[Haskell-cafe] Polyvariadic functions operating with a monoid
I had a situation where I had some related types that all had toString functions. Of course in Haskell, lists all have to be composed of values of exactly the same type, so instead of passing around lists of values with these related types, I created a polyvariadic function polyToString so that I could write: (polyToString value1 value2 value3 ... valueN) which would then become a list of strings: [toString value1, toString value2, ... , toString valueN] I finally figured out how to do this, but it was a bit harder to figure this out than I expected, and I was wondering if it might be possible to create a small utility library to help other developers do this. It seems to me that in the general case, we would be dealing with a Monoid rather than a list of strings. We could have a toMonoid function and then return polyToMonoid value1 value2 ... valueN = (toMonoid value1) `mappend` (toMonoid value2) 'mappend' ... (toMonoid valueN) So anyone who wanted to convert a bunch of values of different types to a Monoid could easily pass them around using polyToMonoid so long as they defined the appropriate toMonoid function. Basically, a generalised list. So I tried writing the following code but GHC said it had undecidable instances. Has this ever been done successfully? class Monoidable a where toMonoid :: Monoid r = a - r polyToMonoid :: (Monoidable a, Monoid r) = a - r polyToMonoid k = polyToMonoid' k mempty class PolyVariadic p where polyToMonoid' :: (Monoidable a, Monoid r) = a - r - p instance Monoid r = PolyVariadic r where polyToMonoid' k ss = (toMonoid k) `mappend` ss instance (Monoidable a, Monoid r) = PolyVariadic (a - r) where polyToMonoid' k ss = (\a - polyToMonoid' k (toMonoid a) `mappend` ss) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A parsec question
Does this one give the expected error message for Parsec3.1 - unfortunately I can't test as I'm still using Parsec 2.1.0.1. parser = block (many digit ? digit) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Non-existing types in existential quantification?
On Sun, 3 Oct 2010, Ben Franksen wrote: Christopher Done wrote: Consider the following program: main = putStrLn $ show $ length [undefined :: a,undefined :: b] A concrete type of the element in list doesn't need to be determined at runtime, or any time. a unifies with b, and that unifies with x in length :: [x] - Int. A simpler example is main = print Nothing This seems to be a different example, because GHCi -Wall says that the type variable defaults to (). Thus 'Nothing' has monomorphic type at runtime. The difference is certainly that 'print' requires a Show instance, whereas Christopher's example does not require a type constraint. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
On 3 October 2010 06:51, Michael Snoyman mich...@snoyman.com wrote: * Does pass.net still exist anywhere? Same for parallel web. I couldn't find any references to pass.net. http://www.haskell.org/haskellwiki/Web/Existing_software I meant that I remember adding it, but I couldn't find any references for it. I.e. it's dead. Maybe we can split that page by active/inactive too. I would recommend *not* qualifying the active/recommended stuff. Maybe Frameworks and Frameworks/Inactive. I personally wouldn't want to group new, unevaluated code with inactive: I think we should give the new players the same publicity as the established products on the main page, but perhaps with a little label explaining how new/untested it is. Trouble is most on the Web/Frameworks page are still /available/ but it's hard to see if they're active i.e. people are still using them, or whether they're still build-able. Perhaps it would be best to create the Frameworks/Inactive page and then at the top of the Frameworks page say Inactive frameworks are listed here. and then when someone is definitely sure something is defunct they can move it. Or what do we do? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
On Sun, Oct 3, 2010 at 11:59 AM, Christopher Done chrisd...@googlemail.com wrote: On 3 October 2010 06:51, Michael Snoyman mich...@snoyman.com wrote: * Does pass.net still exist anywhere? Same for parallel web. I couldn't find any references to pass.net. http://www.haskell.org/haskellwiki/Web/Existing_software I meant that I remember adding it, but I couldn't find any references for it. I.e. it's dead. Maybe we can split that page by active/inactive too. I would recommend *not* qualifying the active/recommended stuff. Maybe Frameworks and Frameworks/Inactive. I personally wouldn't want to group new, unevaluated code with inactive: I think we should give the new players the same publicity as the established products on the main page, but perhaps with a little label explaining how new/untested it is. Trouble is most on the Web/Frameworks page are still /available/ but it's hard to see if they're active i.e. people are still using them, or whether they're still build-able. Perhaps it would be best to create the Frameworks/Inactive page and then at the top of the Frameworks page say Inactive frameworks are listed here. and then when someone is definitely sure something is defunct they can move it. Or what do we do? I would actually do the opposite: we can put the libraries/frameworks that we are sure *are* active into the Active section and put everything else into Inactive. I have a feeling we'll be pretty close on the mark with our guesses; a quick look at the last upload date on Hackage should be sufficient. People are *much* more likely to move stuff from Inactive to Active than the other way around. We can also send out an email to the cafe/web-devel with a list of packages we plan to mark as inactive and see if anyone objects. If no one is willing to stand up for a package, odds are it's dead. Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
On 3 October 2010 12:10, Michael Snoyman mich...@snoyman.com wrote: I would actually do the opposite: we can put the libraries/frameworks that we are sure *are* active into the Active section and put everything else into Inactive. I have a feeling we'll be pretty close on the mark with our guesses; a quick look at the last upload date on Hackage should be sufficient. People are *much* more likely to move stuff from Inactive to Active than the other way around. We can also send out an email to the cafe/web-devel with a list of packages we plan to mark as inactive and see if anyone objects. If no one is willing to stand up for a package, odds are it's dead. That sounds like a good approach. Anyway, it's not the end of the world if a package gets put in inactive. Ones I know are definitely active are: Happstack Haskell on a Horse loli Salvia Snap Yesod Turbinado -- is this still active? Alson Kemp basically ditched Haskell, so... Yesod The others... I don't know. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
On Sun, Oct 3, 2010 at 12:20 PM, Christopher Done chrisd...@googlemail.com wrote: On 3 October 2010 12:10, Michael Snoyman mich...@snoyman.com wrote: I would actually do the opposite: we can put the libraries/frameworks that we are sure *are* active into the Active section and put everything else into Inactive. I have a feeling we'll be pretty close on the mark with our guesses; a quick look at the last upload date on Hackage should be sufficient. People are *much* more likely to move stuff from Inactive to Active than the other way around. We can also send out an email to the cafe/web-devel with a list of packages we plan to mark as inactive and see if anyone objects. If no one is willing to stand up for a package, odds are it's dead. That sounds like a good approach. Anyway, it's not the end of the world if a package gets put in inactive. Ones I know are definitely active are: Happstack Haskell on a Horse loli Salvia Snap Yesod Turbinado -- is this still active? Alson Kemp basically ditched Haskell, so... Yesod The others... I don't know. I think it's fair to say that turbinado is inactive. But keep in mind that we should probably look at more than just the frameworks: servers, templating, etc. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
On 3 October 2010 12:31, Michael Snoyman mich...@snoyman.com wrote: I think it's fair to say that turbinado is inactive. But keep in mind that we should probably look at more than just the frameworks: servers, templating, etc. Sure, it should be a general rule across the board. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: EDSL for Makefile
If you don't want to mention r1 explicitly, but want to refer to target, sources and such only a monadic approach (e.g. Reader Monad) might be what you want. On Oct 3, 6:14 am, C K Kashyap ckkash...@gmail.com wrote: Thanks Emil ... yeah, that works...I was wondering what I could do to not have to mention r1 explicitly. I'll check out Neil's pdf and video now - perhaps I'll find answers there. I checked out the video - nice - but I think, understandably, since its not open source yet, not much of implementations details were mentioned. So, I have this unanswered question nagging in my head. In the example below, how can I let the makefile writer refer to the target name and dependencies. Likr Emil mentioned, I could use target r1 but I want to avoid having to mention r1. http://hpaste.org/40233/haskell_makefile_edsl -- Regards, Kashyap ___ Haskell-Cafe mailing list haskell-c...@haskell.orghttp://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: Non-existing types in existential quantification?
