On Wednesday 19 November 2008 11:38:07 pm David Menendez wrote:
One possibility would be to add minimum and maximum to Ord with the appropriate default definitions, similar to Monoid's mconcat.
This is probably the most sensible way. However, first seeing this, I wanted to see if I could do it with RULES, like so: -- this of course fails for Ords where a <= b, b <=a, but a and b are -- somehow distinguishable. min' :: Ord a => [a] -> [a] -> [a] min' as@(a:at) bs@(b:bt) | a < b = as | b < a = bs | otherwise = a : min' at bt -- arbitrary choice here minimum' :: Ord a => [[a]] -> [a] minimum' = foldl1 min' {-# RULES "min-list" min = min' #-} {-# RULES "minimum-list" minimum = minimum' #-} However, trying this gives me complaints about how Ord a cannot be inferred from Ord [a], so min and minimum aren't providing the right information for min' and minimum'. Perhaps it's unreasonable to expect this to work? Anyhow, barring that problem, that's probably the best hope one has of inserting a fancy procedure for lazy data without reworking the core libraries. This also has the issue of not really solving the problem all the way down. For instance, I think: let a = replicate (2^20) 2 in minimum [[[a]], [[a]], [[1]]] still shows bad behavior. So you need an algorithm more clever than the one I've come up with. :) -- Dan