Re: [Haskell-cafe] Re: [Haskell] Swapping parameters and type classes
On 9/18/07, Simon Marlow [EMAIL PROTECTED] wrote: Ian Lynagh wrote: I proposed renaming haskell@ - haskell-announce@ haskell-cafe@ - haskell@ [snip] but now I have to admit I think haskell-cafe is a big win for the community. To me this suggests renaming haskell@ to haskell-announce@ while leaving haskell-cafe@ as is. Jim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Missing Symbol (2)
Hi Here's another one: f [] = v f (x:xs) = x . f xs The . is supposed to denote a generic operator - f [] maps to some value v and applied to non-empty list applies head to ? operator. Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
Paul, The Hutton book has gotten great reviews, but pouring over a faulty OCRd version can only be counterproductive. I highly recommend Yet Another Haskell Tutorial[1]. After that you can check out all the other wonderful online resources (starting here [2]). At that point a few OCR errors won't be able to stand in your way! Best wishes on you Haskell explorations, Joel [1] http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf [2] http://haskell.org/haskellwiki/Learning_Haskell ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
The book is very well written. I did try to follow the wikibook thingummajig but found the English a bit trying. Cheers Paul t 07:43 19/09/2007, you wrote: Paul, The Hutton book has gotten great reviews, but pouring over a faulty OCRd version can only be counterproductive. I highly recommend Yet Another Haskell Tutorial[1]. After that you can check out all the other wonderful online resources (starting here [2]). At that point a few OCR errors won't be able to stand in your way! Best wishes on you Haskell explorations, Joel [1] http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf [2] http://haskell.org/haskellwiki/Learning_Haskell ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] Functor ((,) a)
Janis Voigtlaender: What do I have to import to get the Functor ((,) a) instance? (redirected to haskell-cafe) Control.Monad.Instances, believe it or not. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] help me ! who has vty ?
who has vty for haskell? I can't find the right one so who can mail one to me ? thanks 2007-09-19 clisper ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Library Process (was Building production stable software in Haskell)
Sven Panne wrote: On Tuesday 18 September 2007 09:44, Dominic Steinitz wrote: This discussion has sparked a question in my mind: What is the process for the inclusion of modules / packages in ghc, hugs and other compilers interpreters? Personal interest of the people working on GHC et. al. ;-) I thought the master plan was that less would come with the compiler / interpreter and the user would install packages using cabal. [...] Although this statement might be a bit heretical on this list, I'll have to repeat myself again that Cabal, cabal-install, cabal-whatever will *never* be the right tool for the end user to install Haskell packages on platforms with their own packaging systems like RPM (the same holds for other systems, I just use RPM as an example here). I think you're identifying a non-problem here. Cabal was never intended to be used instead of the system's packaging tools for installing packages globally on the system, if you look back through the original Cabal design discussions you'll see this. We recognised the critical importance of working with, rather than around, emerge/ports/RPM/apt/whatever. Nowadays from a Cabal package you get make an RPM, a Windows installer, and the Gentoo folks have imported the entirety of Hackage. This is how it's meant to work. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Library Process (was Building production stable software in Haskell)
Building Debian packages seems a bit hard to do for laymen like me. Anyone have a script that takes a .cabal file, some additional meta data and creates a .deb package? It would be really nice if we could make this procedure easier. Every time I read some documentation on how to create a Debian package, I feel like crying. ;) Cheers, Johan On 9/19/07, Simon Marlow [EMAIL PROTECTED] wrote: Sven Panne wrote: On Tuesday 18 September 2007 09:44, Dominic Steinitz wrote: This discussion has sparked a question in my mind: What is the process for the inclusion of modules / packages in ghc, hugs and other compilers interpreters? Personal interest of the people working on GHC et. al. ;-) I thought the master plan was that less would come with the compiler / interpreter and the user would install packages using cabal. [...] Although this statement might be a bit heretical on this list, I'll have to repeat myself again that Cabal, cabal-install, cabal-whatever will *never* be the right tool for the end user to install Haskell packages on platforms with their own packaging systems like RPM (the same holds for other systems, I just use RPM as an example here). I think you're identifying a non-problem here. Cabal was never intended to be used instead of the system's packaging tools for installing packages globally on the system, if you look back through the original Cabal design discussions you'll see this. We recognised the critical importance of working with, rather than around, emerge/ports/RPM/apt/whatever. Nowadays from a Cabal package you get make an RPM, a Windows installer, and the Gentoo folks have imported the entirety of Hackage. This is how it's meant to work. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] help me ! who has vty ?