On Sunday 03 October 2010 10:43:24, Henning Thielemann wrote: On Sun, 3 Oct 2010, Ben Franksen wrote: Christopher Done wrote: Consider the following program: main = putStrLn $ show $ length [undefined :: a,undefined :: b] A concrete type of the element in list doesn't need to be determined at runtime, or any time. a unifies with b, and that unifies with x in length :: [x] - Int. A simpler example is main = print Nothing This seems to be a different example, because GHCi -Wall says that the type variable defaults to (). Thus 'Nothing' has monomorphic type at runtime. The difference is certainly that 'print' requires a Show instance, whereas Christopher's example does not require a type constraint. Yup. Prelude print $ length [undefined :: a, undefined :: b] 2 Prelude print $ length [undefined :: a, undefined :: Num b = b] interactive:1:32: Warning: Defaulting the following constraint(s) to type `Integer' `Num a' arising from an expression type signature at interactive:1:32-54 In the expression: undefined :: (Num b) = b In the first argument of `length', namely `[undefined :: a, undefined :: (Num b) = b]' In the second argument of `($)', namely `length [undefined :: a, undefined :: (Num b) = b]' 2 Prelude print $ length [undefined :: a, undefined :: Num b = c - b] interactive:1:32: Warning: Defaulting the following constraint(s) to type `Integer' `Num b' arising from an expression type signature at interactive:1:32-59 In the expression: undefined :: (Num b) = c - b In the first argument of `length', namely `[undefined :: a, undefined :: (Num b) = c - b]' In the second argument of `($)', namely `length [undefined :: a, undefined :: (Num b) = c - b]' 2 At runtime, a type variable has to be either unconstrained or instantiated with a concrete type? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Coding conventions for Haskell?
Andrew Coppin schrieb: On 30/09/2010 02:56 PM, Henning Thielemann wrote: In Cabal you can write one module per line and need no separator or terminator at all. Really? As far as I can tell, that doesn't work at all... See e.g. http://hackage.haskell.org/packages/archive/utility-ht/0.0.5.1/utility-ht.cabal ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
I just discovered this: http://www.haskell.org/haskellwiki/Performance/Strictness See the Haskell Performance Resource box? That's great! I'm going to make one for our Web articles. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] EDSL for Makefile
I checked out the video - nice - but I think, understandably, since its not open source yet, not much of implementations details were mentioned. Yes, it's unfortunate. So, I have this unanswered question nagging in my head. In the example below, how can I let the makefile writer refer to the target name and dependencies. Likr Emil mentioned, I could use target r1 but I want to avoid having to mention r1. If it helps, written in Shake, this would be: main = do want [file1] file1 * \x - do need [file2] putStrLn Hello putStrLn World Thanks, Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Non-existing types in existential quantification?
On Sun, Oct 3, 2010 at 14:10, Daniel Fischer daniel.is.fisc...@web.de wrote: On Sunday 03 October 2010 10:43:24, Henning Thielemann wrote: On Sun, 3 Oct 2010, Ben Franksen wrote: Christopher Done wrote: Consider the following program: main = putStrLn $ show $ length [undefined :: a,undefined :: b] A concrete type of the element in list doesn't need to be determined at runtime, or any time. a unifies with b, and that unifies with x in length :: [x] - Int. A simpler example is main = print Nothing This seems to be a different example, because GHCi -Wall says that the type variable defaults to (). Thus 'Nothing' has monomorphic type at runtime. The difference is certainly that 'print' requires a Show instance, whereas Christopher's example does not require a type constraint. Yup. Prelude print $ length [undefined :: a, undefined :: b] 2 Prelude print $ length [undefined :: a, undefined :: Num b = b] interactive:1:32: Warning: Defaulting the following constraint(s) to type `Integer' `Num a' arising from an expression type signature at interactive:1:32-54 In the expression: undefined :: (Num b) = b In the first argument of `length', namely `[undefined :: a, undefined :: (Num b) = b]' In the second argument of `($)', namely `length [undefined :: a, undefined :: (Num b) = b]' 2 Prelude print $ length [undefined :: a, undefined :: Num b = c - b] interactive:1:32: Warning: Defaulting the following constraint(s) to type `Integer' `Num b' arising from an expression type signature at interactive:1:32-59 In the expression: undefined :: (Num b) = c - b In the first argument of `length', namely `[undefined :: a, undefined :: (Num b) = c - b]' In the second argument of `($)', namely `length [undefined :: a, undefined :: (Num b) = c - b]' 2 At runtime, a type variable has to be either unconstrained or instantiated with a concrete type? GHC has the 'Any' type, and the docs state: It's also used to instantiate un-constrained type variables after type checking. [1] So I guess at least for GHC, all type variables are instantiated after type checking? Regards, Erik [1] http://www.haskell.org/ghc/docs/6.12.2/html/libraries/ghc-prim-0.2.0.0/GHC-Prim.html#t%3AAny ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: EDSL for Makefile
On Sun, Oct 3, 2010 at 5:22 PM, steffen steffen.sier...@googlemail.com wrote: If you don't want to mention r1 explicitly, but want to refer to target, sources and such only a monadic approach (e.g. Reader Monad) might be what you want. Thanks Steffen ... would you be able to give me an example? -- Regards, Kashyap ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] EDSL for Makefile
Thanks Neil, main = do want [file1] file1 * \x - do need [file2] putStrLn Hello putStrLn World What if I want to mention file1 only once? -- Regards, Kashyap ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] EDSL for Makefile
Hello C, Sunday, October 3, 2010, 6:59:25 PM, you wrote: Thanks Neil, main = do want [file1] file1 * \x - do need [file2] putStrLn Hello putStrLn World What if I want to mention file1 only once? mention_only_once file action = do want [file] file * action main = mention_only_once file1 $ \x - do need [file2] putStrLn Hello putStrLn World -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
