Re: [Haskell-cafe] Type question re: map
map :: (a -> b) -> [a] -> [b] map takes a function and transforms a list of a's to b's. map succ [1,2,3] ==> [succ 1, succ 2, succ 3] ==> [2, 3, 4] In general, map f :: [a] -> [b] where a is domain-type of f and b is image-type of f (f :: a -> b). map map [x, y, z] ==> [map x, map y, map z] so, x,y,z should functions of type (a -> b). and the result list, [map x, map y, map z], should have type [([a] -> [b])] because map x :: [a] -> [b]where x :: a -> b On 3/6/09, R J wrote: > > > Given the following (usual) definition of "map": > > map:: (a -> b) -> [a] -> [b] > map f [] = [] > map f (x : xs) = f x : map f xs > > What's the type of "map map"? > > GHCi's :t command reveals: > > *Main> :t map map > map map :: [a -> b] -> [[a] -> [b]] > > I'd be grateful if anyone could provide a systematic type calculation so > that I can reason through more complicated examples myself. > > Thanks. > > _ > Windows Live™: Life without walls. > http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_032009 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] C-like Haskell
Did you print it? I'm using same code with ghc --make -O2 and it takes forever to finish. On Wed, Jan 28, 2009 at 8:06 PM, Duncan Coutts wrote: > On Wed, 2009-01-28 at 16:42 -0800, drblanco wrote: > >> I do already have the number I wanted, but was wondering how this could be >> made faster, or even why it's so slow. This is all on GHC 6.8.3 under OS X >> Intel, using ghc -O2. > > I'm not exactly sure what's different, but for me it works pretty well. > I put back in the Int64 type signature. > >> For comparison, the C code below runs in <1 second. > > You've got a faster machine than me :-) > > I compiled both the Haskell and C versions to standalone executables > with ghc/gcc -O2 and ran them with time. > > C version: > $ time ./circ > 3141592649589764829 > > real0m2.430s > user0m2.428s > sys 0m0.000s > > Haskell version: > time ./circ2 > 3141592653589764829 > > real0m2.753s > user0m2.756s > sys 0m0.000s > > > Not too bad I'd say! :-) > > I was using ghc-6.10 for this test. It would appear that ghc-6.8 is a > bit slower, I get: > > 3141592653589764829 > > real0m5.767s > user0m5.768s > sys 0m0.000s > > Now the other difference is that I'm using a 64bit machine so perhaps > ghc just produces terrible code for Int64 on 32bit machines. > > Duncan > > ___ > 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] Stupid question, re: overloaded type classes
The following code compiles fine on my ghci
ghci> :l sexpr.hs
[1 of 1] Compiling Sexpr( sexpr.hs, interpreted )
Ok, modules loaded: Sexpr.
$ ghci --version
The Glorious Glasgow Haskell Compilation System, version 6.8.2
-- code
{-# LANGUAGE TypeSynonymInstances #-}
module Sexpr where
data Sexp = List [Sexp]
| Atom String
deriving (Eq, Ord, Show)
class Sexpable a where
toSexp :: a -> Sexp
fromSexp :: Sexp -> Maybe a
instance Sexpable String where
toSexp s = Atom s
fromSexp (Atom s) = Just s
fromSexp _ = Nothing
instance Sexpable a => Sexpable [ a ] where
toSexp lst = List $ map toSexp lst
fromSexp (List lst) = mapM fromSexp lst
fromSexp _ = Nothing
On Sun, Jan 18, 2009 at 2:23 PM, Brian Hurt wrote:
>
> So, I'm working with this simplistic S-expression library of my own design
> (yes, I know, reinventing the wheel). Basically, I have the type:
>
> data Sexp =
>List of [ Sexp ]
>| Atom of String
>
> with the associated parsers and printers which really aren't relevent to the
> question at hand. Then, I want to define the type class of types I can
> convert to and from s-expressions, like:
>
> class Sexpable a where
>toSexp :: a -> Sexp
>fromSexp :: Sexp -> Maybe a
>
> here, fromSexp can return Nothing is the s-expression isn't the right form to
> be parsed into a whatever.
