02 Abstraction and Specification

It's been said that computer science is the study of abstractions, but what is an abstraction?

ab·strac·tion (from middle french & late latin)

verb: abstract

• Forgetting information, so that different things can be treated the same

noun

• Artifacts that result from that process.

When creating abstractions, one important part is writing specifications for them.

spec·i·fi·ca·tion (unknown etymology)

noun

• intended behavior of abstraction

verb: specify

• The act of creating such an artifact (creating a specification)

Audiences of specifications

The different audiences have different specifications

Clients

• What they must guarantee (preconditions)
• What they can assume (postconditions)

Implementers

• What they can assume (preconditions)
• What they must guarantee (postconditions)

Benifits of specifications

• Locality: understand abstraction without needing to understand implementation
• Modifiability: change implementation without breaking client code
• Accountability: clarify who is to blame when something goes wrong

Satisfaction

An implementation satisfies a specification if it provides the described behavior.

Many implementations can satisfy the same specification

• Client must assume it could be any of them
• Implemented gets to pick one

(everyone should act in good faith)

Ambiguity

These factors may create ambiguity, which decrases satisfaction

• The client didn't understand what they wanted you to implement
• The client genuinely didn't care
• You didn't understand the meaning

These can all lead to ambiguity in a specification.

How do we avoid this?

• If you wrote the specification, improve it!
• If the client wrote the specification, seek clarification (but don't make 500 clarifications lol)