rationalize

(rationalize x)

Answers nil as nil, a rational itself, and a double as the exact rational of the shortest decimal it prints as, so (rationalize 0.1) is 1/10 and (rationalize 0.3333333333333333) is 3333333333333333/10000000000000000, like the oracle. NaN and the infinities signal, so does a non-number. As a value a one-argument function.