gistfile1.nim

type A = object
type B = object

proc a(x: A) = echo "a"
proc b(x: B) = echo "b"

proc test(x: A|B) =
  when x is A:
    a(x)
  else:
    b(x)

test(A())

# --- ALTERNATIVE (using type-classes) --- #

type A = object
type B = object

proc a(x: A) = echo "a"
proc b(x: B) = echo "b"

type SomeA = generic x
  a(x)

proc test(x) =
  when x is SomeA:
    a(x)
  else:
    b(x)

test(A())