Hello, I was recently intrigued by this style of argument on haskell cafe: One can write a function Eq a => ((a -> Bool) -> a) -> [a] that enumerates the elements of the set. Because we have universal quantification, this list can not be infinite. Which makes sense, topologically: These so-called searchable sets are topologically compact, and the Eq constraint means the space is discrete. Compact subsets of a discrete space are finite. ------- I've seen arguments like these "in the wild" during Scott topology construction and in some other weird places (hyperfunctions), but I've never seen a systematic treatment of this. I'd love to have a reference (papers / textbook preferred) to self learn this stuff! Thanks Siddharth -- Sending this from my phone, please excuse any typos!