So I went through the Applications_and_libraries/Web_programming page and pulled out any remaining goodness from it into pages under the Web/ umbrella and then set it up as a redirect to Web/ I made an infobox which I put on every Web/ page, which makes it very nice for navigating between the sections. I also added a tiny introduction to the main page. http://www.haskell.org/haskellwiki/Web I also updated the Practical Web Programming in Haskell tutorial to remove irrelevant/out-dated references and links, updated some bits, and generally improved the formatting so that it's not just a huge chunk of headings and whitespace: http://www.haskell.org/haskellwiki/Web/Literature/Practical_web_programming_in_Haskell I think on the Web/ page we should have a hot frameworks or something like that, listing Happstack, Yesod and Snap, preferably with their logos. Do you have a logo for Yesod? I'm thinking of just tastefully taking the cube. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
On Sun, Oct 3, 2010 at 5:33 PM, Christopher Done chrisd...@googlemail.com wrote: So I went through the Applications_and_libraries/Web_programming page and pulled out any remaining goodness from it into pages under the Web/ umbrella and then set it up as a redirect to Web/ I made an infobox which I put on every Web/ page, which makes it very nice for navigating between the sections. I also added a tiny introduction to the main page. http://www.haskell.org/haskellwiki/Web I also updated the Practical Web Programming in Haskell tutorial to remove irrelevant/out-dated references and links, updated some bits, and generally improved the formatting so that it's not just a huge chunk of headings and whitespace: http://www.haskell.org/haskellwiki/Web/Literature/Practical_web_programming_in_Haskell I think on the Web/ page we should have a hot frameworks or something like that, listing Happstack, Yesod and Snap, preferably with their logos. Do you have a logo for Yesod? I'm thinking of just tastefully taking the cube. Well done, it all looks *very* nice. Regarding Yesod: yes, use the cube for now, I may eventually make a better logo, but that's it for the moment. The only concern I have is the practical web programming in Haskell page, which frankly isn't very practical. I personally would put a big fat warning at the top saying most people do not recommend using straight CGI these days. One other possibility is putting together a tutorials section (descending order by date) linking to various blog posts. But I think you've done an amazing job on cleaning things up and getting it presentable. A big thank you! Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] I still cannot seem to get a GUI working under Windows.
On Fri, 01 Oct 2010 13:45:00 +0100, you wrote: I think that the issue is that making things better on Windows (and likely OS X as well) requires co-ordinated and agreed action across a number of areas. This means getting a moderate number of people, most of whom give up their time and effort for free, and for the good of the community, to agree on what is needed and to prioritize it in a co-ordinated way. The action needed by any individual might be relatively small, but without community agreement and action, the end goal cannot be achieved. Thank you. That's basically what I was trying to say: The project is too big for one person, or a small group of people. But it also can't happen unless there's a shared understanding of what is important and why it is important, and that's what seems to be lacking here. -Steve Schafer ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: I still cannot seem to get a GUI working under Windows.
On Sat, 2 Oct 2010 11:02:21 -0700, you wrote: I imagine someone looking at a lovely app and saying, Wow -- great interface! I bet it was programmed in Haskell. While I can agree with the sentiment...well, good luck with that. ;-) -Steve Schafer ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell web development entries on the Wiki
On 3 October 2010 17:41, Michael Snoyman mich...@snoyman.com wrote: Well done, it all looks *very* nice. Regarding Yesod: yes, use the cube for now, I may eventually make a better logo, but that's it for the moment. Righteo. The only concern I have is the practical web programming in Haskell page, which frankly isn't very practical. I personally would put a big fat warning at the top saying most people do not recommend using straight CGI these days. Yeah, it's not the best. One other possibility is putting together a tutorials section (descending order by date) linking to various blog posts. There is the http://www.haskell.org/haskellwiki/Web/Literature labelled Literature (research, talks and blogs) section which contains blog posts, but perhaps a tutorial-centric page would be in order. Really we should link to a yesod, snap, happstack tutorial and cgi on the first page, I think. But I think you've done an amazing job on cleaning things up and getting it presentable. A big thank you! Cheers! I'm looking forward to when the new Wiki version and template is rolled out. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Big Arrays
Dear Group, I am trying to create a (relatively) big array, but it seems I cannot make anything larger than 2^30 or so. Here is the code: import Data.Word import Data.Array.Unboxed import Control.Monad.ST import Data.Array.ST import Control.Exception import Prelude hiding (catch) t1 :: Word64 - UArray Word64 Bool t1 size = runSTUArray (do a - newArray (0,size) True :: ST s (STUArray s Word64 Bool) writeArray a 0 False return a) catchArrayException x = do let err = show (x :: SomeException) putStrLn $ Exception [ ++ err ++ ] return () main = do let a1 = t1 (2^30) a2 = t1 (2^31) a3 = t1 (2^32) catch (print $ (a1!0,a1!1)) catchArrayException catch (print $ (a2!0,a2!1)) catchArrayException catch (print $ (a3!0,a3!1)) catchArrayException This results in: *Main GOA main (False,True) (Exception [Negative range size] (False,Exception [Error in array index; 1 not in range [0..1)] It looks like array ranges can only be Ints, and not Int64 or Word64 types. Any pointers would be appreciated. Best wishes, Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Big Arrays
Hello Henry, Sunday, October 3, 2010, 7:54:49 PM, you wrote: It looks like array ranges can only be Ints, and not Int64 or Word64 types. yes, it's Int internally got efficiency reasons. you can do your own implementation to override this limit :) -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: A parsec question
Stephen Tetley wrote: Does this one give the expected error message for Parsec3.1 - unfortunately I can't test as I'm still using Parsec 2.1.0.1. parser = block (many digit ? digit) Unfortunately, no. Cheers Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Re: Non-existing types in existential quantification?
