June 2010 - Haskell-Cafe

How does one get off haskell?
by Günther Schmidt 25 Jun '10

25 Jun '10

Hi list, I'm facing a really tough problem. About 3 years ago I stopped doing freelance and quite nicely paid projects in Java, PHP and C#. Now I'm dire straits, again, and need to get back into the project market which seems to have picked up again, quite a lot of projects out there and it looks like I could ask again for decent rates. (I personally call them compensation because I never ever enjoyed doing Java etc. but the money was good.) Anyway the problem is that I am totally reluctant to code in anything else but haskell. It has always been a problem to me getting up early in the morning, taking a train to work and coming back in the evening totally exhausted. But I think I could manage that again, at least for 3 or 6 months and then my bank account will be fine again and I can take it easy for another year or so. But this time all this is much harder. I really cannot see myself writing such huge amounts of code over and over again not doing much, well you know the story. BTW this is not meant as a fun post, I'm actually quite serious, ie. I need money, only way of getting it is doing Java, C# or PHP. So how does one get off haskell? Are there people in similar situations that have managed? How did you do it? Günther

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AusHac2010, the first Australian Haskell Hackathon is growing near, and we need you!
by Alex Mason 24 Jun '10

24 Jun '10

AusHac2010 will be held from the 16th to the 18th of July at UNSW, where we'll be working on various haskell projects such as: The LLVM backend for GHC (I'm going to see if we can somehow get in contact with David T during the hackathon so we can discuss what can be done) A Generic graph class The Accelerate and Repa libraries for fast array computations Hubris, the Ruby-Haskell bridge (Mark's done some really awesome work here, but I'm sure could use some help) Leksah, the Haskell IDE written in Haskell MPI bindings, to either bring the previous package alive from its bitrotten state, or create a new one from scratch A Notification library for things like Growl and libnotify A Library re-builder, extending haskell-updater for use with cabal-install, Arch, etc. If you can come for any of the days we'll be hacking, we'd love to see you! But you'll need to sign up first (so you can get access to the UNSW wireless network). If you're not sure if you can make it, sign up anyway, you're not obliged to come, but mark yourself as only possibly being able to come. The signup page can be found at: Sign up!: [http://axman6.wufoo.com/forms/aushac-2010-sign-up-20/] See the AusHac2010 wiki page [http://www.haskell.org/haskellwiki/AusHac2010] for more details on what we'll be doing, and various other details. We've already got 20-30 people coming hopefully, but the more we have the better. If you've already signed up using the previous form, you we still need you to fill out this new 2.0 version of the sign up form If you have questions, please feel free to email me or Ivan (our addresses are the reply-to addresses for this email... I hope), or talk to us (Axman6, ivanm) on #haskell. Cheers, Alex Mason

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Re: [Haskell-cafe] Question regarding deepseq (Control.DeepSeq)
by Frank Moore 24 Jun '10

