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lifting between two StateT monads
by Alexander Vodomerov 01 Jul '06

01 Jul '06

Hello! I've just started learning Haskell and have a simple question regarding monads. Please, don't blame me if it is too obvious (any RTFM links are welcome). Suppose, we have two data types, X and Y. X represent local state, Y -- global one. In some sense, X is contained in Y. For example, Y maybe (X, Z) or [X] or Map Int X. Let's take that (as it is in my case) Y = Map Int X I have function that make some transormation of local state (X) and having some other side effects. f :: StateT X m a where m is another monad, something like WriterT Log IO. Now I want to convert it into function transorming global state, more specifically it's n-th part. So, I want to combine n :: Int and f into g :: StateT Y m a where (Y ! n) is transormed via f, and other parts of Y are not touched. How can this be done? I've looked through StateT documentation and tutorial, but had not found any function from StateT X to StateT Y where Y is not the same as X. With best regards, Alexander.

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Re: ANNOUNCE: HNOP 0.1
by Ashley Yakeley 01 Jul '06

01 Jul '06

> Now we just need the Godot version, in which nothing happens twice. You may have to wait for that. -- Ashley Yakeley Seattle WA

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Re: [Haskell] Re: ANNOUNCE: HNOP 0.1
by Neil Mitchell 01 Jul '06

01 Jul '06

> > It would be interesting to compile it with John Meacham's Jhc, which > > claims to leave very little overhead in its executables. I know the > > current Jhc has a limit on total program size, but the current HNOP at > > least may slip under the wire (and I don't make any use of arrays, so it > > shouldn't be too slow). > > ; jhc -v Main.hs > 6Kb yhc Main.hs ls Main.hbc 113 bytes Thanks Neil

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Re: [Haskell] Re: ANNOUNCE: HNOP 0.1
by Robert Dockins 30 Jun '06

30 Jun '06

[moved to cafe] On Jun 30, 2006, at 4:01 AM, Ashley Yakeley wrote: > In article > <252C89C12FCAD84BA018CAD5268682FD0190C955(a)GBLONXMB02.corp.amvescap.net > >, > "Bayley, Alistair" <Alistair_Bayley(a)invescoperpetual.co.uk> wrote: > >> Cool, that's awesome. But I don't see any Haddock docs? Or a Cabal >> Setup.hs? Would it be much trouble to add them? > > Bear in mind HNOP compiles just to an executable file, so it doesn't > really have a Haskell API. > > One interesting line of development would be to spin off the core > functionality into a separate library, to provide no-op services to > other Haskell applications. I'm sorry; I know this is a serious discussion (either that or everyone involved in this discussion has a more subtle sense of humor than I), but this sentence made me laugh out loud... :-) "no-op services"? That's just great! > I'm thinking something like this: > > noop :: IO () -- generalise to other Monads? > > This would actually not be too hard to write, given my existing work, > and then of course the executable would simply be a thin wrapper. > > -- > Ashley Yakeley > Seattle WA Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG

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Haskell I/O inside: down the rabbit's hole
by Bulat Ziganshin 30 Jun '06

30 Jun '06

Hello Haskell, I'm just wrote exhaustive introduction to Haskell I/O titled HASKELL I/O INSIDE: DOWN THE RABBIT'S HOLE Can you please help me (and Haskell community) by proof-reading this text? Even if you will correct paragraph or two, it will advance the work further. Also feel free to add more explanations, correct me and ask (here) about new topics you need to be explained. The page is http://haskell.org/haskellwiki/IO_inside -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re[2]: [Haskell-cafe] Haskell I/O inside: down the rabbit's hole
by Bulat Ziganshin 30 Jun '06

30 Jun '06

Hello Donald, Friday, June 30, 2006, 1:20:03 PM, you wrote: >> http://haskell.org/haskellwiki/IO_inside > Perhaps mention > http://research.microsoft.com/~simonpj/Papers/marktoberdorf > "Tackling the awkward squad: monadic input/output, concurrency, > exceptions, and foreign-language calls in Haskell" > for people wanting to go further into the area? of course. feel free to add anything you think is useful. like 'Arrays' page, it is collective work, and like this page it can go much further from it's initial state. a have added a section, but it needs further editing -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: Speed of ByteString.Lazy
by Chad Scherrer 30 Jun '06

30 Jun '06

Then I should be thanking Duncan as well (thanks!). I had seen the array fusion idea before in the NDP work, but I hadn't thought of applying to this area. I wonder where else the concept might apply? Is there a typeclass to be built? -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx Cheers! Duncan Coutts and I have put in many long hours in improving the bytestring code over the last 3 months, in particularly the Lazy bytestring library, and array fusion on bytestrings. You can see here, http://www/~dons/images/commits/fps-commits.png<http://www/%7Edons/images/commits/fps-commits.png>, some 200 patches a month :} Its gratifying to find out that people really use the code :) -- Don -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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A question about stack overflow
by Huazhi (Hank) Gong 29 Jun '06

29 Jun '06

Hi, all I'm just a newbie for Haskell and functional programming world. The idea I currently read is quite different and interesting. I have one general question about the recursively looping style. For example: myMax [ ] = error "empty list" myMax [x] = x myMax [x:xs] = if x>= (myMax xs) then x else (myMax xs) I just list out this kind of very simple program. However, if the list size if a big number such as 10000000, the Winhug will report that the stack is overflow. Does it mean that the functional programming is lacking of scalability? I do know that we can manually change the stack size for it. But that's not a good solution according to my opinion. Yours, Hank

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how to write an haskell binding
by minh thu 28 Jun '06

28 Jun '06

hi, hopefully (well, i liked the discussions) this message will not spawn so much messages :) i'd like to know if there exist some general guidelines/information about writing an haskell (or is it *a* haskell ?) binding for a c or a c++ library. i worry about : * implementation tools : use tools like c2hs or others, or do that bare hands * haskell idioms/usage : make good use of monads, type classes and other haskell related things * for c++, is it better to first write a c api for the c++ code before writing the binding * maybe advice about the c api knowing there will be haskell binding * any thing i forgot thanks a lot, thu

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Deducing Show for GADTs
by Joel Björnson 28 Jun '06

28 Jun '06

Hi. I came a cross the following phenomena which, at least to me, occurs kind of awkward. The code below : data MyData a where DC1 :: (Show a ) => a -> MyData a instance Show (MyData a) where show (DC1 a ) = show a yields the ghci error : 'Could not deduce (Show a) from the context (Show (MyData a))' Adding a Show restriction for the instantiation as in instance Show a => Show (MyData a ) where show (DC1 a ) = show a makes the type checker happy. However, this means that all parametrised values over MyData must have a Show type which isn't necessarily what one wants. I would also like to point out that adding a 'wrapper type' as in data Wrap a = Wrap a data MyData a where DC1 :: (Show a ) => a -> MyData (Wrap a) instance Show (MyData a ) where show (DC1 a ) = show a works fine. Even though 'Wrap' does not derive Show. So, if anyone can give me some hints about the reason for this, I will appreciate it :) Thanks /Joel

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