How to write lambda methods in Objective-C?

OS X 10.6 introduced blocks. See AlBlue's answer for examples.

If you're not using Snow Leopard, you can get something close to function composition using various other features.

Example using C function pointers:

void sayHello() {
    NSLog(@"Hello!");
}

void doSomethingTwice(void (*something)(void)) {
    something();
    something();
}

int main(void) {
    doSomethingTwice(sayHello);
    return 0;
}

Example using the command pattern:

@protocol Command <NSObject>
- (void) doSomething;
@end

@interface SayHello : NSObject <Command> {
}
@end

@implementation SayHello
- (void) doSomething {
    NSLog(@"Hello!");    
}
@end

void doSomethingTwice(id<Command> command) {
    [command doSomething];
    [command doSomething];
}

int main(void) {
    SayHello* sayHello = [[SayHello alloc] init];
    doSomethingTwice(sayHello);
    [sayHello release];
    return 0;
}

Example using a selector:

@interface SaySomething : NSObject {
}
- (void) sayHello;
@end

@implementation SaySomething
- (void) sayHello {
    NSLog(@"Hello!");    
}
@end

void doSomethingTwice(id<NSObject> obj, SEL selector) {
    [obj performSelector:selector];
    [obj performSelector:selector];
}

int main(void) {
    SaySomething* saySomething = [[SaySomething alloc] init];
    doSomethingTwice(saySomething, @selector(sayHello));
    [saySomething release];
    return 0;
}

I heard André Pang at NSConference talking about how blocks were going to be introduced with the next version of Objective-C.

This should allow functional programming.

Edit: Since Snow Leopard has been released, this is indeed the case. Objective-C now has Blocks.