Hi who has vty for haskell? I can't find the right one so who can mail one to me ? http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vty-3.0.0 hackage has a lot of packages, and is always a good place to start a search. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] Functor ((,) a)
On 9/19/07, Janis Voigtlaender [EMAIL PROTECTED] wrote: BTW, what would have been the easiest way for me to find this out on my own? The following is probably not the easiest way: I keep a copy of the sources of GHC and the libraries [1] on my disk. When I want to search for something I simply do a grep. In emacs you also have a grep-find function which performs a grep in the background and shows the result in a new buffer. The results are hyperlinked so you can just click on a result and emacs will open the respected file and jump to the found string. In your case you do a 'M-x grep-find' followed by the search string: instance Functor ((,). This will result in: ./Control/Monad/Instances.hs:19:instance Functor ((-) r) where ./_darcs/pristine/Control/Monad/Instances.hs:19:instance Functor ((-) r) where I think you can get rid of the annoying _darcs... by following: [3]. I haven't tried that though. (Note I'm on Linux. I don't know if this works in Windows. It probably does with Cygwin.) Of course the disadvantage of this approach is that you have to know exactly what you are looking for. Then there's also Hoogle [3] with which you can search for names or types. However in your case this wouldn't have helped. regards, Bas [1] http://hackage.haskell.org/trac/ghc/wiki/Building/GettingTheSources [2] http://wiki.darcs.net/DarcsWiki/HintsAndTips#head-c189bd259a3d141796ebb4f8d58b8a7664c52ece [3] http://haskell.org/hoogle ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Why isn't pattern matching lazy by default?
I'm experimenting with functional reactive programming for creating simple 2D/3D video games and interactive apps, trying to develop my own version of it from scratch, for learning Haskell. I got stuck with an endless loop when trying to split a stream into a pair of two streams (a kind of reactive if/then/else). Luckily I first read the Haskell School of Expression so I remembered that pattern matching is not lazy and this could be the cause, which it was (I had to replace (x:xs) by ~(x:xs)) I could also fix the problem by not using pattern matching at all, using head/tail calls instead. Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. Thanks, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
On 9/19/07, Peter Verswyvelen [EMAIL PROTECTED] wrote: Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. AFAIK, pattern matches are desugared to cases, so f (x:xs) = rhs f [] = rhs' is equivalent to f y = case y of (x:xs) - rhs [] - rhs' Case's have to be strict because that's how we look inside the values in Haskell =). Of course I may be somehow mistaken here, as I am learning Haskell, too. -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Hi Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. f ~(x:xs) = rhs f ~[] = rhs' Now guess what f [] does... If you use a where binding then pattern matching is lazy. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] transparent parallelization
The problem with Haskell is not finding opportunities to parallelize, they are legion. Actually, quite the opposite, there's so much that your code ends up slower than a sequential realization. The hard part is making a good cost-model and a good way to create coarser chunks of work. It's not worthwhile to spawn a thread (even a very lightweight one) for virtually every subexpression. Automatic parallelization is easy, efficient parallelization is hard. Absolutely. We had another crack at this recently in an upcoming ICFP paper (draft online at http://research.microsoft.com/~tharris/papers/2007-fdip.pdf). In that paper we start out by collecting profiles of the thunk creation, entry, and update operations for a set of benchmarks and conduct a limit study of how fast these dependencies would allow them to be evaluated (e.g. ignoring interactions through the GC, cache effects, the costs of sparking thunks etc.). We then make the system increasingly more practical, (i) setting a lower bound on the compute-time of thunks that we spark, (ii) making predictions of a thunk's compute-time when it's allocated, and then (iii) building a real implementation based on GHC 6.6. The parallel speedup dwindles at each stage: this kind of automated approach looks plausible for using 2n cores instead of using n, but I'm sceptical about it being able to efficiently exploit n cores instead of 1. Tim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Mmm, yes of course... blush... But shouldn't f ~(x:xs) = rhs give a compile-time error since neither x nor xs is used in the right hand side, and hence nothing will ever get pattern matched when this function is called, which clearly indicates a mistake? That is, if I understand lazy pattern matching correctly... And then in these cases the user would have to annotate the pattern match as being strict, so he is aware of the eager evaluation taking place Oh well, the way it is now is also easy to get used to, one just has to know how it works (just like M-theory ;-) ) Cheers, Peter Neil Mitchell wrote: Hi Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. f ~(x:xs) = rhs f ~[] = rhs' Now guess what f [] does... If you use a where binding then pattern matching is lazy. