Though it is reasonable to make utilities monadic, they lack a crucial property: *laziness*. I have the following datatype: newtype ArbReal = ArbReal (Word -> Integer) This represents arbitrary real numbers by being able to compute arbitrary decimal places after the decimal point. For example, to compute pi, let f be the function passed to constructor ArbReal. Then f 0 = 3, f 1 = 31, f 2 = 314, and so on. I can implement instance Random ArbReal: instance Random ArbReal where random g = let (h, i) = split g d:digits = randomRs (0 :: Word, 9) h getNum [] = 0 getNum (d:ds) = toInteger d + 10 * getNum ds takeDigits n = getNum (reverse (take n digits)) + toInteger (fromEnum (d >= 5)) in (ArbReal (takeDigits . fromIntegral), i) randomR (lo, hi) g = let (x, h) = random g in (lo + x * (hi - lo), h) But I see no way to implement an instance of UniformRange ArbReal, for it relies on randomRs, which is lazy. Neither ST nor IO is able to contain such laziness.