1. 6001 structure & interpretation of computer programs recitation 14/ november 12, 1997

2. topics three representations entity relationship diagram object model
Scheme representation
1/15/2019 daniel jackson

3. levels of abstraction last time we looked at how to
model the real world with an entity-relationship diagram (classes, is-a, associations)
design an object model (classes, inheritance, instance vars, methods)
and we briefly mentioned
a way to simulate objects in Scheme
today
we’ll go more deeply into how these 3 representations are related
using examples from problem set 7
1/15/2019 daniel jackson

4. entity-relationship diagram
classes in an adventure game
named, mobile, place, person, thing, transporter, troll, rover, rock
associations
owns, at, adjoins, transports_to
exercise
arrange the classes in an is-a hierarchy
mark the associations between appropriate classes
note that a class can be associated with itself
what the ER diagram does not mention
how associations are implemented: who has pointers to whom
how state and methods are allocated to classes
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5. object model now allocate instance variables name location things
neighbours
possessions
owner
now invent some methods
note that a subclass need not add instance vars (eg troll)
subclass can override methods
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6. consistency constraints
how they arise
when an association is represented by multiple instance vars
the instance vars of different objects are related
example
the owns association
person has possessions, thing has owner
how they are maintained
cannot be maintained all the time!
when one object changes, it calls methods on the related objects
abstract constraints
not all constraints arise from implementation
example:
destination assocation connects Transporter to Place
might want to say that the destination cannot be a transporter
or that there are no “transporter loops”
1/15/2019 daniel jackson

7. scheme representation of objects
how objects are represented in the problem set
an object O is represented by a chain of Scheme closures, one for each class in the path upwards in the inheritance hierarchy
each subclass closure contains a pointer to its superclass closure
each closure has a self pointer that always points to the object O
confusing terminology
the superclass closure is said to be a “part” of the subclass closure
but not a part in the modelling sense: Vehicle is a part of Car, Wheel is not a part of Car!
how parts differ from objects
their self pointers don’t point to themselves
each is accessible only to the one closure that contains it
example
see figure 1 of PS7
1/15/2019 daniel jackson

8. closures revisited a closure is a procedure and an environment
the environment is accessible only to the procedure
so closure represents encapsulated state
example: bank account with one operation (withdraw)
(define (make-withdraw bal) (lambda (i) (set! bal (- bal i)) bal))
(define w1 (make-withdraw 100))
(define w2 (make-withdraw 100))
(w1 10) ==> 90
(w1 10) ==> 80
(w2 10) ==> 90
draw environment diagram
1/15/2019 daniel jackson

9. dispatch many bank accounts, many operations
procedure that makes accounts:
(define (make-account bal) (define (withdraw amt) (set! bal (- bal amt)) bal) (define (deposit amt) (set! bal (+ bal amt)) bal) (define (dispatch m) (cond ((eq? m ‘withdraw) withdraw) ((eq? m ‘deposit) deposit))) dispatch)
example of use
(define a1 (make-account 50))
((a1 ‘deposit) 40)
((a1 ‘withdraw) 60)
1/15/2019 daniel jackson

10. putting local procedures in a new frame
in the problem set object representation
the method procedures are bound in a fresh frame:
(define (make-account bal) (let ((withdraw (lambda (amt) …))) (let ((dispatch (lambda (m) …))) dispatch)
1/15/2019 daniel jackson

11. puzzle for student presentation
build a “shell” with history that works like this:
(define m (mk-shell))
(m ‘execute p) ; executes the procedure p and sets p to be the current proc
(m ‘back) ; sets the current proc back one
(m ‘forward) ; sets the current proc forward one
(m ‘run) ; executes the current proc
hint
use the circular buffer we designed last time!
1/15/2019 daniel jackson