[Haskell-cafe] Re: [Haskell] Arrows GUI Library Based on GTK+

[Kevin Atkinson]

On Tue, 22 Mar 2005 ross at soi.city.ac.uk wrote:

> On Tue, Mar 22, 2005 at 09:52:04AM -0700, Kevin Atkinson wrote:
> > I must admit that I am baffled by what this is doing.  But I don't think
> > it has the semantics I want.  When I try substituting your
> > code in I get "Exception: <<loop>>".
>
> I could have made it a bit simpler:
>
> instance ArrowLoop FG' where
>         loop (FG' f) = FG' $ \ c x -> do
>                 (c', ~(x', _)) <- mfix $ \ ~(_, ~(_, y)) -> f c (x, y)
>                 return (c', x')
>
> This executes f once only, with y bound to the third component of the
> output of f.  This isn't available until f finishes, so any attempt
> to examine it while f is running will lead to <<loop>>, but f can pass
> it around, and store it in data structures; it can even create cyclic
> structures.  (Under the hood, the IO instance of mfix starts with y bound
> to an exception, and updates it when f finishes, a bit like what you're
> trying to do with IORef's, except that existing references to y then
> point at the updated thing.)  Your definition runs f once with undefined
> as the last argument to get a value for y to supply to a second run.
> Presumably the things you're doing with Control need to change too,
> and I don't understand all that, but I expect that the mfix version
> could be made to work, and would do less work.

I think I understand it more, but I am not sure it will do what I want.
For one thing f still needs to know when it can examine its input thus,
still needing an initializing pass.

Please have a look at the new code.  I have reworked how loops are handled
and no longer use Control.  Also the state variable is now needed in
separate functions.  Thus I am not sure I can use the mfix trick to hide
the state.