Henning Thielemann wrote: On Sun, 3 Oct 2010, Ben Franksen wrote: Christopher Done wrote: Consider the following program: main = putStrLn $ show $ length [undefined :: a,undefined :: b] A concrete type of the element in list doesn't need to be determined at runtime, or any time. a unifies with b, and that unifies with x in length :: [x] - Int. A simpler example is main = print Nothing This seems to be a different example, because GHCi -Wall says that the type variable defaults to (). Thus 'Nothing' has monomorphic type at runtime. The difference is certainly that 'print' requires a Show instance, whereas Christopher's example does not require a type constraint. Right. I always forget about defaulting. This is an obscure feature of the language. Are there any programs that rely on defaulting and could not be easily re-written so as not to? Cheers Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] A question regarding cmdargs package
How can I disable the standard arguments 'help' and 'version'? Cheers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell Helper
Hello, I 'm developing a new language in haskell and I need someone to help me. Anyone would like to help? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell Helper
Is the language secret? because I'm sure lots of people would like to help on this mailing list, but you'd need to tell us what help you need first :) On Sun, Oct 3, 2010 at 8:38 PM, Eduardo Ribeiro asaferibei...@ymail.comwrote: Hello, I 'm developing a new language in haskell and I need someone to help me. Anyone would like to help? ___ 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 Helper
No, it is not secret. I'm having trouble to define functions. Take a look at my code(please be gentle) http://haskell.1045720.n5.nabble.com/file/n3100036/hai1.hs hai1.hs -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-Helper-tp3093854p3100036.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Polyvariadic functions operating with a monoid
On Sun, Oct 3, 2010 at 1:24 AM, Kevin Jardine kevinjard...@gmail.com wrote: I had a situation where I had some related types that all had toString functions. Of course in Haskell, lists all have to be composed of values of exactly the same type, so instead of passing around lists of values with these related types, I created a polyvariadic function polyToString so that I could write: (polyToString value1 value2 value3 ... valueN) which would then become a list of strings: [toString value1, toString value2, ... , toString valueN] First of all, you are not using the monoidal structure of String at all. This trick ought to work for any type whatsoever -- you're just throwing them in a list. Other than a few brackets, commas, and a repeated identifier (which you can let-bind to shorten), what benefit is it giving you? I strongly recommend against polyvariadic functions. While you get a little bit of notational convenience, you lose composability. There are pains when you try to write a function that takes a polyvariadic function as an argument, or when you try to feed the function values from a list, etc. The mechanisms to create polyvariadic functions are brittle and hacky (eg. you cannot have a polymorphic return type, as you want in this case). Since all your values are known statically, I would recommend biting the bullet and doing it the way you were doing it. [ s value1, s value2, s value3, ... ] where s x = toString x (I had to eta expand s so that I didn't hit the monomorphism restriction) When you want to be passing around heterogeneous lists, it usually works to convert them before you put them in the list, like you were doing. I finally figured out how to do this, but it was a bit harder to figure this out than I expected, and I was wondering if it might be possible to create a small utility library to help other developers do this. It seems to me that in the general case, we would be dealing with a Monoid rather than a list of strings. We could have a toMonoid function and then return polyToMonoid value1 value2 ... valueN = (toMonoid value1) `mappend` (toMonoid value2) 'mappend' ... (toMonoid valueN) So anyone who wanted to convert a bunch of values of different types to a Monoid could easily pass them around using polyToMonoid so long as they defined the appropriate toMonoid function. Basically, a generalised list. So I tried writing the following code but GHC said it had undecidable instances. Has this ever been done successfully? class Monoidable a where toMonoid :: Monoid r = a - r polyToMonoid :: (Monoidable a, Monoid r) = a - r polyToMonoid k = polyToMonoid' k mempty class PolyVariadic p where polyToMonoid' :: (Monoidable a, Monoid r) = a - r - p instance Monoid r = PolyVariadic r where polyToMonoid' k ss = (toMonoid k) `mappend` ss instance (Monoidable a, Monoid r) = PolyVariadic (a - r) where polyToMonoid' k ss = (\a - polyToMonoid' k (toMonoid a) `mappend` ss) ___ 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] Polyvariadic functions operating with a monoid
On Sun, Oct 3, 2010 at 1:26 PM, Luke Palmer lrpal...@gmail.com wrote: On Sun, Oct 3, 2010 at 1:24 AM, Kevin Jardine kevinjard...@gmail.com wrote: I had a situation where I had some related types that all had toString functions. Of course in Haskell, lists all have to be composed of values of exactly the same type, so instead of passing around lists of values with these related types, I created a polyvariadic function polyToString so that I could write: (polyToString value1 value2 value3 ... valueN) which would then become a list of strings: [toString value1, toString value2, ... , toString valueN] First of all, you are not using the monoidal structure of String at all. This trick ought to work for any type whatsoever -- you're just throwing them in a list. Oops, sorry for not reading your message more closely. You were indeed talking about the monoidal structure of list. So... nevermind about this comment. :-P Other than a few brackets, commas, and a repeated identifier (which you can let-bind to shorten), what benefit is it giving you? I strongly recommend against polyvariadic functions. While you get a little bit of notational convenience, you lose composability. There are pains when you try to write a function that takes a polyvariadic function as an argument, or when you try to feed the function values from a list, etc. The mechanisms to create polyvariadic functions are brittle and hacky (eg. you cannot have a polymorphic return type, as you want in this case). Since all your values are known statically, I would recommend biting the bullet and doing it the way you were doing it. [ s value1, s value2, s value3, ... ] where s x = toString x (I had to eta expand s so that I didn't hit the monomorphism restriction) When you want to be passing around heterogeneous lists, it usually works to convert them before you put them in the list, like you were doing. I finally figured out how to do this, but it was a bit harder to figure this