Specifications: the Good, the Bad, and the Ugly

One can define a prime number as an integer greater than 1 that has no divisors other than 1 and itself. This is a declarative definition: it says what a prime number is rather than how to get one. One can also appeal to the Sieve of Eratosthenes. This would be a procedural definition. It is the opposite of declarative and it only tells how to get prime numbers.

In Software Engineering it is considered bad form to specify a function by pseudocode or by a reference implementation; a proper specification is supposed to be declarative. This blanket preference for the declarative assumes that such specifications are always better: easier to understand and to write. This is indeed the case with prime numbers, but not always. In the case of the Reef Knot, the Bowline, or any other kind of knot, a declarative definition, though perhaps possible, is likely to be less useful than being shown how to make one, which is procedural. Here I consider an example where it is easy to compare the merits of the Good (declarative), the Bad (procedural), and an even less respectable third approach.

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