24 Jun '10

Daniel, >This means *when meanOver2 is evaluated*, then evaluate (mean as). >Binding it in a let is lazy, so it won't be evaluated until it's needed >(for printing in this case). >Also note that (mean as) is a Double, so deepseq is just seq in this case >(but I suppose this is just a boiled down example and you also want to time >computations with results where deepseq does strictly more than seq). Thanks for the help, I think I understand now. This is indeed a boiled down example. I have some data that I am trying to compute in parallel, and I want to control how much evaluation to force. To do this, I was trying to use NFData and deepseq, since I thought bang patterns were a little clunky (but maybe they are less clunky than forcing evaluations with seq and deepseqs). Are there best practices for keeping track of which variables are bound where, when writing a complex program? >There are two standard ways to achieve what you want, >1. > let meanOver2 = ... > end <- meanOver2 `deepseq` getCurrentTime >2. put {-# LANGUAGE BangPatterns #-} at the top of the file and write > let !meanOver2 = ... > end <- getCurrentTime >The bang on meanOver2 means "evaluate this expression now (to weak head >normal form, i.e. to the outermost constructor)". Sometimes for me, whnf is not quite enough. For example, if I have a newtype P (with type constructor P) that is a for Map M Int. Is NFData and deepseq the way to do this, or should I be trying to implement my own Strategy along the lines of rwhnf in Control.Parallel.Strategies? >Another thing, for timing computations, wall-clock time is not appropriate, >better use >System.CPUTime.getCPUTime >to get only the CPU-time the process took, and not also what your browser >or whatever used in the meantime. Thanks for the tip. Frank > On Thu, Jun 24, 2010 at 9:38 PM, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: >> >> On Friday 25 June 2010 02:57:31, Frank Moore wrote: >> > Hello Haskellers, >> > >> > I am new to programming in Haskell and I am having trouble understanding >> > exactly when statements become evaluated. My goal is to try and measure >> > how long a computation takes without having to use a show function. The >> > code I am trying to use is below (taken in part from RWH chapter 25) >> > >> > ---------------------------------- >> > import Data.List (foldl') >> > import Data.Time.Clock (diffUTCTime, getCurrentTime) >> > import Control.DeepSeq (deepseq) >> > >> > mean :: [Double] -> Double >> > mean xs = s / fromIntegral n where >> > (n,s) = foldl' k (0,0) xs >> > k (n,s) x = n `seq` s `seq` (n+1,s+x) >> > >> > main = do >> > let as = [1..1e7] :: [Double] >> > start <- getCurrentTime >> > let meanOver2 = deepseq (mean as) `seq` (mean as) / fromIntegral 2 >> >> This means *when meanOver2 is evaluated*, then evaluate (mean as). >> Binding it in a let is lazy, so it won't be evaluated until it's needed >> (for printing in this case). >> Also note that (mean as) is a Double, so deepseq is just seq in this case >> (but I suppose this is just a boiled down example and you also want to time >> computations with results where deepseq does strictly more than seq). >> >> There are two standard ways to achieve what you want, >> >> 1. >> let meanOver2 = ... >> end <- meanOver2 `deepseq` getCurrentTime >> >> 2. put {-# LANGUAGE BangPatterns #-} at the top of the file and write >> >> let !meanOver2 = ... >> end <- getCurrentTime >> >> The bang on meanOver2 means "evaluate this expression now (to weak head >> normal form, i.e. to the outermost constructor)". >> >> > end <- getCurrentTime >> > putStrLn (show (end `diffUTCTime` start)) >> > putStrLn (show meanOver2) >> > ------------------------------------- >> >> Another thing, for timing computations, wall-clock time is not appropriate, >> better use >> >> System.CPUTime.getCPUTime >> >> to get only the CPU-time the process took, and not also what your browser >> or whatever used in the meantime. >> >> > >> > My understanding of deepseq was that it evaluates (mean as) completely >> > before continuing, and then the show would not take any time, but >> >> No, it evaluates (mean as) completely *when meanOver2 is demanded*, not >> before. >> >> > instead all the time is spent in the show meanOver2 function. I feel >> > like I am missing something fundamental here. Any suggestions? Thanks >> > for your help. >> > >> > Frank >> > > > > -- > Dr. W. Frank Moore > H. C. Wang Assistant Professor > Department of Mathematics, Cornell University > 310 Malott Hall, Ithaca NY 14853-4201, USA > > Office: Malott 587, Phone: +1 607 255 4030 > Email: frankmoore(a)math.cornell.edu -- Dr. W. Frank Moore H. C. Wang Assistant Professor Department of Mathematics, Cornell University 310 Malott Hall, Ithaca NY 14853-4201, USA Office: Malott 587, Phone: +1 607 255 4030 Email: frankmoore(a)math.cornell.edu

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Question regarding deepseq (Control.DeepSeq)
by Frank Moore 24 Jun '10

24 Jun '10

Hello Haskellers, I am new to programming in Haskell and I am having trouble understanding exactly when statements become evaluated. My goal is to try and measure how long a computation takes without having to use a show function. The code I am trying to use is below (taken in part from RWH chapter 25) ---------------------------------- import Data.List (foldl') import Data.Time.Clock (diffUTCTime, getCurrentTime) import Control.DeepSeq (deepseq) mean :: [Double] -> Double mean xs = s / fromIntegral n where (n,s) = foldl' k (0,0) xs k (n,s) x = n `seq` s `seq` (n+1,s+x) main = do let as = [1..1e7] :: [Double] start <- getCurrentTime let meanOver2 = deepseq (mean as) `seq` (mean as) / fromIntegral 2 end <- getCurrentTime putStrLn (show (end `diffUTCTime` start)) putStrLn (show meanOver2) ------------------------------------- My understanding of deepseq was that it evaluates (mean as) completely before continuing, and then the show would not take any time, but instead all the time is spent in the show meanOver2 function. I feel like I am missing something fundamental here. Any suggestions? Thanks for your help. Frank

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Tiger compiler: variable escaping analysis phase
by José Romildo Malaquias 24 Jun '10

