Re: How can I implement this arrow? Thanks
In article <[EMAIL PROTECTED]>, Derek Elkins <[EMAIL PROTECTED]> wrote: > It's not an arrow the way Yu Di wanted it, but it is an arrow. Which > arrow it is was part of my point in the latter paragraphs. Oh yes, you're right. For some reason I wasn't thinking of the String as state. -- Ashley Yakeley, Seattle WA ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How can I implement this arrow? Thanks
On Tue, 16 Sep 2003 23:28:43 -0700 Ashley Yakeley <[EMAIL PROTECTED]> wrote: > In article <[EMAIL PROTECTED]>, > Derek Elkins <[EMAIL PROTECTED]> wrote: > > > > I don't think this type is an arrow. For a "product arrow", i.e. > > > an instance of Hughes' "Arrow" class with "first" defined, you can > > > define this: > > > > Oh, it's definitely an arrow. > > I don't think you can make it an instance of Hughes' Arrow without > some function for combining Strings. It's not an arrow the way Yu Di wanted it, but it is an arrow. Which arrow it is was part of my point in the latter paragraphs. Here's the StateFunctor from Hughes' paper: type StateFunctor s a b c = a (b,s) (c,s) Using String for s and (->) for a we get: type StringStateArrow b c = (b,String) -> (c,String) alternatively, looking at the Kleisli arrow with m being the State monad we get: type Kleisli m a b = a -> m b type StateM s a = s -> (a,s) type StateA s a b = Kleisli (StateM s) a b type StateA s a b = a -> s -> (b,s) type StateA s a b = (a,s) -> (b,s) type StringState a b = (a,String) -> (b,String) or, if we do combine Strings we get something like the Writer monad: type Monoid m => WriterM m a = (a,m) type Monoid m => WriterA m a b = Kleisli (WriteM m) a b type Monoid m => WriterA m a b = a -> (b,m) type StringWriterA a b = a -> (b,String) where mempty = [] and mappend = (++) In either of these cases, there is not much reason to use an arrow over a monad if it's the only arrow you are using and there are reasonable reasons to use a monad over an arrow. Neither of these do what Yu Di apparently wanted. In the more recent post by Yu Di, an arrow or monad looks more appropriate, though for different reasons and in a different way, which just goes to show that when posting one should state what the ultimate goal is. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How can I implement this arrow? Thanks
In article <[EMAIL PROTECTED]>, Derek Elkins <[EMAIL PROTECTED]> wrote: > > I don't think this type is an arrow. For a "product arrow", i.e. an > > instance of Hughes' "Arrow" class with "first" defined, you can define > > this: > > Oh, it's definitely an arrow. I don't think you can make it an instance of Hughes' Arrow without some function for combining Strings. -- Ashley Yakeley, Seattle WA ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How can I implement this arrow? Thanks
On Tue, Sep 16, 2003 at 02:32:48PM -0500, Yu Di wrote: > What I am trying to do is to use Haskell to simulate some process, and > meanwhile collecting information about the data-flow throughout the whole > process into the output. The (String, a) example is just a simplified > version of that. Thanks for pointing out my mistakes, I will try to do it > as a decorated-data combinator. This sounds a bit like the example in http://www.soi.city.ac.uk/~ross/talks/fop.4.ps.gz The idea is to define your network as a generic arrow, and instantiate it differently for different interpretations. For simulation, one can use Stream i -> Stream o, though there are other possibilities. To get a map, you use a state transformer (with the map as the state), and pass edge labels through the arrows. I think the reason you're getting in a tangle is that you're trying to do both with the same interpretation. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How can I implement this arrow? Thanks
On Tue, 16 Sep 2003 14:32:48 -0500
"Yu Di" <[EMAIL PROTECTED]> wrote:
> What I am trying to do is to use Haskell to simulate some process, and
> meanwhile collecting information about the data-flow throughout the
> whole process into the output. The (String, a) example is just a
> simplified version of that. Thanks for pointing out my mistakes, I
> will try to do it as a decorated-data combinator.
>
> Thanks again,
If all you want to do is trace the execution of some computation, you
can use the Writer/Output monad. This will associate the outputting
with the functions that process the simulation. It looks something
like,
foo a b = do
tell ("Entering foo with "++show a++" "++show b)
doStuff a
someMoreStuff b
tell "Leaving foo"
You can write things other than strings.
