> The difference (in work) between map Wrapped and conv is the difference > between map id and id :: [a] -> [a]. In the absence of any fusion/rewrite > rules, the former breaks down a list, and builds up a new one with exactly the > same elements (or, every element x becomes an id x thunk, perhaps). So, in a > lazy language, inspecting each cons cell carries an additional O(1) overhead > over inspecting the corresponding cons cell in the original list (because > inspecting the former implicitly inspects the latter, and then yields a new > cons cell with the same values for inspection).
On a related note, I've been occasionally worried about conversions like 'map convert huge' where 'convert' is from one newtype to another with the same underlying type. I tried some simple examples and looking at core it seems like the 'map id huge' is optimized away. However, I'm guessing that's only because of a 'map id xs -> id xs' rewrite rule involved, and it won't work for all data structures. It seems like a better solution than relying on rewrite rules would be to lift the newtype up one level, e.g. convert 'M (Newtype x)' to 'Newtype (M x)'. Actually what I really want is to replace every function that goes M x -> x with M x -> Newtype x, but we don't have parameterized modules and doing this for something like Data.Map means a lot of boilerplate. Surely there is some more general approach? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe