corespond to emit_action in my > > example) > > > >> let (world',node') = ... > >> put (world',node') > > > > I need to return a value and an Action and NodeState to the simulation > > algorithm. and then, after the simulation algorithm calculates a new > > WorldState it will want the mobility_model to where it left off, but with > > a new WorldState. > > > > I hope I'm clear about what I wish to achieve: each emit_action should > > return a value (Action, NodeState) and maybe a function > > mobility_model_cont which I then could call with the new WorldState to > > continue from where emit_action returned. > > I'm not entirely sure ... but I think it doesn't matter that much :) > Here is why. > > This was just an exemple : > mobility_model :: OtherInput -> State (WorldState,NodeState) () > > You could also have > mobility_model :: OtherInput -> NodeState -> State WorldState > (NodeState,Action) > or whatever. > > In fact, the State monad makes it easy to thread (in this context, it > means 'pass around') an argument to many functions, providing a nice > syntax reminiscent of imperative language. But it lets you completely > free of what is passed around. It depends on what you want to be > explicitely passed by argument, and what you want to pass in the state > of the monad (that is, what you want to appear, inside the monad only, > as some global variable). > > So in your code, if you often need to pass a WorldState to a function > which should return a modified WorldState, it makes sense to put > WorldState inside the state monad. But, maybe, if there is just a few > functions which act on NodeState, it has not to be part of the state > carried by the state monad. > > I'm not entirely sure of what is a problem to you : is it the use of > the State monad, or something else ? > If it can help you to formulate your question you can post some code > (or past it to http://hpaste.org/) Threading the state is not the problem. Maybe this will help: what I have now: fsm world state = case state of first -> do_stuff_one (move_up, succ state) second -> do_stuff_two (move_left, succ state) third -> do_stuff_three (move_right, first) what I'd want to have is to say: fsm world state = do do_stuff_one yield move_up do_stuff_two yield move_left do_stuff_three yield move_right fsm world state and have it "translated" to: fsm world state = do_stuff_one (move_up, \world' state' -> do_stuff_two (move_left, \world'' state'' -> do_stuff_three (move_right, fsm world'' state'') Thanks! Marcin Kosiba

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