Message Passing: Alternative to Generics

• data is “active” — knows how to compute• dispatch on operation

(define (rat (n <integer>) (d <integer>)) (method ((op <symbol>)) (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 respectto an environment

Bindingidentifier : value

Frameunordered set of bindings

Environmentordered list of frames

x:10 +:{add}y:20z:30

x:() w:foobar

x:ohy:(list of stuff)

globalenvironment(top level frame)

(machine code foradding numbers)

The Lookup Rule

• the value of an identifier x is the value associated with x in the first (lowest) frame containing abinding 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:20z:30

x:() w:foobar

x:ohy:(list of stuff)

x:10 +:{add}y:20 d:12z:30

x:() w:foobar

x:ohy:(list of stuff)

(define d (+ 5 7))

before after

x:10 +:{add}y:20z:30

x:() w:foobar

x:ohy:(list of stuff)

x:10 +:{add}y:20z:30

x:() w:foobar

x:myy:(list of stuff)

(set! x 'my)

before after