In this chapter, we learned how to write our own simple methods and we saw how to use the substitution model of evaluation to understand method calls.
We saw that methods both abstract over expressions, in the same way as names, and also generalize over expressions, allowing us to represent a group of related expressions with one name.
We wrote some interesting methods, but we still have more repeated code than is desirable (think about the repeated calls to
circle in the exercises.)
In the next chapter, we will learn how we can generalize over this using structural recursion over the natural numbers.