input.b

DEBUG = True

macro ASSERT(e):
    return \(if DEBUG:
                 if not $e:
                     raise AssertionError($(stringify(e))))


def assert_equal(x, z):
    # x and z are compared so they must have the same type
    ASSERT(x == z)

def foo(x):
    # x is used in an addition to an integer, so it gets unified with integer
    y = x + 1
    
    # assert_equal takes two parameters of the same type, so these must have the same type
    assert_equal(x, "test")

output.txt

ERROR CHECKING TYPES
18:17: Cannot unify int and string
    assert_equal(x, "test")
                ^