out than I expected, and I was wondering if it might be possible to create a small utility library to help other developers do this. It seems to me that in the general case, we would be dealing with a Monoid rather than a list of strings. We could have a toMonoid function and then return polyToMonoid value1 value2 ... valueN = (toMonoid value1) `mappend` (toMonoid value2) 'mappend' ... (toMonoid valueN) So anyone who wanted to convert a bunch of values of different types to a Monoid could easily pass them around using polyToMonoid so long as they defined the appropriate toMonoid function. Basically, a generalised list. So I tried writing the following code but GHC said it had undecidable instances. Has this ever been done successfully? class Monoidable a where toMonoid :: Monoid r = a - r polyToMonoid :: (Monoidable a, Monoid r) = a - r polyToMonoid k = polyToMonoid' k mempty class PolyVariadic p where polyToMonoid' :: (Monoidable a, Monoid r) = a - r - p instance Monoid r = PolyVariadic r where polyToMonoid' k ss = (toMonoid k) `mappend` ss instance (Monoidable a, Monoid r) = PolyVariadic (a - r) where polyToMonoid' k ss = (\a - polyToMonoid' k (toMonoid a) `mappend` ss) ___ 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] Suggestions for improvement
I am reading the book `The Haskell Road to Math, Logic, One of the exercises in the first chapter asks for a function that maps a string abcd to abbccc and bang! to baannn!. Since such a function f fixes the empty word, and maps wa to f(w)a^(length(w)+1) for any word w and any letter a, I came up with the following solution: -- Map abcd to abbccc and bang! to baannn!. blowup :: String - String blowup [] = [] blowup x = blowup (allButLast x) ++ lastToTheLength x -- Map abcd to abc. allButLast :: String - String allButLast [] = [] allButLast [x] = [] allButLast (x : xs) = x : allButLast xs -- Map abcd to d^4 = . lastToTheLength :: String - String lastToTheLength [] = [] lastToTheLength [x] = [x] lastToTheLength (_ : xs) = lastToTheLength xs ++ [last xs] One question I have is whether I can eliminate points in the above definition of blowup, and write something like blowup = (++) . (blowup . allButLast, lastToTheLength) thinking of (++) as a function String x String - String. Also, I can't figure out whether it is possible to get a shorter solution using fold. I have tried Hlint on my file, but it gave no suggestions. I am sure there are better ways, and would like some pointers and any general suggestions for improvement. Thanks and regards, Raghavendra. -- N. Raghavendra ra...@mri.ernet.in | http://www.retrotexts.net/ Harish-Chandra Research Institute | http://www.mri.ernet.in/ See message headers for contact and OpenPGP information. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Big Arrays
On Sun, Oct 3, 2010 at 11:18 AM, Bulat Ziganshin bulat.zigans...@gmail.com wrote: Hello Henry, Sunday, October 3, 2010, 7:54:49 PM, you wrote: It looks like array ranges can only be Ints, and not Int64 or Word64 types. yes, it's Int internally got efficiency reasons. you can do your own implementation to override this limit :) A good place to start when rolling your own is the primitive package[1] on Hackage. It is intimately tied to GHC, however. Antoine [1] http://hackage.haskell.org/package/primitive ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Polyvariadic functions operating with a monoid
Luke, I had no idea polyvariadic functions would be controversial. Yes, in my original case the function would be trivial: toMonoid k = [toString k] I do prefer the less cluttered look. I think that (poly value1 value2 value3) is easier to follow when the values are all of related types (in my case, all in the same type class). But in the more general Monoid case there would not necessarily be a list result. Eg. sumOf 1 2 3 could also be implemented using a Monoid approach with mempty = 0 and mappend = + Kevin On Oct 3, 9:30 pm, Luke Palmer lrpal...@gmail.com wrote: On Sun, Oct 3, 2010 at 1:26 PM, Luke Palmer lrpal...@gmail.com wrote: On Sun, Oct 3, 2010 at 1:24 AM, Kevin Jardine kevinjard...@gmail.com wrote: I had a situation where I had some related types that all had toString functions. Of course in Haskell, lists all have to be composed of values of exactly the same type, so instead of passing around lists of values with these related types, I created a polyvariadic function polyToString so that I could write: (polyToString value1 value2 value3 ... valueN) which would then become a list of strings: [toString value1, toString value2, ... , toString valueN] First of all, you are not using the monoidal structure of String at all. This trick ought to work for any type whatsoever -- you're just throwing them in a list. Oops, sorry for not reading your message more closely. You were indeed talking about the monoidal structure of list. So... nevermind about this comment. :-P Other than a few brackets, commas, and a repeated identifier (which you can let-bind to shorten), what benefit is it giving you? I strongly recommend against polyvariadic functions. While you get a little bit of notational convenience, you lose composability. There are pains when you try to write a function that takes a polyvariadic function as an argument, or when you try to feed the function values from a list, etc. The mechanisms to create polyvariadic functions are brittle and hacky (eg. you cannot have a polymorphic return type, as you want in this case). Since all your values are known statically, I would recommend biting the bullet and doing it the way you were doing it. [ s value1, s value2, s value3, ... ] where s x = toString x (I had to eta expand s so that I didn't hit the monomorphism restriction) When you want to be passing around heterogeneous lists, it usually works to convert them before you put them in the list, like you were doing. I finally figured out how to do this, but it was a bit harder to figure this out than I expected, and I was wondering if it might be possible to create a small utility library to help other developers do this. It seems to me that in the general case, we would be dealing with a Monoid rather than a list of strings. We could have a toMonoid function and then return polyToMonoid value1 value2 ... valueN = (toMonoid value1) `mappend` (toMonoid value2) 'mappend' ... (toMonoid valueN) So anyone who wanted to convert a bunch of values of different types to a Monoid could easily pass them around using polyToMonoid so long as they defined the appropriate toMonoid function. Basically, a generalised list. So I tried writing the following code but GHC said it had undecidable instances. Has this ever been done successfully? class Monoidable a where toMonoid :: Monoid r = a - r polyToMonoid :: (Monoidable a, Monoid r) = a - r polyToMonoid k = polyToMonoid' k mempty class PolyVariadic p where polyToMonoid' :: (Monoidable a, Monoid r) = a - r - p instance Monoid r = PolyVariadic r where polyToMonoid' k ss = (toMonoid k) `mappend` ss instance (Monoidable a, Monoid r) = PolyVariadic (a - r) where polyToMonoid' k ss = (\a - polyToMonoid' k (toMonoid a) `mappend` ss) ___ Haskell-Cafe mailing list haskell-c...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list haskell-c...@haskell.orghttp://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] Suggestions for improvement