>
> Now, here's the problem. I want to define a bunch of default instances, and
> two in particular I want to define are:
>
> instance Sexpable String where
>toSexp s = Atom s
>fromSexp (Atom s) = Just s
>fromSexp _ = Nothing
>
> instance Sexpable a => Sexpable [ a ] where
>toSexp lst = List $ map toSexp lst
>fromSexp (List lst) = mapM fromSexp lst
>fromSexp _ = Nothing
>
> Note that I am not implementing Sexpable Char anywhere, so the only valid
> transform for [Char] should be the String one. But this still causes a
> compiler error due to the overloaded instances on [Char].
>
> There are two solutions to this that I already know of. One is to play games
> with newtype, which I don't like because it simply adds complexity in my case
> and doesn't help anything else. The second possibility is to compile with
> -fallow-incoherent-instances, which I'm slightly afraid of because I'm not
> sure what (if any) possible errors adding this option might allow.
>
> So my question is twofold: 1) what errors might be allowed if I add
> -fallow-incoherent-instances, and 2) is there some third choice that avoids
> both solutions I already know about?
>
> Thanks.
>
> Brian
>
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Re: [Haskell-cafe] Simplification of this function?
Sorry it won't work because list should be homogeneous.
I think your myFunc is standard.
This guy names it "thread":
http://alaska-kamtchatka.blogspot.com/2009/01/essence-of-concatenative-languages.html
On Fri, Jan 16, 2009 at 10:32 AM, sam lee wrote:
> @pl myFunc f (a,b) (c,d) = (f a c, f b d)
> myFunc = (`ap` snd) . (. fst) . flip flip snd . ((flip . (ap
> .)) .) . flip flip fst . ((flip . ((.) .)) .) . (flip =<< (((.) . flip .
> (((.) . (,)) .)) .))
>
> why not use zipWith?
>
> [a,b] `g` [c,d] where g = zipWith f
>
> 2009/1/16 Andrew Wagner
>
>> I've been playing around with this, but haven't been able to come up with
>> anything.
>> myFunc f (a,b) (c,d) = (f a c, f b d)
>>
>> It feels as if there should be a nice simple version of this using some
>> combination of {un,}curry, "on", &&&, ***, or something else.
>>
>> Any thoughts?
>>
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>>
>>
>
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Re: [Haskell-cafe] Simplification of this function?
@pl myFunc f (a,b) (c,d) = (f a c, f b d)
myFunc = (`ap` snd) . (. fst) . flip flip snd . ((flip . (ap .))
.) . flip flip fst . ((flip . ((.) .)) .) . (flip =<< (((.) . flip . (((.) .
(,)) .)) .))
why not use zipWith?
[a,b] `g` [c,d] where g = zipWith f
2009/1/16 Andrew Wagner
> I've been playing around with this, but haven't been able to come up with
> anything.
> myFunc f (a,b) (c,d) = (f a c, f b d)
>
> It feels as if there should be a nice simple version of this using some
> combination of {un,}curry, "on", &&&, ***, or something else.
>
> Any thoughts?
>
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Re: [Haskell-cafe] Time for a new logo?
