How can I implement this arrow? Thanks

[Derek Elkins]

On Tue, 16 Sep 2003 01:52:03 -0700
Ashley Yakeley <ashley at semantic.org> wrote:

> In article <Sea1-F33yfxRydC3hSs00038d72 at hotmail.com>,
>  "Yu Di" <diyu01 at hotmail.com> 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.