To reinforce Yichao's point, using Nullables in the way you propose comes
with substantial performance risks. The problem is that your strategy
globally poisons type inference: type-uncertain functions don't just force
run-time dispatch near the location of their function calls, they also
force run-time dispatch in every downstream function call that depends upon
the results of that function call.
Consider code like the following:
function slow()
x = uncertain_type_generator()
for i in 1:10_000_000
do_something_with(x)
end
end
Every call to do_something_with() is made type-uncertain because the type
of x can't be inferred at method-compile-time. So your hot loop is poisoned
by the use of a once-called type-uncertain function.
-- John
