Algebraic Specification

Now that we are proficient at proofs about functions, we can tackle a bigger challenge: proving the correctness of a data structure, such as a stack, queue, or set.

Correctness proofs always need specifications. In proving the correctness of iterative factorial, we used recursive factorial as a specification. By analogy, we could provide two implementations of a data structure---one simple, the other complex and efficient---and prove that the two are equivalent.
That would require us to introduce ways to translate between the two implementations. For example, we could prove the correctness of a dictionary implemented as a red-black tree relative to an implementation as an association list, by defining functions to convert trees to lists. Such an approach is certainly valid, but it doesn't lead to new ideas about verification for us to study.

Instead, we will pursue a different approach based on equational specifications, aka algebraic specifications. The idea with these is to

• define the types of the data structure operations, and
• to write a set of equations that define how the operations interact with one another.

The reason the word "algebra" shows up here is (in part) that this type-and-equation based approach is something we learned in high-school algebra. For example, here is a specification for some operators:

0 : int
1 : int
- : int -> int
+ : int -> int -> int
* : int -> int -> int

(a + b) + c = a + (b + c)
a + b = b + a
a + 0 = a
a + (-a) = 0
(a * b) * c = a * (b * c)
a * b = b * a
a * 1 = a
a * 0 = 0
a * (b + c) = a * b + a * c


The types of those operators, and the associated equations, are facts you learned when studying algebra. (And if you take an abstract algebra course in college, you will learn even more about them.)

Our goal is now to write similar specifications for data structures, and use them to reason about the correctness of implementations.