One question I have is whether I can eliminate points in the above definition of blowup, and write something like blowup = (++) . (blowup . allButLast, lastToTheLength) thinking of (++) as a function String x String - String. Actually (++) is of type String - String - String. When you want something of the type you mean (you normally write that as (String, String) - String in Haskell, then you can use (uncurry (++)). Additionally, you can't combine the functions (blowup . allButLast) and lastToTheLength into a function that returns a pair like you seem to attempt. You need a function like the following for that: comma :: (a - b) - (a - c) - a - (b,c) comma f g x = (f x, g x) Then you could say: blowup = (uncurry (++)) . comma (blowup . allButLast) lastToTheLength Ignore this if you haven't read about Applicative or type classes yet, but using the Applicative instance for arrow types (-) a, you can also write comma = liftA2 (,) or blowup = (uncurry (++)) . liftA2 (,) (blowup . allButLast) lastToTheLength Also, I can't figure out whether it is possible to get a shorter solution using fold. I have tried Hlint on my file, but it gave no suggestions. I am sure there are better ways, and would like some pointers and any general suggestions for improvement. By the way, shorter is not always better. Trying to recognize abstraction patterns in your code is never a bad thing though. Dominique ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Suggestions for improvement
On 10/3/10 1:45 PM, Dominique Devriese wrote: Additionally, you can't combine the functions (blowup . allButLast) and lastToTheLength into a function that returns a pair like you seem to attempt. You need a function like the following for that: comma :: (a - b) - (a - c) - a - (b,c) comma f g x = (f x, g x) Then you could say: blowup = (uncurry (++)) . comma (blowup . allButLast) lastToTheLength It is worth noting that such a function already exists in the standard libraries; it is the operator in Control.Arrow: blowup = uncurry (++) . (blowup . allButLast lastToTheLength) Cheers, Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell Platform, Hackage and Cabal : The 2nd Year : Status Report
Matthias Kilian k...@outback.escape.de writes: http://www.vimeo.com/15462768 And is there any way to just *download* the video? For people not using adobe flash? +1. I'd like to watch video offline on my phone, so Flash isn't really a good option. It doesn't work on my computer either, at least not without requiring me to download Flash 10 from Adobe. -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell Platform, Hackage and Cabal : The 2nd Year : Status Report
2010/10/3 Ketil Malde ke...@malde.org: Matthias Kilian k...@outback.escape.de writes: http://www.vimeo.com/15462768 And is there any way to just *download* the video? For people not using adobe flash? +1. I'd like to watch video offline on my phone, so Flash isn't really a good option. It doesn't work on my computer either, at least not without requiring me to download Flash 10 from Adobe. http://ossguy.com/?p=172 Cheers, Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Suggestions for improvement
Gregory, 2010/10/3 Gregory Crosswhite gcr...@phys.washington.edu: On 10/3/10 1:45 PM, Dominique Devriese wrote: Additionally, you can't combine the functions (blowup . allButLast) and lastToTheLength into a function that returns a pair like you seem to attempt. You need a function like the following for that: comma :: (a - b) - (a - c) - a - (b,c) comma f g x = (f x, g x) Then you could say: blowup = (uncurry (++)) . comma (blowup . allButLast) lastToTheLength It is worth noting that such a function already exists in the standard libraries; it is the operator in Control.Arrow: blowup = uncurry (++) . (blowup . allButLast lastToTheLength) Or you can write it as (liftA2 (,)) as I noted a few lines further in my mail ;) Dominique ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell Helper
c8h10n4o2 wrote: No, it is not secret. I'm having trouble to define functions. Take a look at my code(please be gentle) http://haskell.1045720.n5.nabble.com/file/n3100036/hai1.hs hai1.hs Can you explain in a few words what the Func constructor should represent why it has three arguments? I ask because I am not sure whether it represents function definition or function call. Maybe yuou can give a small example for a function definition as well as a function application (call) in your Hai language. Cheers Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell Helper
The problem is there. A function in Hai would be function-name, arg1,argn=body. Func stores function name,arguments and body as Strings(I was thinking to put Func String String String). The parser func that I wrote so far try to parse a function definition, not a function call. But when I try to store the function on my Map I get a error with somthing called 'functional dependencies'(which I don't know what is). -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-Helper-tp3093854p3117672.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Suggestions for improvement
On 10/3/10 2:24 PM, Dominique Devriese wrote: Or you can write it as (liftA2 (,)) as I noted a few lines further in my mail ;) Dominique I know, I just mentioned it to increase awareness of the fact that the instance methods for all the classes in Control.Arrow can equivalently be interpreted as useful pre-defined combinators for ordinary functions. Cheers, Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Suggestions for improvement
I suggest to pay more attention to haskell's standard library. allButLast is called init in Data.List module. Second, do not use explicit recursion. You can capture recursion using some high-order function like map, filter, foldr and so on: lastToTheLength xs = map f xs where f = const . last $ xs And last, your type signatures are too restrictive. You can apply your functions to arbitrary lists. lastToTheLength :: [a] - [a] Standard library knowledge is very helpful in producing short and clear definitions. blowup = concat . zipWith replicate [1..] On Mon, Oct 4, 2010 at 1:24 AM, Dominique Devriese dominique.devri...@cs.kuleuven.be wrote: Gregory, 2010/10/3 Gregory Crosswhite gcr...@phys.washington.edu: On 10/3/10 1:45 PM, Dominique Devriese wrote: Additionally, you can't combine the functions (blowup . allButLast) and lastToTheLength into a function that returns a pair like you seem to attempt. You need a function like the following for that: comma :: (a - b) - (a - c) - a - (b,c) comma f g x = (f x, g x) Then you could say: blowup = (uncurry (++)) . comma (blowup . allButLast) lastToTheLength It is worth noting that such a function already exists in the standard libraries; it is the operator in Control.Arrow: blowup = uncurry (++) . (blowup . allButLast lastToTheLength) Or you can write it as (liftA2 (,)) as I noted a few lines further in my mail ;) Dominique ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Victor Nazarov ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell Helper