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Hi Peter, Mmm, yes of course... blush... But shouldn't f ~(x:xs) = rhs give a compile-time error since neither x nor xs is used in the right hand side, and hence nothing will ever get pattern matched when this function is called, which clearly indicates a mistake? That is, if I understand lazy pattern matching correctly... And then in these cases the user would have to annotate the pattern match as being strict, so he is aware of the eager evaluation taking place Well if you defined f: f ~(x:xs) = 1 + 2 f ~[] = 42 Then you will get a warning stating that the pattern matches of (x:xs) and [] are overlapped. It may not be a mistake though, so possibly a bold error for the compiler to throw, it just means that 1+2 will always be evaulated no matter what list you throw at it (provided of course that the result of f is needed to evaluate the rest of the program). It's interesting to note that if you had: f ~(x:xs) = x + 2 f ~[] = 42 Then f [] would give a complie error: Irrefutable pattern failed for pattern (x : xs) Hope that gives some insight. Chris. Oh well, the way it is now is also easy to get used to, one just has to know how it works (just like M-theory ;-) ) Cheers, Peter Neil Mitchell wrote: Hi Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. f ~(x:xs) = rhs f ~[] = rhs' Now guess what f [] does... If you use a where binding then pattern matching is lazy. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
Joel YAHT is full of symbols which cannot be rendered by the Adobe text extractor. I appreciate your kind advice but you must also bear in mind that I am using the Hutton book for a reason. Did you not think it'd occurred to me also that there were other tutorials out there? Sincerely, Paul At 08:28 19/09/2007, you wrote: The book is very well written. I did try to follow the wikibook thingummajig but found the English a bit trying. Cheers Paul t 07:43 19/09/2007, you wrote: Paul, The Hutton book has gotten great reviews, but pouring over a faulty OCRd version can only be counterproductive. I highly recommend Yet Another Haskell Tutorial[1]. After that you can check out all the other wonderful online resources (starting here [2]). At that point a few OCR errors won't be able to stand in your way! Best wishes on you Haskell explorations, Joel [1] http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf [2] http://haskell.org/haskellwiki/Learning_Haskell ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
This looks like foldr, specifically: f = foldr (.) v so really, there's no way of telling what the operator could be, other than that: (.) :: a - b - b where f :: [a] - b v :: b BTW, I find that most of your emails are in a very large font size. Do other people notice this? It's not very nice to read. Gmail's 'Show Original' feature tells me that the HTML version of your message contains font size=3 near the start. On 19/09/2007, PR Stanley [EMAIL PROTECTED] wrote: Hi Here's another one: f [] = v f (x:xs) = x . f xs The . is supposed to denote a generic operator - f [] maps to some value v and applied to non-empty list applies head to ? operator. Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
f ~(x:xs) = x + 2 f ~[] = 42 Then f [] would give a complie error: Irrefutable pattern failed for pattern (x : xs) Sorry, that should be *runtime* error! Chris. Hope that gives some insight. Chris. Oh well, the way it is now is also easy to get used to, one just has to know how it works (just like M-theory ;-) ) Cheers, Peter Neil Mitchell wrote: Hi Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. f ~(x:xs) = rhs f ~[] = rhs' Now guess what f [] does... If you use a where binding then pattern matching is lazy. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Library Process (was Building production stable software in Haskell)
On Wed, Sep 19, 2007 at 11:05:05AM +0200, Johan Tibell wrote: Building Debian packages seems a bit hard to do for laymen like me. Anyone have a script that takes a .cabal file, some additional meta data and creates a .deb package? It would be really nice if we could make this procedure easier. Every time I read some documentation on how to create a Debian package, I feel like crying. ;) http://urchin.earth.li/pipermail/debian-haskell/2007-June/000322.html Thanks Ian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] (win)hipe for Haskell?
During a googling session, I can across (Win)HIPE, a visualization program for the functional language HOPE. See http://dalila.sip.ucm.es/~cpareja/winhipe IMHO a similar tool would be a nice for learning/teaching Haskell; does that exist, or something else that comes close? Thanks, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (win)hipe for Haskell?
Hi Peter, During a googling session, I can across (Win)HIPE, a visualization program for the functional language HOPE. See http://dalila.sip.ucm.es/~cpareja/winhipe IMHO a similar tool would be a nice for learning/teaching Haskell; does that exist, or something else that comes close? Yes, Hat does this http://www.haskell.org/hat/ (if you can get it to work, I typically have little success) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (win)hipe for Haskell?
Yes, Hat does this http://www.haskell.org/hat/ (if you can get it to work, I typically have little success) Thanks. WinHIPE uses graphics and animation. If briefly encountered Hat before, but I had the impression it did not visualize the graphs using graphics, only text. Is this correct? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (win)hipe for Haskell?