http://i35.tinypic.com/mjon83.png used this: http://www.simwebsol.com/ImageTool/Default.aspx 2008/12/14 George Pollard : > ? > > > ___ > 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] Parsec and (then?) type-check
I use my type checking monad, which is separate from Parsec's monad. So, I can't think of a way to type check during parsing in Parsec's monad. Anyways, this is what I did: data Expr = ... | At SourcePos Expr SourcePos is from Parsec. Basically, my parse actions will return (At pos e). And I pass At pos e to typeCheck action. typeCheck (At pos e) = do put pos -- typeCheck is in State monad. -- in case of error, I'll pos <- get and report source position. typeCheck e typeCheck (Variable a) = do ... check out this: http://www.lipl.googlepages.com/index.html#source On Fri, Dec 12, 2008 at 5:06 PM, Greg Fitzgerald wrote: > Parser gurus, > > When you write a parser with a library like Parsec, do you typically > type-check while parsing, or afterward in a separate pass? The latter > is more modular, but it means labeling every element in the AST with > the parser position so that you can give good error messages. Do you > find the added modularity worth the hassle or just pack type-checking > into the parser pass? > > Thanks, > Greg > ___ > 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] SDL program under MinGW
Yes, with some modifications. I used SDL-1.2.13, not SDL-1.2.12. Also, my Include-Dirs has include\SDL, not include: C:\Users\client\code\SDL-1.2.13\include\SDL If I don't put include\SDL, I get errors like Graphics\UI\SDL\General.hsc:1:17: SDL.h: No such file or directory And I used the following command to configure: C:\Users\client\Downloads\SDL-0.5.4>runghc Setup.lhs configure --prefix=C:\Users\client\haskell --datadir=$prefix\share --user it builds and installs fine. I only get linker error (undefined reference to SDL_main) when I compile a program that uses SDL binding. On Sun, Dec 7, 2008 at 4:23 AM, Bit Connor <[EMAIL PROTECTED]> wrote: > Did you follow the instructions described in the WIN32 file? > > On Sat, Dec 6, 2008 at 2:05 PM, sam lee <[EMAIL PROTECTED]> wrote: >> Hi. >> >> I am using Windows Vista. >> I installed the following: >> >> - ghc 6.8.3 (using official Windows binary installer) >> - MinGW (from http://nuwen.net/mingw.html) >> - SDL binding (from >> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL) >> >> I am trying to compile the following program: >> >>module Main where >> >>import qualified Graphics.UI.SDL as SDL >> >>main = do >>SDL.init [SDL.InitEverything] >>SDL.quit >> >> I get the following error: >> >> C:\Users\client\code\sandbox>ghc --make Example.hs >> Linking Example.exe ... >> C:\MinGW\lib/libSDLmain.a(SDL_win32_main.o)(.text+0x409):SDL_win32_main.c: >> undefined reference to `SDL_main' >> >> According to SDL faq >> (http://www.libsdl.org/faq.php?action=listentries&category=4#48) I >> should use "int main(int argc, char *argv[])", not "int main()" nor >> "WinMain()". >> >> Can anyone compile the above snippet on windows machine? >> I am using SDL library that is shipped with MinGW distribution that I'm >> using. >> I also recompiled SDL haskell binding with official SDL windows binary >> (http://www.libsdl.org/release/SDL-devel-1.2.13-mingw32.tar.gz). But I >> get the same undefined reference to SDL_main linker error. >> >> Do I need to have specific main IO action so that SDL would work in Haskell? >> >> Thank you. >> Sam. >> ___ >> 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] SDL program under MinGW
I can compile http://darcs.haskell.org/~lemmih/hsSDL/hssdl/Examples/MacOSX/Test.hs But I can't find the "small Cabal file" and c_main.c . Does anyone have those files? I want to try if Test.hs links well on windows platform. > I think the binding needs to not include SDLMain and instead do the initialization itself; see the rest of that FAQ question. I can't grep for SDLMain in the binding. How should I modify the binding code to use WinMain + proper initialization? Thanks. Sam. On Sat, Dec 6, 2008 at 5:19 PM, Ryan Ingram <[EMAIL PROTECTED]> wrote: > From the link you provided > (http://www.libsdl.org/faq.php?action=listentries&category=4#48) > >> Q:I get "Undefined reference to 'SDL_main'" ... >> A:Make sure that you are declaring main() as: >> >> #include "SDL.h" >> >> int main(int argc, char *argv[]) > > In particular notice that they want you