complex

(complex real &optional imag)

Answers the canonical complex value for a real part and an optional imaginary part (defaulting to zero): a rational zero imaginary part demotes to the real itself, while a float zero stays complex (an integer zero beside a float part becomes 0.0); a rational part beside a float part is coerced to a float, so exact rationals stay exact only when both parts are rational. Complexes read with #C(real imag) and print the same way. A non-real part signals a catchable type error. Works on all four backends.