Hi Yes, Hat does this http://www.haskell.org/hat/ (if you can get it to work, I typically have little success) Thanks. WinHIPE uses graphics and animation. If briefly encountered Hat before, but I had the impression it did not visualize the graphs using graphics, only text. Is this correct? Yes, mainly. There is a graphical user interface for Hat, but its not very well developed. http://www-users.cs.york.ac.uk/~ndm/hat/ Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
It seems GHC does give a warning at compile-time about it, so you did get it right the first time :-) Well the warning happens at compile time certainly. But the irrefutable pattern error only occurs at runtime. cmb21$ ghc --make Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Main.hs:3:0: Warning: Pattern match(es) are overlapped In the definition of `f': f ~[] = ... Linking Main ... cmb21$ ./a.out a.out: Main.hs:(3,0)-(4,10): Irrefutable pattern failed for pattern (x : xs) Cheers, Chris. Thanks for the info, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
It's probably \oplus then. The font size according to my trusty screen reader is 11 point which is not particularly large. Still, if anybody else finds it annoying I'll reduce it to 9pt. Paul At 13:21 19/09/2007, you wrote: This looks like foldr, specifically: f = foldr (.) v so really, there's no way of telling what the operator could be, other than that: (.) :: a - b - b where f :: [a] - b v :: b BTW, I find that most of your emails are in a very large font size. Do other people notice this? It's not very nice to read. Gmail's 'Show Original' feature tells me that the HTML version of your message contains font size=3 near the start. On 19/09/2007, PR Stanley [EMAIL PROTECTED] wrote: Hi Here's another one: f [] = v f (x:xs) = x . f xs The . is supposed to denote a generic operator - f [] maps to some value v and applied to non-empty list applies head to ? operator. Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] downtime for monk ({darcs, hackage, cvs}.haskell.org) Monday from 3pm UTC
On Wed, Sep 19, 2007 at 09:16:56AM +0100, Bayley, Alistair wrote: From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Ian Lynagh The machine called monk ... will be down from 3pm UTC on Monday 24th for an OS and RAM upgrade. Do you have any estimate for how long the upgrades will take? That depends on how many things go wrong. I'll send an e-mail when we believe it's all up and working again. Thanks Ian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
as provided, this won't compile because v hasn't been defined. f [] = 0 f (x:xs) = x . f xs compiles but gives f :: (Num (a - c)) = [c - c] - a - c which seems wrong. maybe I'm not understanding what v is not supposed to be. I am thinking maybe you want iterate *Main take 5 $ iterate (+1) 0 [0,1,2,3,4] It would be easier to understand your desired function if you would provide an example of desired usage. t. PR Stanley [EMAIL PROTECTED] Sent by: [EMAIL PROTECTED] 09/19/2007 02:17 AM To haskell-cafe@haskell.org cc Subject [Haskell-cafe] Missing Symbol (2) Hi Here's another one: f [] = v f (x:xs) = x . f xs The . is supposed to denote a generic operator - f [] maps to some value v and applied to non-empty list applies head to ? operator. Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] Functor ((,) a)
On Wed, Sep 19, 2007 at 10:24:50AM +0200, Janis Voigtlaender wrote: BTW, what would have been the easiest way for me to find this out on my own? Somehow, I would have hoped to be able to navigate to the appropriate point from the mentioned place in the online docs, where it is stated that the instance in question exists. Usually the instance for C (T a1 ... an) should be in either the module defining C or the one defining T (so if you import both you have it); if T is from the Prelude, then the instance should be together with C. But Control.Monad.Instances is a special case, because exists to hold instances of Haskell 98 classes for H98 types that are not defined in H98. Putting them in the proper place would break compatibility with H98. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
Hi Paul, Sorry that my suggestion wasn't so practicable. On the other hand, I see that the YAHT tex source is available on darcs.haskell.org/yaht. Maybe we can do something about those unextractable symbols. Anyone know more about this? It really is a great resource for beginners. -- Joel On 9/19/07, PR Stanley [EMAIL PROTECTED] wrote: Joel YAHT is full of symbols which cannot be rendered by the Adobe text extractor. I appreciate your kind advice but you must also bear in mind that I am using the Hutton book for a reason. Did you not think it'd occurred to me also that there were other tutorials out there? Sincerely, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
Hi Paul, Sorry that my suggestion wasn't so practicable. On the other hand, I see that the YAHT tex source is available on darcs.haskell.org/yaht. Maybe we can do something about those unextractable symbols. Anyone know more about this? It really is a great resource for beginners. -- Joel On 9/19/07, PR Stanley [EMAIL PROTECTED] wrote: Joel YAHT is full of symbols which cannot be rendered by the Adobe text extractor. I appreciate your kind advice but you must also bear in mind that I am using the Hutton book for a reason. Did you not think it'd occurred to me also that there were other tutorials out there? Sincerely, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
C.M.Brown wrote: f ~(x:xs) = x + 2 f ~[] = 42 Then f [] would give a complie error: Irrefutable pattern failed for pattern (x : xs) Sorry, that should be *runtime* error! Chris. It seems GHC does give a warning at compile-time about it, so you did get it right the first time :-) Thanks for the info, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
On Wed, 19 Sep 2007, Peter Verswyvelen wrote: I got stuck with an endless loop when trying to split a stream into a pair of two streams (a kind of reactive if/then/else). Luckily I first read the Haskell School of Expression so I remembered that pattern matching is not lazy and this could be the cause, which it was (I had to replace (x:xs) by ~(x:xs)) ... Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. It's even more confusing that pattern matching in 'let' _is_ lazy. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Missing Symbol (2)
