world POP
26nov07
1
The can be modelled as processed and objects. Objects remain persistent over time; processes occur for a period of time, and then have no meaning outside that. Objects are hard to model natively so you describe them in terms of their properties. Imagine trying to model a processor; you might model it as its set of registers, and the current instruction being executed. Processes are the events that change the object's parameters over time. In fact, in any functional language, you need to procedure to represent an object! So, a different paradigm is needed.
Assignment This allows static data to persist indefinitely in the current scope. (set! name expression) will change the value bound to name to be expression. So the variable must already exist it must have been defined first. This a is side-effect operation. It has serious consequences on verifying a program's correctness. This new command transforms Scheme into an imperative programming language. For example: > (set! x (* 23 2)) > x 46 > (set! x (+ x x)) > x 96 In the last example, `define' would have produced a recursive definition. Set evaluated the expression before assigning it. It also works with `let' statements: > (let ( (z 10) ) (sequence (set! z 20) z ) ) 20 There is another version o...