Message Passing: Alternative to Generics data is “active” — knows how to compute dispatch on operation (define (rat (n ) (d )) (method ((op )) (cond ((= op ’numer) n) ((= op ’denom) d) (else: (error ”Bogus operation")))))) ((rat 3 4) ’numer) => 3

In Java: String m = new String("CS212");... if (m.equals("CS211")) {... if (m == "CS211") {...

New Special Form — set! (set! identifier expression)

set! trashes the substitution model: (define put-in-bowl (lambda (thing) (set! thing (cons thing '(in a bowl))) thing)) (put-in-bowl 'goldfish) => (goldfish in a bowl)

Which variable does set! set? (define x 'barney) => x (let ((x 'purple) (y (set! x 'monster))) x) => ??? x => ???

Which variable does set! set? (define x 'barney) => x (let* ((x 'purple) (y (set! x 'monster))) x) => monster x => barney

An example not involving set! (define x 10) (define foo (lambda () x)) (let ((x 15)) (foo)) => ???

An example not involving set! (define x 10) (define foo (lambda () x)) (let ((x 15)) (foo)) => 10

Static vs. Dynamic Scoping Static scoping: free variables resolved in the environment of the function definition Dynamic scoping: free variables resolved in the environment of the function call Closure: when a function object is created, the environment is packaged with it

Referential Transparency replace equal values with equal values set! destroys referential transparency

The Environment Model an extension of the substitution model more "operational" fully explains static scoping and the process by which variable names are resolved in Scheme (and in almost any other programming language.) An expression only has a value with respect to an environment

Binding identifier : value Frame unordered set of bindings Environment ordered list of frames

x:10 +:{add} y:20 z:30 x:() w:foobar x:oh y:(list of stuff)  global environment (top level frame) (machine code for adding numbers)

The Lookup Rule the value of an identifier x is the value associated with x in the first (lowest) frame containing a binding for x undefined if there aren't any

Define rule To evaluate (define ident expr): evaluate expr in the current environment add a binding to the top-level frame (the global environment) Set! rule To evaluate (set! ident expr): evaluate expr in the current environment look up ident using lookup rule change that binding to value of expr

x:10 +:{add} y:20 z:30 x:() w:foobar x:oh y:(list of stuff) x:10 +:{add} y:20 d:12 z:30 x:() w:foobar x:oh y:(list of stuff) (define d (+ 5 7)) beforeafter

x:10 +:{add} y:20 z:30 x:() w:foobar x:oh y:(list of stuff) x:10 +:{add} y:20 z:30 x:() w:foobar x:my y:(list of stuff) (set! x 'my) beforeafter