defined

set of values for which function is defined

letting f be function(-1 --> 10, 0 --> 20, 1 --> 30)
find x : int(-2..2) such that x in defined(f) $[{"x": -1}, {"x": 0}, {"x": 1}]

f(-1) = 10, f(0) = 20, f(1) = 30

The function f is defined at -1, 0, and 1.

See it demonstrated here.