I think some functions with exhaustively enumerable could be good candidates as well (I guess somewhat rare though?). For example an Int->(...) or Float->(...)

I think some functions with exhaustively enumerable could be good candidates as well (I guess somewhat rare though?). For example an Int->(...) or Float->(...) function (Doubles are not exhaustively enumerable though, I guess, since 2^64 ~= 10^19, 10,000-Peta items). If the programmer can give an error threshold, e.g. in bits of precision (maximum relative error), that should work quite well too.