c8h10n4o2 wrote: The problem is there. A function in Hai would be function-name, arg1,argn=body. Func stores function name,arguments and body as Strings(I was thinking to put Func String String String). The parser func that I wrote so far try to parse a function definition, not a function call. But when I try to store the function on my Map I get a error with somthing called 'functional dependencies'(which I don't know what is). You mean: hai1.hs:41:6: Couldn't match expected type `Hai' against inferred type `[Hai]' Expected type: Map.Map Hai Hai Inferred type: Map.Map [Hai] Hai When using functional dependencies to combine MonadState (Map.Map [Hai] Hai) m, arising from a use of `get' at hai1.hs:52:17-19 MonadState (Map.Map Hai Hai) m, arising from a use of `get' at hai1.hs:47:16-18 When generalising the type(s) for `w' The type checker tells you that you are using the same Map with different key types: at 52:17-19 the key has type [Hai], whereas at 47:16-18 it has type Hai. The latter is in your Func case: e -return $ Map.insert (a :[b]) c d where you use a :[b] which is the same as [a,b] for the key. Everywhere else, the key has type Hai. This in itself is questionable: do you really want to use arbitrary expressions as keys? Usually one would have a Map String Hai representing a map from variable (or function) names to expressions. For functions you then want data Hai = ... | Func [String] Hai | ... so that Func args body represents the (anonymous) function with the formal arguments args and the resulting expression body . The function gets a name by inserting it into the variable map. This means that a definition function-name,arg1,...,argn=body actually defines a variable named function-name which, when it gets looked up in the environment, yields the value Func [arg1,...,argn] body . Cheers Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: A parsec question
On Sun, Oct 3, 2010 at 11:55 AM, Ben Franksen ben.frank...@online.de wrote: Stephen Tetley wrote: Does this one give the expected error message for Parsec3.1 - unfortunately I can't test as I'm still using Parsec 2.1.0.1. parser = block (many digit ? digit) Unfortunately, no. Cheers Ben Hey folks, sorry about this one - my changes to parsec in 3.1 made these error messages worse. I've sent a patch off to the maintainer which fixes the examples in this thread. Antoine ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
I like it! Are the other sections available as well, e.g., (if False then else Cafe) Haskell -- Cafe - Conal On Sat, Oct 2, 2010 at 11:23 AM, Max Bolingbroke batterseapo...@hotmail.com wrote: Hi Cafe, I implemented the proposed Haskell' feature lambda-case/lambda-if [1] during the Haskell Implementors Workshop yesterday for a bit of fun. The patches are online [2, 3]. The feature is demonstrated in this GHCi session: $ inplace/bin/ghc-stage2 --interactive -XLambdaCase GHCi, version 7.1.20101002: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude (if then Haskell else Cafe) False Cafe Prelude (case of 1 - One; _ - Not-one) 1 One Prelude :q Do you like this feature and think it would be worth incorporating this into GHC? Or is it too specialised to be of use? If there is enough support, I'll create a ticket and see what GHC HQ make of it. Max [1] http://hackage.haskell.org/trac/haskell-prime/ticket/41 [2] http://www.omega-prime.co.uk/files/LambdaCase-Testsuite.patch [3] http://www.omega-prime.co.uk/files/LambdaCase-Compiler.patch ___ 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: Re: A parsec question
Antoine Latter wrote: On Sun, Oct 3, 2010 at 11:55 AM, Ben Franksen ben.frank...@online.de wrote: Stephen Tetley wrote: Does this one give the expected error message for Parsec3.1 - unfortunately I can't test as I'm still using Parsec 2.1.0.1. parser = block (many digit ? digit) Unfortunately, no. Hey folks, sorry about this one - my changes to parsec in 3.1 made these error messages worse. I've sent a patch off to the maintainer which fixes the examples in this thread. Thanks! I hope we get a new minor release with these fixes soon. I love parsec-3 very much, especially since you fixed the speed problems. Cheers Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Haskell Helper
Ben Franksen wrote: The type checker tells you that you are using the same Map with different key types: at 52:17-19 the key has type [Hai], whereas at 47:16-18 it has type Hai. The latter is in your Func case: s/latter/former/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Suggestions for improvement
On 10/3/10 5:52 PM, Victor Nazarov wrote: I suggest to pay more attention to haskell's standard library. allButLast is called init in Data.List module. Second, do not use explicit recursion. You can capture recursion using some high-order function like map, filter, foldr and so on: lastToTheLength xs = map f xs where f = const . last $ xs And just to play a little Haskell golf: lastToTheLength = ap (flip map) (const . last) -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Problem with monad transformer stack
I'm having a problem with a simple monad transformer stack that has me
stumped. Here's the sample code:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Control.Monad.Error
import Control.Monad.State
import Data.Typeable
data SomeError =
Error1
| Error2
| ErrorFail
deriving (Eq, Show, Typeable)
data MyData a = MyData [a]
instance Error SomeError where
noMsg = ErrorFail
{- This works: -}
{-
newtype StateError e s a =
StateError ((StateT s (Either e) a))
deriving (Monad,
MonadState s,
MonadError e,
Typeable)
type MyMonad a = StateError SomeError (MyData a) a
-}
{- This doesn't work: -}
newtype MyMonad a =
MyMonad ((StateT (MyData a) (Either SomeError) a))
deriving (Monad,
MonadState (MyData a),
MonadError SomeError,
Typeable)
--
Basically, the more abstracted (commented-out) version works, but the
more specific one gives this error:
Weird.hs:33:12:
Can't make a derived instance of `Monad MyMonad'
(even with cunning newtype deriving):
cannot eta-reduce the representation type enough
In the newtype declaration for `MyMonad'
Weird.hs:34:12:
Cannot eta-reduce to an instance of form
instance (...) = MonadState (MyData a) MyMonad
In the newtype declaration for `MyMonad'
Weird.hs:35:12:
Can't make a derived instance of `MonadError SomeError MyMonad'
(even with cunning newtype deriving):
cannot eta-reduce the representation type enough
In the newtype declaration for `MyMonad'
These error messages mean nothing to me. What's going on? Can the more
specific code be made to work? This is with ghc 6.12.3.