to include SDL.h. I suspect > they are doing something scary like > >> #define main(ac, av) SDL_Main(ac, av, maybe some other args) > > The problem is that the library itself is defining WinMain, so you've > got a fight between the Haskell compiler (which wants to define the > entry point for your program to start up the runtime), and the SDL > library (which is trying to be "helpful" and failing). > > I think the binding needs to not include SDLMain and instead do the > initialization itself; see the rest of that FAQ question. > > In fact, the "MACOSX" readme file in the Haskell SDL binding talks > about exactly this problem. > > -- ryan > > On Sat, Dec 6, 2008 at 2:05 PM, sam lee <[EMAIL PROTECTED]> wrote: >> Hi. >> >> I am using Windows Vista. >> I installed the following: >> >> - ghc 6.8.3 (using official Windows binary installer) >> - MinGW (from http://nuwen.net/mingw.html) >> - SDL binding (from >> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL) >> >> I am trying to compile the following program: >> >>module Main where >> >>import qualified Graphics.UI.SDL as SDL >> >>main = do >>SDL.init [SDL.InitEverything] >>SDL.quit >> >> I get the following error: >> >> C:\Users\client\code\sandbox>ghc --make Example.hs >> Linking Example.exe ... >> C:\MinGW\lib/libSDLmain.a(SDL_win32_main.o)(.text+0x409):SDL_win32_main.c: >> undefined reference to `SDL_main' >> >> According to SDL faq >> (http://www.libsdl.org/faq.php?action=listentries&category=4#48) I >> should use "int main(int argc, char *argv[])", not "int main()" nor >> "WinMain()". >> >> Can anyone compile the above snippet on windows machine? >> I am using SDL library that is shipped with MinGW distribution that I'm >> using. >> I also recompiled SDL haskell binding with official SDL windows binary >> (http://www.libsdl.org/release/SDL-devel-1.2.13-mingw32.tar.gz). But I >> get the same undefined reference to SDL_main linker error. >> >> Do I need to have specific main IO action so that SDL would work in Haskell? >> >> Thank you. >> Sam. >> ___ >> 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] SDL program under MinGW
Hi. I am using Windows Vista. I installed the following: - ghc 6.8.3 (using official Windows binary installer) - MinGW (from http://nuwen.net/mingw.html) - SDL binding (from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL) I am trying to compile the following program: module Main where import qualified Graphics.UI.SDL as SDL main = do SDL.init [SDL.InitEverything] SDL.quit I get the following error: C:\Users\client\code\sandbox>ghc --make Example.hs Linking Example.exe ... C:\MinGW\lib/libSDLmain.a(SDL_win32_main.o)(.text+0x409):SDL_win32_main.c: undefined reference to `SDL_main' According to SDL faq (http://www.libsdl.org/faq.php?action=listentries&category=4#48) I should use "int main(int argc, char *argv[])", not "int main()" nor "WinMain()". Can anyone compile the above snippet on windows machine? I am using SDL library that is shipped with MinGW distribution that I'm using. I also recompiled SDL haskell binding with official SDL windows binary (http://www.libsdl.org/release/SDL-devel-1.2.13-mingw32.tar.gz). But I get the same undefined reference to SDL_main linker error. Do I need to have specific main IO action so that SDL would work in Haskell? Thank you. Sam. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] bug in time libs?
I can't find document for System.Time .
But I can import System.Time . It's weird...
I can't find document for TimeDiff and related functions. I guess this
is all deprecated.
A related bug report: http://hackage.haskell.org/trac/ghc/ticket/2519
I would use Data.Time.Clock
Prelude> :m + Data.Time.Clock
Prelude Data.Time.Clock> getCurrentTime
Prelude Data.Time.Clock> t0 <- getCurrentTime
Prelude Data.Time.Clock> t1 <- getCurrentTime
Prelude Data.Time.Clock> t0
Prelude Data.Time.Clock> diffUTCTime t0 t1
-2.912s
Prelude Data.Time.Clock> diffUTCTime t1 t0
2.912s
Prelude Data.Time.Clock> :t diffUTCTime t1 t0
diffUTCTime t1 t0 :: NominalDiffTime
On Mon, Dec 1, 2008 at 10:05 PM, Jason Dusek <[EMAIL PROTECTED]> wrote:
> I'm on 6.10.1, using the libs provided with the binary of GHC
> for Mac OS X. I was fooling around with the time package in
> GHCi and something seems to be off. I have included a
> transcript.
>
> If I take a time diff, then the seconds are positive when the
> picos are negative and vice versa.
