motz
dilluns, 26. de novembre 2001

"You can't get a fried burger here"

semantic invariants from the pragmatic programmers g

Let's suppose you want to model a very simple transaction, for example, purchasing a hamburger at a fast food establishment. Suppose you have an interface (or abstract class) for "Buying a Burger". The constraints on such a transaction might be expressed as:

Pre-condition to "Buying A Burger": 1) The customer has sufficient money in local currency to purchase a burger. 2) Customer may request options on the burger, any of: cheese, lettuce, or tomato.

Post-condition to "Buying A Burger": 1) Customer will receive a burger with the requested option(s).

Invariant for Burgers: 1) All burgers are made of a beef and kangaroo mixture, and come on a 1.5" diameter roll. 2) All burgers are flame-broiled. 3) All burgers weigh 0.08oz before cooking.

You can't get a fried burger here. If you request a burger with ketchup the supplier is under no obligation to give it to you. If you request a cheese-burger and have the money, you are guaranteed to receive it.

So here we have the pre- and postconditions that define "Buying A Burger". What defines a burger? ...

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