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.