YAHT is available from wikibooks in HTML form. 19.09.2007, в 18:30, Joel Koerwer писал(а): Hi Paul, Sorry that my suggestion wasn't so practicable. On the other hand, I see that the YAHT tex source is available on darcs.haskell.org/yaht. Maybe we can do something about those unextractable symbols. Anyone know more about this? It really is a great resource for beginners. -- Joel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. It's even more confusing that pattern matching in 'let' _is_ lazy. No, it's not. See, in let or where constructs you don't have a choice; you can't do different things depending on whether some value is Just x or Nothing. Therefore, there is no need to perform pattern matching strictly. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
2007/9/19, Miguel Mitrofanov [EMAIL PROTECTED]: Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. It's even more confusing that pattern matching in 'let' _is_ lazy. No, it's not. See, in let or where constructs you don't have a choice; you can't do different things depending on whether some value is Just x or Nothing. Therefore, there is no need to perform pattern matching strictly. In other words, the pattern matching in case can't be lazy by default since the primary purpose of case is to choose an alternative and for that you NEED to evaluate the matched value. In let or where, the pattern matching is only there to allow convenient deconstruction of the value, but you don't make any choice based on the form of the value, so you can afford to be lazy (which is the default in the language everywhere it makes sense, it doesn't in a normal case construct). If you want to use your pattern matching in a case only to deconstruct the value but not to choose an alternative, you need to specify it using ~ . So, IMHO, the rules on strictness of the pattern matching are perfectly consistent and clear. You just need to reflect on the purpose of the constructs where you use it.. -- Jedaï ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
On Wed, 19 Sep 2007, Miguel Mitrofanov wrote: Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. It's even more confusing that pattern matching in 'let' _is_ lazy. No, it's not. See, in let or where constructs you don't have a choice; you can't do different things depending on whether some value is Just x or Nothing. Therefore, there is no need to perform pattern matching strictly. Then why are patterns in lambdas not lazy? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
It's even more confusing that pattern matching in 'let' _is_ lazy. Then why are patterns in lambdas not lazy? I'm learning a lot here! :-) Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Henning Thielemann wrote: Then why are patterns in lambdas not lazy? Because they should allow for more branches! ;-)) null = \ [] - True _ - False -- Zun. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
On 9/19/07, Roberto Zunino [EMAIL PROTECTED] wrote: Henning Thielemann wrote: Then why are patterns in lambdas not lazy? Because they should allow for more branches! ;-)) null = \ [] - True _ - False See http://hackage.haskell.org/trac/haskell-prime/ticket/114 for a relevant proposal for Haskell Prime regarding something like this. Bas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Hi Miguel, See, in let or where constructs you don't have a choice; you can't do different things depending on whether some value is Just x or Nothing. Therefore, there is no need to perform pattern matching strictly. This is not entirely true. This is actually one of those niches in Haskell where the left to right is not quite the same as right to left. A let can be converted to a where but the other way round may require a case introduction. So just like you can define: f (Just x) = x f Nothing = error Nothing You can also define: f x = g x where g (Just x) = x g Nothing = error Nothing g is strict in its first argument. Declared in a let it would look like: f x = let g x = case x of (Just y) - y Nothing - error Nothing in g x Again, g must be strict in its first argument. Chris. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Chaddaï Fouché wrote: 2007/9/19, Miguel Mitrofanov [EMAIL PROTECTED]: Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. It's even more confusing that pattern matching in 'let' _is_ lazy. No, it's not. See, in let or where constructs you don't have a choice; you can't do different things depending on whether some value is Just x or Nothing. Therefore, there is no need to perform pattern matching strictly. In other words, the pattern matching in case can't be lazy by default since the primary purpose of case is to choose an alternative and for that you NEED to evaluate the matched value. In let or where, the pattern matching is only there to allow convenient deconstruction of the value, but you don't make any choice based on the form of the value, so you can afford to be lazy (which is the default in the language everywhere it makes sense, it doesn't in a normal case construct). If you want to use your pattern matching in a case only to deconstruct the value but not to choose an alternative, you need to specify it using ~ . So, IMHO, the rules on strictness of the pattern matching are perfectly consistent and clear. You just need to reflect on the purpose of the constructs where you use it.. Except that newtype deconstruction doesn't introduce strictness - and you can't necessarily see from the syntax of a pattern-match whether it's newtype or data. Isaac ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
On Wednesday 19 September 2007, C.M.Brown wrote: g is strict in its first argument. Declared in a let it would look like: f x = let g x = case x of (Just y) - y Nothing - error Nothing in g x Again, g must be strict in its first argument. Actually, f x = let g (Just x) = x g Nothing = error Nothing in g x is a valid definition. A let expression can have multiple definitions just like a where clause. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] GHC 6.7 on Windows / containers-0.1 package?
