How can I implement this arrow? Thanks

[Derek Elkins]

On Tue, 16 Sep 2003 14:32:48 -0500
"Yu Di" <diyu01 at hotmail.com> 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.