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Re: How can I implement this arrow? Thanks
What I am trying to do is to use Haskell to simulate some process, and meanwhile collecting information about the data-flow throughout the whole process into the output. The (String, a) example is just a simplified version of that. Thanks for pointing out my mistakes, I will try to do it as a decorated-data combinator. Thanks again, Di, Yu 9.16 From: Derek Elkins <[EMAIL PROTECTED]> To: Yu Di <[EMAIL PROTECTED]> CC: [EMAIL PROTECTED], Ashley Yakeley <[EMAIL PROTECTED]> Subject: Re: How can I implement this arrow? Thanks Date: Tue, 16 Sep 2003 12:24:17 -0400 On Tue, 16 Sep 2003 01:52:03 -0700 Ashley Yakeley <[EMAIL PROTECTED]> wrote: > In article <[EMAIL PROTECTED]>, > "Yu Di" <[EMAIL PROTECTED]> wrote: Replying to both: > > Hi, I want to create an arrow which is essentially > > > > data MyArrow a b = MyArrow ((String, a) -> (String,b)) > > I don't think this type is an arrow. For a "product arrow", i.e. an > instance of Hughes' "Arrow" class with "first" defined, you can define > this: Oh, it's definitely an arrow. > > i.e. there is an "information" asscioated with each piece of data > > (represented by the string), So make a data type that sticks together something and it's information. > > and I want to pass it around. What's wrong with the support Haskell already has for passing around things? Why do you think you need an arrow? From how you read the meaning of the MyArrow type, you are apparently misunderstanding what arrows are. Arrows generalize functions. As such things of arrow type are intuitively transformers and the arrow framework standardizes how to stick transformations together to make a larger transformation. The things in an arrow's representation are what's required for the implementations of the transformations. Therefore, things in an arrows representation are, in a sense, "owned" by the arrow computation. This is evidenced by a) arrows have a fully polymorphic type, they place no restrictions on their input; within the computation, the String in MyArrow is not even visible let alone required to be provided, and b) the above arrow is the State arrow specialized to Strings. The String is the state and is owned by the computation. You are having trouble defining 'first' as you'd like because what you want is to duplicate the state of the computation. -The- (there is only one at any time) String being passed around isn't associated with the objects. From the sounds of it you simply want a combinator library that operates on some decorated data. A monad or arrow may be useful to support that, e.g. the environment monad, but a monad/arrow isn't that. You haven't really given much detail to what you're ultimately trying to achieve so I can't really provide much advice. _ Use custom emotions -- try MSN Messenger 6.0! http://www.msnmessenger-download.com/tracking/reach_emoticon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How can I implement this arrow? Thanks
On Tue, 16 Sep 2003 01:52:03 -0700 Ashley Yakeley <[EMAIL PROTECTED]> wrote: > In article <[EMAIL PROTECTED]>, > "Yu Di" <[EMAIL PROTECTED]> wrote: Replying to both: > > Hi, I want to create an arrow which is essentially > > > > data MyArrow a b = MyArrow ((String, a) -> (String,b)) > > I don't think this type is an arrow. For a "product arrow", i.e. an > instance of Hughes' "Arrow" class with "first" defined, you can define > this: Oh, it's definitely an arrow. > > i.e. there is an "information" asscioated with each piece of data > > (represented by the string), So make a data type that sticks together something and it's information. > > and I want to pass it around. What's wrong with the support Haskell already has for passing around things? Why do you think you need an arrow? >From how you read the meaning of the MyArrow type, you are apparently misunderstanding what arrows are. Arrows generalize functions. As such things of arrow type are intuitively transformers and the arrow framework standardizes how to stick transformations together to make a larger transformation. The things in an arrow's representation are what's required for the implementations of the transformations. Therefore, things in an arrows representation are, in a sense, "owned" by the arrow computation. This is evidenced by a) arrows have a fully polymorphic type, they place no restrictions on their input; within the computation, the String in MyArrow is not even visible let alone required to be provided, and b) the above arrow is the State arrow specialized to Strings. The String is the state and is owned by the computation. You are having trouble defining 'first' as you'd like because what you want is to duplicate the state of the computation. -The- (there is only one at any time) String being passed around isn't associated with the objects. >From the sounds of it you simply want a combinator library that operates on some decorated data. A monad or arrow may be useful to support that, e.g. the environment monad, but a monad/arrow isn't that. You haven't really given much detail to what you're ultimately trying to achieve so I can't really provide much advice. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How can I implement this arrow? Thanks
In article <[EMAIL PROTECTED]>,
"Yu Di" <[EMAIL PROTECTED]> wrote:
> Hi, I want to create an arrow which is essentially
>
> data MyArrow a b = MyArrow ((String, a) -> (String,b))
I don't think this type is an arrow. For a "product arrow", i.e. an
instance of Hughes' "Arrow" class with "first" defined, you can define
this:
arrProduct :: arrow p q -> arrow p r -> arrow p (q,r);
arrProduct apq apr =
arr (\p -> (p,p)) >>>
first apr >>>
arr (\(r,p) -> (p,r)) >>>
first apq;
This has a certain intuitive symmetry in its arguments, though for
instance for a Kleisli arrow (a -> m b for some monad m), arrProduct
will essentially execute one before the other. But if you try to define
this directly for MyArrow, you'll find you need to pick one of the two
Strings (losing information) or else combine them somehow. Perhaps this:
arrProduct (MyArrow f1) (MyArrow f2) = MyArrow (\sa -> let
{
(s1,b1) <- f1 sa;
(s2,b2) <- f2 sa;
} in (s1 ++ s2,(b1,b2)));
In general, a type of the form (f a -> f b) is an arrow if f is a
Functor that has one of these:
fApply :: f (a -> b) -> f a -> f b;
fProduct :: f a -> f b -> f (a,b);
I call this class of Functors "FunctorApply", but maybe someone has a
better name. I'm not sure what the attached laws are, but I imagine
they're fairly straightforward.
