Initializing reduce

Tasks such as adding integers or finding maximum values have a common thread: the input values and the accumulated values are of the same type. When two numbers are added, the result is a number; when a maximum or a minimum is chosen between two numbers, the result is still a number. When we use reduce to add numbers together, the running total is a number just like all the other inputs. In the examples so far, the first function call that reduce makes takes the first two items in the sequence. We can break a reduce call into its successive function calls:

(reduce + [1 2 3 5 7])
(+ 1 2)
(+ 3 3)
(+ 6 5)
(+ 11 7)

We actually don't need the anonymous function that we used in the previous examples, because + takes numbers as arguments, and returns a number:

Figure 5.3: The arguments and the return value are all of the same type

In each of our examples so far, three different things are all of the same type:

• The values in the...