Since I'm hopelessly stuck with an weird infinite loop (that consumes 0% CPU time!), and since I can't find the problem using trace, I wanted to try GHC 6.7, which seems to container a debugger (woohoo!!!). Currently I'm using GHC-6.6.1 on Windows. So I grabbed ghc-6.7.20070824 (=the latest one for Windows I could find) and the extra-libs, compiled and installed the GLUT package (which I needed), but when I compile my library, I get Could not find module `Data.Map': it is a member of package containers-0.1, which is hidden So I tried to expose it using GHC-PKG expose containers-0.1 which worked find, because GHC-PKG list prints d:/app/ghc/ghc-6.7.20070824\package.conf: Cabal-1.1.7, GLUT-2.1.1, HUnit-1.1.1, OpenGL-2.2.1, QuickCheck-1.0.1, Win32-2.1, array-0.1, arrows-0.2.1, base-2.1, bytestring-0.1, cgi-3001.1.5, containers-0.1, directory-1.0, fgl-5.4.1, filepath-1.0, (ghc-6.7.20070824), haskell-src-1.0.1, haskell98-1.0, hpc-0.5, html-1.0.1, mtl-1.0.1, network-2.0.1, old-locale-1.0, old-time-1.0, packedstring-0.1, parallel-1.0, parsec-2.0, pretty-1.0, process-1.0, random-1.0, regex-base-0.72, rts-1.0, stm-2.1, template-haskell-0.1, xhtml-3000.0.2 Anybody has an idea how I can continue? Thanks (again!), Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
C.M.Brown wrote: Hi Miguel, See, in let or where constructs you don't have a choice; you can't do different things depending on whether some value is Just x or Nothing. Therefore, there is no need to perform pattern matching strictly. This is not entirely true. This is actually one of those niches in Haskell where the left to right is not quite the same as right to left. A let can be converted to a where but the other way round may require a case introduction. So just like you can define: f (Just x) = x f Nothing = error Nothing You can also define: f x = g x where g (Just x) = x g Nothing = error Nothing g is strict in its first argument. Declared in a let it would look like: f x = let g x = case x of (Just y) - y Nothing - error Nothing in g x Again, g must be strict in its first argument. or f x = let g (Just x) = x g Nothing = error Nothing in g x also works perfectly fine. The (Just x) and Nothing are not let-patterns, they are function-definition-patterns, and of course are strict since they make a decision. Isaac ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC 6.7 on Windows / containers-0.1 package?
Hi Peter, So I grabbed ghc-6.7.20070824 (=the latest one for Windows I could find) and the extra-libs, compiled and installed the GLUT package (which I needed), but when I compile my library, I get Could not find module `Data.Map': it is a member of package containers-0.1, which is hidden All dependencies etc. have changed when going to 6.7/6.8 - you are probably better off using 6.6.1 for now. I also don't think that the debugger will help you track down infinite loop style errors. You might be better off posting the code and asking for help. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC 6.7 on Windows / containers-0.1 package?
