Hi Andrew, On Mon, Oct 06, 2008 at 09:48:51PM +0100, Andrew Coppin wrote:
data ResultSet x = Pack {unpack :: [[x]]} deriving (Eq)
Your ResultSet monad is roughly equivalent to newtype Nat = Nat Int instance Monoid Nat where mempty = Nat 0 (Nat x) `mappend` (Nat y) = Nat (x+y) type ResultSet' = WriterT Nat [] -- ResultSet' x = [(x, Nat)] where unpack :: ResultSet' x -> [[x]] gives a list whose nth element is the list of alternatives whose "cost" (Nat data) is n (with trailing [] lists removed). Except that using [[x]] internally lets you be lazy about handling items of high cost. (This is kind of neat actually.) I'd therefore guess that if there is an associated monad transformer ResultSetT, it's similarly equivalent to ResultSetT' m x = WriterT Nat (ListT m x) where ListT is some version of "ListT done right". But on the other hand, as I understand "ListT done right", you can think of ListT m x as a "list" of xs where you have to perform an action in the m monad to get each successive value in the list. The equivalence converting ResultSet' to ResultSet sort of "tears up" the list in a way I'm not sure is compatible with inserting a monad like that. Once again, all this high-falutin' nonsense corresponds to really concrete questions about what you want your code to *actually do*. Consider your original problem
run :: State -> Foo -> Either ErrorType (ResultSet State)
run_and :: State -> Foo -> Foo -> Either ErrorType (ResultSet State) {- some Either-ified version of run_and :: State -> Foo -> Foo -> ResultSet State run_and s0 x y = do s1 <- run s0 x s2 <- run s1 y return s2 -}
Say run s0 x returns many different possibilities s1 (with varying costs). And suppose run s1 y is a (Left err) for some of these s1 and a (Right whatever) for others. When should the overall result of run_and be a Left and when should it be a Right? And *which error* should you return if there's more than one Left? Do you really want to check whether every run s1 y is a (Right whatever)? In that case you are not gaining much from the laziness of ResultSet and might as well use ResultSet'. Until you decide the answer to questions of this kind, you can't know how to best structure your code. Regards, Reid Barton