24 Jun '10

Hello. I have been teaching an introductory course on compiler construction to our undergraduates students using Appel's "Modern Compiler Implementation in Java". There are also versions of the book in ML and C. The books explain how to write a compiler for the Tiger programming language. Now I want to implement a Tiger compiler in Haskell. The lexer and parser (built with the help of alex and happy) and the type checker are already working. Next step is implementing a variable escape analysis phase, needed for the intermediate code generator. The objective of this phase is to find out if each variable escapes or not. In general an escaping variable should be allocated in the stack frame (in memory). Non escaping variables may be allocated in the stack frame or in a register. Generally, a variable escapes if it is passed by reference, its address is taken (using C's & operator, for instance), or it is accessed from a nested function. Only the last is possible with Tiger. The approach adopted by Appel in his books is easy: a muttable field in the abstract syntax of variable declarations, of for expressions, and of function formal parameters, which introduce new variables, is used to collect the escaping information of the variable. In the ML version of the book this is a reference to boolean (bool ref). Initially, in a conservative approach, the reference is initialized to true. In the variable escaping analysis phase of the compiler, a function findEscape looks for escaping variables and record this information in the escape fields of the abstract syntax. To do this the entire abstract syntax tree is traversed looking for escaping uses of every variable, and, when found, the field is set to true. findEscape uses a symbol table to accomplish its work, binding variable names to a reference to boolean (the same reference used in the abstract syntax tree). When processing a variable declaraction, for instance, it inserts the variable name into the symbol table, binding it to its escaping field. When processing an expression which is a simple variable, if the variable occurs in a nested function, its binding in the symbol table is set to true. This reflects directly in the abstract syntax tree of the variable declaration, as the escape field in the variable declaration and the binding in the symbol table are the same reference to bool. I am look for good ways to implement the variable escaping analysis phase in Haskell, which is a pure language. I see two alternatives: 1) adopt the same approach as Appel used in his ML version of the compiler: use a mutable reference in the IO monad (Data.IORef) to hold the variable escaping information, and write everything inside the IO monad 2) build a new abstract syntax tree with updated information regarding variable escaping The second option is more elegant in my point of view, but would be much less efficient than the first one. So I want to know what advices Haskell programmers has to me about implementing this phase of the Tiger compiler in Haskell. Regards, Romildo -- Computer Science Department Universidade Federal de Ouro Preto, Brazil

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When the unknown is unknown
by Martin Drautzburg 24 Jun '10

24 Jun '10

Hello all, I am currently playing with Paul Hudak's Euterpea (a music program, formely called Haskore) and I am trying to teach it about rhythm. I said that a rhythm is a series of Moments (or Beats), each expressed as fractions of a bar. But each Moment also has volume. So I could model rhythm as Pairs of (Moment, Volume). However I certanly do not want to specify both the Moments and the Volume, but rather compute one from the other. Often the Moments are known and I need to compute the Volumes, but this is not always the case. I might as well start with the volume and compute the moments. The latter would be particularly interesting when trying to find rhythms which are suitable for certain lyrics. In that case I must even be prepared to find more than one series-of-moments which "fit" to the given series-of-volumes. There are countless other problems like this, e.g. when trying to match harmony, melody and tension. In that case I even have three variables and I may want to specify tension first, then melody and have the harmony computed. At first glance this looks like a Prolog-like problem to me. I could say that certain things are always true for [(Moment, Volume)] and let an inference engine figure out the options which are still in consideration. From which angle would you approach problems like this? Should I get my hands on a prolog-in-haskel implementation (which indeed seems to exist)? Or should I roll my own poor-man's prolog? Or is this a constraint-satisfaction-problem? Or is there even a more straight-forward more haskellish pattern, which solves such problems? Any pointers would be much appreciated. -- Martin

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Continuations and coroutines
by Yves Parès 24 Jun '10

24 Jun '10

Hello, I saw on the haskell wikibook that coroutines could be implemented by using continuations : http://en.wikibooks.org/wiki/Haskell/Continuation_passing_style#Example:_co…, the section is empty) Since I'm actually learning the wonders of continuations, I just wonder : how ?

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ANN: statechart-0.0.0
by Tom Hawkins 24 Jun '10

24 Jun '10

Statechart [1] is a program that compiles Rhapsody [2] statechart diagrams [3] into C. Rhapsody is a UML <cough, choke, gag...> tool from IBM intended for embedded systems development. If you use Rhapsody, and its code generator makes your eyes bleed, statechart may provide some relief. -Tom [1] http://hackage.haskell.org/package/statechart [2] http://www-01.ibm.com/software/rational/products/rhapsody/developer/ [3] http://en.wikipedia.org/wiki/State_diagram#Harel_statechart

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Unknown symbol: __fixunsdfdi
by Michael Snoyman 24 Jun '10

24 Jun '10

Hi all, I'm trying to get some code to compile on Windows, and have run into an unknown symbol. Has anyone else run into an unknown __fixunsdfdi? I'm using GHC 6.12.3. I'm trying to use my persistent-sqlite package. To reproduce, cabal install persistent-sqlite and then try running the example program[1]. Thanks, Michael [1] http://github.com/snoyberg/persistent/raw/ef31e6ddece659b1b8240756a1a47360f…

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Network of GUI Controls - using MonadFix?
by Günther Schmidt 24 Jun '10

24 Jun '10

Hello list, let's say I would like to create 4 buttons where any one button can respond to a click and notify every other button. Ie. where the 1st button is wired with the 2nd, the 2nd with the 3rd ... and the 4th button with the 1st. In short I would like these buttons to be connected as in a cyclic list. Is that something that MonadFix is meant to be used for? Günther

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