>
> --
> _jsn
>
>
> |...transcript...|
>
>
> Prelude System.Time> :m + System.Time
> Prelude System.Time> t0 <- System.Time.getClockTime
> Prelude System.Time> t1 <- System.Time.getClockTime
> Prelude System.Time> diffClockTimes t0 t1
> TimeDiff { tdYear = 0
> , tdMonth = 0
> , tdDay = 0
> , tdHour = 0
> , tdMin = 0
> , tdSec = -7
> , tdPicosec = 23884600
> }
> Prelude System.Time> diffClockTimes t1 t0
> TimeDiff { tdYear = 0
> , tdMonth = 0
> , tdDay = 0
> , tdHour = 0
> , tdMin = 0
> , tdSec = 7
> , tdPicosec = -23884600
> }
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Re: [Haskell-cafe] '#' in literate haskell
# is significant because it can be sh-bang line or pre-processor.
The only way I can think of is:
alias lhspp="sed 's/^#//'"
ghc --make -F -pgmF lhspp File.lhs
On Sat, Nov 29, 2008 at 10:07 PM, John MacFarlane <[EMAIL PROTECTED]> wrote:
> Can anyone explain why ghc does not treat the following
> as a valid literate haskell program?
>
> - test.lhs
> # This is a test
>
>> foo = reverse . words
>
>
>
> When I try to load this in ghci (or compile it using ghc),
> I get:
>
> test.lhs:1:2: lexical error at character 'T'
>
> It seems that the problem is the '#' character in the first
> column. Replacing it with something else, or moving it to
> the right one space, solves the problem.
>
> The following literate haskell program, from
> http://notvincenz.blogspot.com/2008/01/literate-haskell-and-c.html,
> also fails to load for me, for the same reason (the leading
> '#' in line 8).
>
> literate-haskell-and-c.lhs ---
> /* c and lhs file
>
>>
>> module Foo where
>> main = print "Haskell"
>>
>
> */
> #include
>
> int main() {
> printf("C\n");
> return 0;
> }
>
>
> I've reproduced this with ghc 6.10.1 and ghc 6.8.3 (linux binaries
> from haskell.org) and with ghc 6.8.2 (Ubuntu intrepid).
> Interestingly, hugs (September 2006 version) has no trouble with
> test.lhs. I haven't tried ghc 6.6.
>
> I care about this because I'd like to use markdown conventions
> to format the comment parts of literate haskell programs. Markdown
> supports atx-style headers, which begin with strings of '#'
> characters starting in the first column. I know that some people use
> markdown with literate haskell, so there must be something basic here
> that I'm missing!
>
> John
>
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Re: [Haskell-cafe] SOEGraphics in GHC?
http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HGL or http://hackage.haskell.org/cgi-bin/hackage-scripts/package/soegtk 2008/11/24 Dmitri O.Kondratiev <[EMAIL PROTECTED]>: > Please help, to locate in GHC distribution SOEGraphics library from > Paul Hudak, book "The Haskell School of Expression" > (http://www.haskell.org/soe/ ) > > Thanks! > > ___ > 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] What *not* to use Haskell for
I haven't found multitrack audio recording applications written in Haskell. These are usually written in C++ using Jack audio or ASIO. This probably means that it is not a good idea to write real time audio applications in Haskell at the moment. So, probably avoid writing applications that use a high frequency timer and IO that should be synchronous to the timer in Haskell. On Tue, Nov 11, 2008 at 9:02 PM, wren ng thornton <[EMAIL PROTECTED]> wrote: > Dave Tapley wrote: >> >> Hi everyone >> >> So I should clarify I'm not a troll and do "see the Haskell light". But >> one thing I can never answer when preaching to others is "what does >> Haskell not do well?" >> >> Usually I'll avoid then question and explain that it is a 'complete' >> language and we do have more than enough libraries to make it useful and >> productive. But I'd be keen to know if people have any anecdotes, >> ideally ones which can subsequently be twisted into an argument for >> Haskell ;) > > With the appropriate caveats about particular subdomains (see final > paragraph), I wouldn't use Haskell for scripting. That is, (1) for > Bash-style programming where 95% of the code is just invoking *nix jobs, or > (2) for very simple yet regex-heavy scripts where Perl/Awk/Sed is