On Sat, Dec 8, 2018 at 7:46 PM Jeffrey Brown wrote:

The following expressions both cause GHCI to hang:

...

S.intersection (S.fromList [1,2]) (S.fromList [1..]) fromList ^CInterrupted. S.intersection (S.fromList [1..]) (S.fromList [1,2]) fromList ^CInterrupted.

Is there a smarter way to take the intersection of sets when at least one of them is small (but I don't know which one that is)?

Taking the intersection of already-constructed, finite sets is very fast if one of them is small (should be O(n lg m) where n is the size of the smaller set and m the size of the larger one). However, Data.Set doesn't handle infinite sets as it tries to construct balanced trees and is strict in the keys. Even fromAscList won't help you here. To represent infinite (or bigger than you actually want to construct) sets you'll likely need to roll your own custom representation, and I suspect it will in general need to either have limitations on the key space or worse asymptotic performance. That said, you can often get the behavior you're looking for by representing big sets by a membership predicate and using filter. Here's an (untested) example of the right sort of thing: data MySet k = Pred (k -> Bool) | Concrete (S.Set k) intersection (Pred a) (Pred b) = Pred (\k -> a k && b k) intersection (Concrete a) (Pred b) = Concrete (S.filter b a) intersection (Pred a) (Concrete b) = Concrete (S.filter a b) intersection (Concrete a) (Concrete b) = Concrete (S.intersection a b) Note that a concrete set "concretizes" anything it touches. Don't take unions of these sets, though, it'll just be a mess. -Jan-Willem Maessen

-- Jeff Brown | Jeffrey Benjamin Brown Website https://msu.edu/~brown202/ | Facebook https://www.facebook.com/mejeff.younotjeff | LinkedIn https://www.linkedin.com/in/jeffreybenjaminbrown(spammy, so I often miss messages here) | Github https://github.com/jeffreybenjaminbrown

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Not like that, no. The Set type is explicitly for *finite* sets only. fromList [1..] is bottom and will run out of memory. You'd need a *very* different implementation to be able to support infinite sets at all, and even then you'd only catch certain special cases. On Sat, Dec 8, 2018, 7:46 PM Jeffrey Brown

The following expressions both cause GHCI to hang:

...

S.intersection (S.fromList [1,2]) (S.fromList [1..]) fromList ^CInterrupted. S.intersection (S.fromList [1..]) (S.fromList [1,2]) fromList ^CInterrupted.

Is there a smarter way to take the intersection of sets when at least one of them is small (but I don't know which one that is)?

-- Jeff Brown | Jeffrey Benjamin Brown Website https://msu.edu/~brown202/ | Facebook https://www.facebook.com/mejeff.younotjeff | LinkedIn https://www.linkedin.com/in/jeffreybenjaminbrown(spammy, so I often miss messages here) | Github https://github.com/jeffreybenjaminbrown

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