Foundations of Objective-C
André Peng

--

Get out a piece of paper
Write a factorial function. Use whatever language you like

Don't cheat

int fact(int n)
{
 /* Write your code here */
}

fact(3) = 3 * 2 * 1 == 6
fact(4) = 4 * 3 * 2 * 1 == 24
fact(5) = 5 * 4 * 3 * 2 * 1 == 128

fact(0) = 1

--
What is an object?

int i;
i = 0xdeadbeef
 [de][ad][be][ef]

--

typedef struct _IntContainer {
 int i;
} IntContainer

IntContainer ic;

ic.i=0xdeadbeef;

 [de][ad][be][ef]

--

What is an object?

typedef struct _NSPoint {
 CGFloat x;
 CGFloat y;
} NSPoint;

NSPoint p;

p.x = 1.0; p.y = 2.0;

[3f][80][00][00][00][40][00][00][00]
           x               y


--

int *pi;
*pi = 0xdeadbeef;

        pi  [08][03][91][70]
0x08039170  [de][ad][be][ef]

--

struct NSObject {
 Class isa;
}

--

struct NSObject {
 struct objc_class* isa;
}


struct objc_class {
 Class isa;
 Class super_class;
 const char* name;
 long version;
 long info;
 long instance_size;
 struct objc_ivar_list *ivars;
 struct objc_method_list **methodLists;
 struct objc_cache *cache;
 struct objc_protocol_list *protocols;
}

(gdb) p NSApp
$2 = (struct objc_object *)
0x8039170

(gdb) p *NSApp
$3 = {
 isa = 0x15f8e0
}

(gdb) p NSApp->isa
$4 = (struct objc_class *)
0x15f8e0
(gbd) p *NSApp->isa
$5 = {
 isa = 0x160de0,
 super_class = 0x22d3a0,
 name = 0x1322de "RWApplication",
 version = 0,
 info = 12206145,
 instance_size = 100,
 ivars = 0x169720,
 methodLists = 80391e0,
 cache = 809d710,
 protocols = 15b064
}

--

@interface NSObject {
 class isa;
}

@interface MySubClass : NSObject {
 int i;
}

@interface MySubSubClass : MySubClass {
 float f;
}

struct NSObject {
 Class isa;
}

struct MySubClass {
 Class isa;
 int i;
}

struct MySubSubClass {
 Class isa;
 int i;
 float f;
}

--

Messaging

--

@implementation NSMutableString
(void) appendString:(NSString*)aString
{
 ...;
}

@end

=>

void | NSMutableString appendString:|(id self, SEL _cmd, NSString* aString) 
{
 ...;
}

--

% nm /usr/lib/libSystem.dylib | grep strcmp
06093b0 T _strcmp

nm Foundation.framework/Foundation | grep NSString compare
0C02bbf0 t -|NSString compare:|
0C06c200 t -|NSString compare:options|
0C00d480 t -|NSString compare:options:range|
0C00d4e0 t -|NSString compare:options:range:locale:|

--

I'm sorry that I long ago coined the term "objects" for this topic because it gets many people to focus on the lesser idea.

The big idea is "messaging".
 - Alan Kay

--

http://www.paulgraham.com/power.html

--

UNIFORMITY IS POWER

--

for(int i = 0; i<=10; i++) {
 NSLog(@"%d", i);
}

1 to:10 do:[:i|
 Transcript show: (i asString);
].

--

Everything is an object, including code.
Messaging is the exclusive means of object communication.

--

if (x < y) {
 foo;
} else {
 bar;
}

(x < y) ifTrue: [
 foo
] ifFalse: [
bar
]

--

Simpler to reason about
Simpler to implement
Re-think the problem

--

NSArray* array = ...;
|array do:^(id obj){ NSLog("%@", obj) }];
|NSThread performOnMainThread:^{ ... }]
|array pdo:^(id obj){ NSLog("%@", obj) }];

--

Alonzo &
Alan &
Stephen &
Kurt.

--

f(x) = x * 2
f(3) == 6

λ x. x * 2
λ (x. x * 2) 3 == 6

--

x
λ...
(λ...) y

--

0 := λ f x. x
1 := λ f x. f x
2 := λ f x. f (f x)
3 := λ f x. f (f (f x))
PLUS := λ m n f x. n f (m f x)
MULT := λ m n f. n (n f)
TRUE := λ x y. x
FALSE := λ x y. y
AND := λ p q. p q p
OR := λ p q. p q p
NOT := λ p a b. p b a
IFTHENELSE := λ p a b. p a b
PAIR := λ x y f. f x y
FIRST := λ p. p TRUE
SECOND := λ p. p FALSE
NIL := λ x. TRUE
NOTNIL := λ p. p (λx y. FALSE)

--

Get out a piece of paper
Write a factorial function. Use whatever language you like

Don't cheat

int fact(int n)
{
 /* Write your code here */
}

fact(3) = 3 * 2 * 1 == 6
fact(4) = 4 * 3 * 2 * 1 == 24
fact(5) = 5 * 4 * 3 * 2 * 1 == 128

fact(0) = 1

--

int fact(int n)
{
 int result = 1;
 int i;
 for(int i=0; i<=n; i++) 
 {
  result *=i;
 }

 return result;
}

int fact(int n)
{
 if (n == 0) return 1;
 else return n * fact(n-1);
}

--

Though OOP came from many motivations, two were central... the small scale one was to find a more flexible version of assignment, and then try to eliminate it altogether.
 - Alan Kay

Assignment statements - even abstract ones - express very low-level goals, and moreof them will be needed to get anything done. Generally, we don't want the programmer to be messing around with state, whether simulated or not.
 - Alan Kay

--

Pure functions
Give the same output when given the same inputs
Have no observable side-effects

--

f(x) = x * 2
f(string) = newString

--

Pure functions
Are far, far easier to reason about (i.e. maintain!)
Are far more composable than objects+methods
 They are more uniform
 They are smaller
Naturally concurrent

--

Lessons for Objective C
Minimal state, in both methods and objects
Every |var you add is a burden
Design your data so that invalid state is impossible
Use const liberally in methods
Prefer immutable objects over mutable

--

Lessons for Objective C
If you need a comment to explain something, move the code into a new function
Search for uniformity, in data design and method calls

--

Thanks!
André Peng
W http://algorithm.com.au
E ozone@algorithm.com.au
T http://twitter.com/andrepang