often > used. > > Re #1: Honestly, I don't see anything other than a dedicated competitor > being able to unseat Bourne/Bash at this task. Certainly a competitor would > have much room for improvement-- what with being able to replace > string-rewriting semantics with term-rewriting semantics, vastly improving > type safety and catching innumerable bugs. However, with unsavory frequency, > it is exactly those type-unsafe substitutions which can make shell scripting > cleaner and more direct than a type-safe alternative. Having type safety as > well as this sort of non-compositional structure would take a good deal of > work to get right. > > Re #2: People often complain about spooky Perl that uses things like > implicit $_ or other hidden variables. While these constructs can make any > sizable project unmaintainable, for the quick and dirty jobs they're just > what's needed to get things done with clarity. While ByteString code using > regexes is just as fast in Haskell, it's often more than twice as long as > the Perl, Sed, or Awk equivalents because many of the basic control > structures (like Perl's -n, -p, -l,... flags) aren't already provided. > > > That said, this isn't necessarily a bad thing for Haskell. "Real" > programming languages often don't do so well in these areas (Perl being the > exception), and they don't feel too bad about it. Both varieties of shell > scripting are very much of the DSL nature; for programs with a significant > amount of "actual logic" instead of mere plumbing or regexing, Haskell can > certainly outshine these competitors. On the one hand, C and friends fight > dirty and much work has been done so Haskell can join in on the bit-bashing > glory. However, shell scripting is a whole different kind of imperative muck > and very little work (that I've seen) has tried to get Haskell to jump down > in the sewers with them. > > -- > Live well, > ~wren > ___ > 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] An interesting curiosity
I think it's because + is left associative. and pattern matching is
top to bottom.
1 + 1 + 1 => (1 + 1) + 1 => found match: 1 + 1 = 3 => 3 + 1 => found
match: 3 + 1 = 3 => 3
On Thu, Sep 18, 2008 at 11:01 PM, Steven Shaw <[EMAIL PROTECTED]> wrote:
> 2008/9/19 Evan Laforge <[EMAIL PROTECTED]>
>> let {1 + 1 = 3; 3 + 1 = 3} in 1 + 1 + 1
>
> Which gives 3.
>
> Perhaps even more confusing is:
>
> let {1 + 1 = 3; 3 + 1 = 3} in 3 + 1 + 1
>
> which gives 3
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Re: [Haskell-cafe] How to do this in FP way?
I can think of 2 ways.
> module Main where
>
> import Control.Monad.State
First, normal way:
> diff (now, old) = (now - old, now)
diff takes now and old and returns result (now - old) and modified old (now).
For example,
diff (diff (1,0))
==> diff (1 - 0, 1)
==> diff (1, 1)
==> (1 - 1, 1)
==> (0, 1)
I think people use the word "threaded" to describe what diff is doing:
the variable "old" is threaded through many calls to diff.
> testDiff = diff . diff . diff . diff . diff . diff $ (2, 1)
testDiff returns (2,1)
Second way is using monads:
> diff' now = do
> old <- get
> put now
> return (now - old)
diff' uses State monad.
If you're not familiar with monads, State monad does similar to what
diff function does (it threads the variable "old").
But, being a monadic action, diff' looks like imperative version
syntactically. It gives illusion of having global variable (old).
> testDiff' = do
> result <- diff' 2
> result <- diff' result
> result <- diff' result
> result <- diff' result
> result <- diff' result
> result <- diff' result
> return result
>
> runTestDiff' = runState testDiff' 1
runTestDiff' also returns (2,1)
2008/6/15 Magicloud Magiclouds <[EMAIL PROTECTED]>:
> Hello,
> I am getting familiar with FP now, and I have a "program design" kind of
> question.
> Say I have something like this in C:
> static int old;
> int diff (int now) { /* this would be called once a second */
> int ret = now - old;
> old = now;
> return ret;
> }
> Because there is no "variable" in Haskell. So how to do this in a FP
> way?
>
> Thanks.
