> There are 16 functions mapping 2 bits -> 1 bit. Think 4 possible inputs, 2 possible output values for each input (0 or 1), so there are 2^4 = 16 such function

> There are 16 functions mapping 2 bits -> 1 bit. Think 4 possible inputs, 2 possible output values for each input (0 or 1), so there are 2^4 = 16 such functions. When people talk about algebraic data types, they tend to forget about functions. The tuple `(a, b)` is a product type because it has #a * #b possible values. The union type `a | b` is a sum type because it has #a + #b possible values. the function `a -> b` is an exponential type because it has #b ^ #a possible values.