On Wed, Feb 17, 2010 at 6:58 AM, Heinrich Apfelmus
Ah, I meant to use the union' from your previous message, but I think that doesn't work because it doesn't have the crucial property that the case
union (VIP x xs) ys = ...
does not pattern match on the second argument.
Ahh yes, my original union' has a bit that looks like this union' (VIP x xs) (VIP y ys) = case cmp x y of LT -> VIP x (union' xs (VIP y ys)) EQ -> VIP x (union' xs ys) GT -> error "Data.List.Ordered.unionAll: assumption violated!" union' (VIP x xs) (Crowd ys) = VIP x (union' xs (Crowd ys)) For whatever reason, this works in the case of an infinite number of lists with my original version, but not the simplified version. By applying a standard transformation to make this lazier, we can rewrite these clauses as union' (VIP x xs) ys = VIP x $ case ys of Crowd _ -> union' xs ys VIP y yt -> case cmp x y of LT -> union' xs ys EQ -> union' xs yt GT -> error msg In the original case, we have this strictness property union' (VIP x xs) ⊥ == ⊥ The revised verison is a bit lazier: union' (VIP x xs) ⊥ == VIP x ⊥ And so the simplified unionAll now works again on an infinite number of lists. I've uploaded data-ordlist-0.4.2 to fix the bug introduced with data-ordlist-0.4.1, and added a regression test to the suite. Best, Leon