>
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>
>
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Re: [Haskell-cafe] M1 + M2 = M3 where both computations in M1 and M2 can be used?
> tInfer :: MonadTypeInfer m => Expr -> m Type > eval :: MonadEval m => Expr -> m Expr That solves! I should've left out type annotation. On Mon, May 12, 2008 at 10:38 AM, Twan van Laarhoven <[EMAIL PROTECTED]> wrote: > sam lee wrote: > > > > > > Hi. > > > > I want to compose two monads to build another monad where > > computations of the two monads can be used inside. > > > > I have: > > > > - MonadTypeInfer : interface (class) for TypeInfer monad > > - TypeInfer : a monad that has Map String Type (association of names and > types) > > - TypeInferT : transformer of above monad > > - MonadEval : interface (class) for Eval monad > > - Eval : a monad that has Map String Expr (association of names and > > code/function body) > > - EvalT : transformer of Eval > > - tInfer :: Expr -> TypeInfer Type -- given expr, returns type of it > > in TypeInfer monad > > - eval :: Expr -> Eval Expr -- given expr, returns normalized expr in Eval > monad > > > > > > Is there a way to build a monad where you could use sub-monads' > > (monads used to build current monad) computations? > > > > A solution to this problem is to use type classes, and in particular > MonadTrans. You can then give an instance of MonadTypeInfer for EvalT m > where m is an instance of MonadTypeInfer, and similarly an instance > MonadEval for TypeInferT m. How this is implemented depends on the Monads in > question, but if you use the monad transformer library with newtype deriving > you can just add "deriving MonadTrans". > > class Monad m => MonadTypeInfer m where > -- functions -- > tiStuff :: X -> m Whatever > > class Monad m => MonadEval m where > -- functions -- > > instance Monad m => MonadTypeInfer (TypeInferT m) where > -- implementation -- > tiStuff = ... > > instance Monad m => MonadEval (EvalT m) where > -- implementation -- > > instance MonadEval m => MonadTypeInfer (EvalT m) where > -- lift the functions from TypeInfer through the EvalT type, > -- MonadTrans from the mtl might help here > tiStuff x = lift (tiStuff x) > > tInfer :: MonadTypeInfer m => Expr -> m Type > eval :: MonadEval m => Expr -> m Expr > > > Twan > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] M1 + M2 = M3 where both computations in M1 and M2 can be used?
Hi.
I want to compose two monads to build another monad where
computations of the two monads can be used inside.
I have:
- MonadTypeInfer : interface (class) for TypeInfer monad
- TypeInfer : a monad that has Map String Type (association of names and types)
- TypeInferT : transformer of above monad
- MonadEval : interface (class) for Eval monad
- Eval : a monad that has Map String Expr (association of names and
code/function body)
- EvalT : transformer of Eval
- tInfer :: Expr -> TypeInfer Type -- given expr, returns type of it
in TypeInfer monad
- eval :: Expr -> Eval Expr -- given expr, returns normalized expr in Eval monad
Problem: in repl, when user defines a function, it should type check
and register type of the function to TypeInfer monad's Map String
Type.
Also, it should store the expression of the function in Eval monad.
I build REPL monad using TypeInferT and EvalT.
> newtype REPL a = REPL { runREPL :: TypeInferT (EvalT IO) a }
> deriving(Monad, Functor, MonadIO, MonadTypeInfer, MonadEval)
> repl :: REPL ()
> repl = do
> input <- prompt ">>> "
> case parse input of
> Left err -> -- handle error
> Right expr -> do
> t <- tInfer expr -- BAD!! tInfer :: TypeInfer Type
> println (show t)
> result <- eval expr -- BAD!! eval :: Eval Expr
> println (show result)
> repl
Should I make tInfer :: REPL Type, eval :: REPL Expr?
Is there a way to build a monad where you could use sub-monads'
(monads used to build current monad) computations?
I prefer keeping tInfer :: TypeInfer Type, eval :: Eval Expr
because tInfer never uses actions in Eval monad and vice versa.
It seems like what I am asking is to break the type system.
Maybe I should just make them run in REPL monad.
Thank you.
Sam.
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