On Wed, Sep 19, 2007 at 10:24:24PM +0100, Neil Mitchell wrote: Hi Peter, So I grabbed ghc-6.7.20070824 (=the latest one for Windows I could find) and the extra-libs, compiled and installed the GLUT package (which I needed), but when I compile my library, I get Could not find module `Data.Map': it is a member of package containers-0.1, which is hidden All dependencies etc. have changed when going to 6.7/6.8 - you are probably better off using 6.6.1 for now. I also don't think that the debugger will help you track down infinite loop style errors. You might be better off posting the code and asking for help. You said 0% CPU. That's *very* important. It means that you are using the threaded runtime (GHCi?), and that you triggered a blackhole. You should be able to handle this by compiling your program with -prof (do *not* use -threaded!), and running with +RTS -xc. With luck, that will give you a backtrace to the infinite loop. PS. blackholes are a serious dark corner of GHC's execution model, chances are better than even that if you try to use the debugger for this you will discover a new and (for you) crippling bug. I wouldn't recommend it. Stefan signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Hi Isaac or f x = let g (Just x) = x g Nothing = error Nothing in g x That's interesting, thanks! Chris. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
On 9/19/07, C.M.Brown [EMAIL PROTECTED] wrote: f x = let g (Just x) = x g Nothing = error Nothing in g x That's interesting, thanks! FYI, you may also use guards, even if you don't have a parameter. For example, you may turn f x = let k = if null x then 10 else head x in froobar $ quox k into f x = let k | null x= 10 | otherwise = head x in froobar $ quox k Okay, this example is ugly, however it serves the purpose of illustrating the idea =). -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Interesting folds over bytestring lists?
I have a data structure which is a list of bytestrings, but externally it looks like one big string. One of the operations I want to support takes a section of the string, starting at some arbitrary index and ending somewhere further down the line. In implementing the function I came up with the two functions below, dropTo and takeTo. In my mind, dropTo moves over the list of bytestrings until it reaches the starting point, and then returns the rest. takeTo, in contrast, scans over the list until it has seen enough bytes to return the amount requested. In both cases I am trying to share structure as much as possible in order to avoid unnecessary copying and space leaks. I thought these two functions were interested and am looking for feedback, comments, improvements, etc. Thanks! import qualified Data.ByteString as S import qualified Data.ByteString.Lazy as L import qualified Data.ByteString.Base as B import Test.QuickCheck import Data.List (foldl') import Data.Word dropTo :: Int - [L.ByteString] - [L.ByteString] dropTo _ [] = [] dropTo amt strs = let dropTo' :: (Int, [L.ByteString]) - L.ByteString - (Int, [L.ByteString]) dropTo' (rem, acc) ss | rem == 0 = (0, acc) | otherwise = let chunks = L.toChunks ss in case foldl' dropStricts (rem, chunks) chunks of (!n, rest) | null rest - (n, drop 1 acc) | otherwise - (0, L.fromChunks rest : drop 1 acc) dropStricts :: (Int, [S.ByteString]) - S.ByteString - (Int, [S.ByteString]) dropStricts (rem, acc) str | rem == 0 = (0, acc) | rem - S.length str == 0 = (0, drop 1 acc) | rem - S.length str 0 = (0, S.drop rem str : (drop 1 acc)) | otherwise = (rem - S.length str, drop 1 acc) (_, rest) = foldl' dropTo' (amt, strs) strs in rest takeTo :: Int - [L.ByteString] - [L.ByteString] takeTo _ [] = [] takeTo amt strs = let countLazies :: (Int, Int, L.ByteString) - L.ByteString - (Int, Int, L.ByteString) countLazies (rem, !total, lazyLeftover) ss | rem == 0 = (0, total, lazyLeftover) | otherwise = let chunks = L.toChunks ss in case foldl' countStricts (rem, 0, S.empty) chunks of (!n, amt, strictLeftover) | S.null strictLeftover - (n, total + 1, L.empty) | otherwise - (n, total, L.fromChunks (take amt chunks ++ [strictLeftover])) countStricts :: (Int, Int, S.ByteString) - S.ByteString - (Int, Int, S.ByteString) countStricts (rem, !total, leftover) str | rem == 0 = (0, total, leftover) | rem - S.length str == 0 = (0, total + 1, S.empty) | rem - S.length str 0 = (0, total, S.take rem str) | otherwise = (rem - S.length str, total + 1, S.empty) in case foldl' countLazies (amt, 0, L.empty) strs of (_, total, leftover) | L.null leftover - take total strs | otherwise - take total strs ++ [leftover] prop_dropToNonEmpty :: [[Word8]] - Int - Property prop_dropToNonEmpty strs amt = amt = 0 (all (not . null) strs) == all (not . L.null) (dropTo amt (map (L.pack) strs)) prop_dropToCorrect :: [[Word8]] - Int - Property prop_dropToCorrect strs amt = let lazyStr = L.drop amt64 (toLazyBS strs) amt64 = fromIntegral amt in amt = 0 (all (not . null) strs) == (L.concat (dropTo amt (map (L.pack) strs))) == lazyStr prop_takeToNonEmpty :: [[Word8]] - Int - Property prop_takeToNonEmpty strs amt = amt = 0 (all (not . null) strs) == all (not . L.null) (takeTo amt (map (L.pack) strs)) prop_takeToCorrect :: [[Word8]] - Int - Property prop_takeToCorrect strs amt = let lazyStr = L.take amt64 (toLazyBS strs) amt64 = fromIntegral amt in amt = 0 (all (not . null) strs) == (L.concat (takeTo amt (map (L.pack) strs))) == lazyStr -- Functions and instances for testing purposes toLazyBS :: [[Word8]] - L.ByteString toLazyBS = L.concat . map L.pack instance Arbitrary Word8 where arbitrary = elements [minBound .. maxBound] coarbitrary = undefined main = do putStrLn prop_takeToNonEmpty quickCheck prop_takeToNonEmpty putStrLn prop_takeToCorrect quickCheck prop_takeToCorrect putStrLn prop_dropToCorrect quickCheck prop_dropToCorrect putStrLn prop_dropToNonEmpty quickCheck prop_dropToNonEmpty ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] length defined with foldr