Thanks,
Mike
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Re: [Haskell-cafe] Problem with monad transformer stack
On 4 October 2010 03:40, Michael Vanier mvanie...@gmail.com wrote: newtype MyMonad a = MyMonad ((StateT (MyData a) (Either SomeError) a)) deriving (Monad, MonadState (MyData a), MonadError SomeError, Typeable) I think it's the `a'. I think it needs to be a concrete type. E.g. the following is OK: newtype MyMonad a = MyMonad ((StateT (MyData ()) (Either SomeError) a)) deriving (Monad, MonadState (MyData ()), MonadError SomeError, Typeable) But newtype MyMonad a = MyMonad ((StateT (MyData ()) (Either SomeError) [a])) deriving (Monad, MonadState (MyData ()), MonadError SomeError, Typeable) is not. This reminds me of the restriction that impredicative types remove, but I don't think it's related. These error messages mean nothing to me. What's going on? Can the more specific code be made to work? This is with ghc 6.12.3. It seems like eta-reducing `X' or `x' is enough, but Foo x,, i.e. a parametrized type with a type variable isn't enough. I think that's what's going on, but I don't know why. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problem with monad transformer stack
On Sun, Oct 3, 2010 at 9:40 PM, Michael Vanier mvanie...@gmail.com wrote:
{- This doesn't work: -}
newtype MyMonad a =
MyMonad ((StateT (MyData a) (Either SomeError) a))
deriving (Monad,
MonadState (MyData a),
MonadError SomeError,
Typeable)
This simply isn't allowed by the generalised newtype derivation machinery,
because the type variable a appears in one of the classes you're deriving.
In fact, I'm not sure how you're hoping for your type to actually work as a
monad. If you try using (=) on your type synonym that currently appears to
typecheck, you'll find that the only value that can inhabit the state
parameter is bottom. Try writing out and using a definition of (=) by hand
to understand your confusion.
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Re: [Haskell-cafe] Big Arrays
On Sun, Oct 3, 2010 at 11:54 AM, Henry Laxen nadine.and.he...@pobox.comwrote: I am trying to create a (relatively) big array, but it seems I cannot make anything larger than 2^30 or so. Here is the code: Use a 64-bit machine, where Int is 64 bits wide. Trying to create a larger array on a 32-bit machine doesn't make any sense. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problem with monad transformer stack
On 10/3/10 7:06 PM, Bryan O'Sullivan wrote:
On Sun, Oct 3, 2010 at 9:40 PM, Michael Vanier mvanie...@gmail.com
mailto:mvanie...@gmail.com wrote:
{- This doesn't work: -}
newtype MyMonad a =
MyMonad ((StateT (MyData a) (Either SomeError) a))
deriving (Monad,
MonadState (MyData a),
MonadError SomeError,
Typeable)
This simply isn't allowed by the generalised newtype derivation
machinery, because the type variable a appears in one of the classes
you're deriving.
In fact, I'm not sure how you're hoping for your type to actually work
as a monad. If you try using (=) on your type synonym that currently
appears to typecheck, you'll find that the only value that can inhabit
the state parameter is bottom. Try writing out and using a definition
of (=) by hand to understand your confusion.
I disagree with your second point. I have this in working code:
--
newtype StateErrorIO e s a =
StateErrorIO { runS :: (StateT s (ErrorT e IO) a) }
deriving (Monad,
MonadIO,
MonadState s,
MonadError e,
Typeable)
--
I can assure you that it works on non-bottom types.
As for the first point, that makes sense. So if I do this:
--
newtype MyMonadS s a =
MyMonad ((StateT s (Either SomeError) a))
deriving (Monad,
MonadState s,
MonadError SomeError,
Typeable)
type MyMonad a = MyMonadS (MyData a) a
--
it type checks. And yeah, writing out the instances by hand is the best
way to understand what's going on.
Mike
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Re: Re[2]: [Haskell-cafe] EDSL for Makefile
mention_only_once file action = do want [file] file * action main = mention_only_once file1 $ \x - do need [file2] putStrLn Hello putStrLn World Thanks Bulat I guess even this should work - main = do let file1=file1 want [file1] file1 * \x - do need [file2] putStrLn Hello putStrLn World -- Regards, Kashyap ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Big Arrays
On Sun, Oct 3, 2010 at 19:09, Bryan O'Sullivan b...@serpentine.com wrote: On Sun, Oct 3, 2010 at 11:54 AM, Henry Laxen nadine.and.he...@pobox.com wrote: I am trying to create a (relatively) big array, but it seems I cannot make anything larger than 2^30 or so. Here is the code: Use a 64-bit machine, where Int is 64 bits wide. Trying to create a larger array on a 32-bit machine doesn't make any sense. Sure it does; a 32-bit system can address much more than 2**30 elements. Artificially limiting how much memory can be allocated by depending on a poorly-specced type like 'Int' is a poor design decision in Haskell and GHC. OP: for this particular use case (unboxed Word64), an easily solution is to have some structure like (data BigArray a = BigArray (UArray Word32 a) ((UArray Word32 a) ((UArray Word32 a) ((UArray Word32 a)), with each array containing 2^30 elements. You'll need to write custom indexing and modification functions, to process the index and pass it to the appropriate array. Something like: idxBig :: BigArray a - Word32 - (UArray Word32 a, Word32) idxBig (BigArray a0 a1 a2 a3) i | i 2^30 = (a0, i) | i 2^31 = (a1, i - 2^30) | i 2^30 + 2^31 = (a2, i - 2^31) | i 2^32 = (a3, i - 2^31 - 2^30) Then wrap the existing array functions: idx :: BigArray a - Word32 - a idx arr i = let (arr', i') = idxBig arr i in arr' ! i' ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Suggestions for improvement
At 2010-10-03T22:45:30+02:00, Dominique Devriese wrote: Additionally, you can't combine the functions (blowup . allButLast) and lastToTheLength into a function that returns a pair like you seem to attempt. You need a function like the following for that: comma :: (a - b) - (a - c) - a - (b,c) comma f g x = (f x, g x) Then you could say: blowup = (uncurry (++)) . comma (blowup . allButLast) lastToTheLength Thanks, I'll try that. Ignore this if you haven't read about Applicative or type classes yet, but using the Applicative instance for arrow types (-) a, you can also write comma = liftA2 (,) I hadn't come up to that point, but will read about it now. Regards, Raghavendra. -- N. Raghavendra ra...@mri.ernet.in | http://www.retrotexts.net/ Harish-Chandra Research Institute | http://www.mri.ernet.in/ See message headers for contact and OpenPGP information. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