--
Ashley Yakeley, Seattle WA
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Re: How can I implement this arrow? Thanks
Am Dienstag 16 September 2003 04:57 schrieb Yu Di: > Hi, I want to create an arrow which is essentially > > data MyArrow a b = MyArrow ((String, a) -> (String,b)) > > i.e. there is an "information" asscioated with each piece of data > (represented by the string), and I want > to pass it around. But I have a problem about how to define "pure" and > "first". At first, I declared > > pure f = MyArrow (\(s, x) -> (s, f x)) > first (MyArrow f) = MyArrow (\(s, (x, y)) -> let (s', z) = f (s, x) in (s', > (z, y))) > > this seems to work, but then I begin to have problems with the > "data-plumbing" pure arrows, e.g. in > > pure (\x -> (x, x)) >>> first someArrow pure (\(_, x) -> x) > > Ideally, this arrow will preserve whatever information I put there for the > input, but because "first > someArrow" will change the whole information associated with the pair of > result, I can't find any > way to let "pure (\(_, x)->x)" (which is an extremely generic function) > retrieve the part of information for the second piece in the pair tuple. what does the compiler say? or is it a runtime error? how did you implement "(>>>) :: a b c -> a c d -> a b d"? (MyArrow f1) >>> (MyArrow f2) = MyArrow (f2 . f1) does this compile?: (pure (\x -> (x, x)) :: MyArrow a (a,a)) >>> (first (someArrow :: MyArrow a b) :: MyArrow (a,a) (b,a)) >>> (pure (\(_, x) -> x) :: MyArrow (b,a) a) pure and first seem to be correct. but ... just as an (slow) alternative: -- first :: a x fx -> a (x, y) (fx, y) first (MyArrow f) = MyArrow $ (\((fs,fx),y)->(fs,(fx,y))) . (\(sx,y)->(f sx,y)) . (\(s,(x,y))->((s,x),y)) - marc > > Of course I can create specialized arrows for the tasks \x -> (x, x) and > \(_, x) -> x which passes the information around, but this will become > tedious as I will have to define specialized arrows for a lot of similar > tasks one by one, and I won't be able to use the arrow pre-processor at all. > > So how can I implement this? Thanks very much! > > Di, Yu > 9.15 > > _ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
How can I implement this arrow? Thanks
Hi, I want to create an arrow which is essentially data MyArrow a b = MyArrow ((String, a) -> (String,b)) i.e. there is an "information" asscioated with each piece of data (represented by the string), and I want to pass it around. But I have a problem about how to define "pure" and "first". At first, I declared pure f = MyArrow (\(s, x) -> (s, f x)) first (MyArrow f) = MyArrow (\(s, (x, y)) -> let (s', z) = f (s, x) in (s', (z, y))) this seems to work, but then I begin to have problems with the "data-plumbing" pure arrows, e.g. in pure (\x -> (x, x)) >>> first someArrow pure (\(_, x) -> x) Ideally, this arrow will preserve whatever information I put there for the input, but because "first someArrow" will change the whole information associated with the pair of result, I can't find any way to let "pure (\(_, x)->x)" (which is an extremely generic function) retrieve the part of information for the second piece in the pair tuple. Of course I can create specialized arrows for the tasks \x -> (x, x) and \(_, x) -> x which passes the information around, but this will become tedious as I will have to define specialized arrows for a lot of similar tasks one by one, and I won't be able to use the arrow pre-processor at all. So how can I implement this? Thanks very much! Di, Yu 9.15 _ Get 10MB of e-mail storage! Sign up for Hotmail Extra Storage. http://join.msn.com/?PAGE=features/es ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