Hi length = foldr (. n . 1 + n) 0 Any idea how to define length with foldr. The above definition doesn't make much sense. Many thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] length defined with foldr
On Thu, Sep 20, 2007 at 04:17:54AM +0100, PR Stanley wrote: Hi length = foldr (. n . 1 + n) 0 Any idea how to define length with foldr. The above definition doesn't make much sense. Many thanks, Paul length = foldr (λ_ n → 1 + n) 0 or, in ASCII concrete syntax length = foldr (\_ n - 1 + n) 0 Stefan signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] length defined with foldr
On Sep 19, 2007, at 23:17 , PR Stanley wrote: length = foldr (. n . 1 + n) 0 Any idea how to define length with foldr. The above definition doesn't make much sense. Still fighting with the botched OCR? length = foldr (\_ n - 1 + n) 0 -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] length defined with foldr
Still fighting with the botched OCR? I am, as a matter of fact! Is that okay with you? *smile* length = foldr (\_ n - 1 + n) 0 -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Spot the difference!
Hi \_ n - 1 + n \_ - (\n - 1 + n) The outcome seems to be identical. is there a substantive difference between the two definitions? The second one seems to be more expressive of the currying principle. Any thoughts? Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Spot the difference!
On Sep 20, 2007, at 0:03 , PR Stanley wrote: \_ n - 1 + n \_ - (\n - 1 + n) The outcome seems to be identical. is there a substantive difference between the two definitions? The second one seems to be more expressive of the currying principle. Any thoughts? I *think* the former is internally converted to the latter; this is how currying works. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Spot the difference!
I must confess that the use of \_ had me a little confused at first but thinking about it I can see that it makes perfect sense to have an argument or a wildcard character for any value. Cheers, Paul On Sep 20, 2007, at 0:03 , PR Stanley wrote: \_ n - 1 + n \_ - (\n - 1 + n) The outcome seems to be identical. is there a substantive difference between the two definitions? The second one seems to be more expressive of the currying principle. Any thoughts? I *think* the former is internally converted to the latter; this is how currying works. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Spot the difference!
On Sep 20, 2007, at 0:16 , PR Stanley wrote: I must confess that the use of \_ had me a little confused at first but thinking about it I can see that it makes perfect sense to have an argument or a wildcard character for any value. Sure. Remember, arguments are pattern matches in Haskell (the degenerate case being a variable name which is an irrefutable match resulting in a lambda binding). As such, it's useful to have a wildcard pattern which matches irrefutably without binding. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Spot the difference!
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 PR Stanley wrote: I must confess that the use of \_ had me a little confused at first but thinking about it I can see that it makes perfect sense to have an argument or a wildcard character for any value. Cheers, Paul FYI If \_ - foo confuses you, you might wish to use const foo instead. Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFG8fizmnpgrYe6r60RAqKBAKCKQ76HMtJ8jsgJ5nmX8ECROOcirQCfQgKS OQflRMDqHqX2TUWibG4oiB4= =L6C4 -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Spot the difference!
test1 n _ = 1 + n test2 n = \_ - 1 + n I don't know if it's still the case, but GHC used to compile different code for these at high optimisation levels. The first was essentially compiled to: test1 = \n _ - 1+n And the second to: test2 = \n - let x = n+1 in \_ - x The difference is that test1 is faster if it's usually fully applied, test2 is fully lazy. Fully lazy? Can you elaborate please? Thanks, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Spot the difference!
G'day all. Quoting PR Stanley [EMAIL PROTECTED]: Fully lazy? Can you elaborate please? Sure. that code again: test1 = \n _ - 1+n test2 = \n - let x = n+1 in \_ - x Suppose we have: f g x = g x + g x And we try two options: f (test1 4) 3 f (test2 4) 3 In the first case, (1+4) will be evaluated twice. In the second case, it will only be evaluated once. Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
