Promise of an API

Promise of an API
Maxim Zaks (@iceX33)
CocoaHeads Berlin

Agenda
— Consume a promise
— Produce a promise
— Swift

- (NSNumber*)addFirstNumber:(NSNumber*)number1
toSecondNumber:(NSNumber*)number2;

toSecondNumber:(NSNumber*)number2
withError:(NSError**)error;

- (void)addFirstNumber:(NSNumber*)number1
withBlock:(void (^)(NSNumber *sum, NSError *error))block;

withBlock:(void (^)(NSNumber *sum, NSError *error))block;
success:(void (^)(NSNumber *sum))success
failure:(void (^)(NSError *error))failur;

And now with a real promise!
- (OMPromise*)addFirstNumber:(NSNumber*)number1

What is a promise
(OMPromise)?
It's just an object

It has state
— Unfulﬁlled
— Fulﬁlled
— Failed

It has other properties
— result
— error
— progress*

It has methods
- (OMPromise *)fulfilled:(void (^)(id result))fulfilHandler;
- (OMPromise *)failed:(void (^)(NSError *error))failHandler;
- (OMPromise *)progressed:(void (^)(float progress))progressHandler;

It lets you call handlers on a custom queue
- (OMPromise *)fulfilled:(void (^)(id result))fulfilHandler on:(dispatch_queue_t)queue;
- (OMPromise *)failed:(void (^)(NSError *error))failHandler on:(dispatch_queue_t)queue;
- (OMPromise *)progressed:(void (^)(float progress))progressHandler on:(dispatch_queue_t)queue;

OMPromise *sum = [calc addFirstNumber:@3 toSecondNumber:@2];
[[sum fulﬁlled:^(id result){
NSLog(@"Sum: %@", result);
}] failed:^(NSError *error){
NSLog(@"Failed to calculate because of: %@", error);
}];

Concatenation
sum1 = 3 + 2
sum2 = sum1 + 2
sum3 = sum1 + 3

Concatenation
OMPromise *sum1 = [calc addFirstNumber:@3 toSecondNumber:@2];
OMPromise *sum2 = [sum1 than:^(NSNumber *n1){
return [calc addFirstNumber:n1 toSecondNumber:@2];
}];
OMPromise *sum3 = [sum1 than:^(NSNumber *n1){
}];
[[[OMPromise all:@[sum1, sum2, sum3]] fulﬁlled:^(NSArray *results){
NSLog(@"Sum1: %@, Sum2: %@, Sum3: %@", results[0], results[1], results[2]);
} on:dispatch_get_main_queue()] failed:^(NSError *error){
NSLog(@"Failed to calculate because of: %@", error);
} on:dispatch_get_main_queue()];

Deep chaining
OMPromise *sum = [OMPromise chain:[
^(NSNumber *n1){
return [calc addFirstNumber:@2 toSecondNumber:@3];
},
^(NSNumber *n1){
},
^(NSNumber *n1){
}
] initial:nil];

Recovering from failed promise
OMPromise *sum1 = [calc addFirstNumber:@3 toSecondNumber:@2];
OMPromise *rescuedSum = [sum1 rescue:^(NSError *error){
NSLog(@"Shit happens! %@", error);
return @0;
}];

Recap 1/3
@interface OMPromise : NSObject
@property(assign, readonly, nonatomic) OMPromiseState state;
@property(readonly, nonatomic) id result;
@property(readonly, nonatomic) NSError *error;
@property(assign, readonly, nonatomic) ﬂoat progress;

Recap 2/3
- (OMPromise *)fulfilled:(void (^)(id result))fulfilHandler;
- (OMPromise *)failed:(void (^)(NSError *error))failHandler;
- (OMPromise *)progressed:(void (^)(float progress))progressHandler;
- (OMPromise *)then:(id (^)(id result))thenHandler;
- (OMPromise *)rescue:(id (^)(NSError *error))rescueHandler;

Recap 3/3
+ (OMPromise *)chain:(NSArray *)thenHandlers initial:(id)result;
+ (OMPromise *)any:(NSArray *)promises;
+ (OMPromise *)all:(NSArray *)promises;

Now what's about creating a
promise?

Convenience methods on OMPromise
+ (OMPromise *)promiseWithResult:(id)result;
+ (OMPromise *)promiseWithResult:(id)result after:(NSTimeInterval)delay;
+ (OMPromise *)promiseWithError:(NSError *)error;
+ (OMPromise *)promiseWithTask:(id (^)())task;

Example for success
- (OMPromise *)addFirstNumber:(NSNumber*)number1
{
return [OMPromise promiseWithTask:^{
return @([n1 intValue] + [n2 intValue])
}];
}

Example for error
{
return [OMPromise promiseWithTask:^{
return [NSError new];
}];
}

And what is the non convenient
way?

Defered object
@interface OMDeferred : OMPromise
+ (OMDeferred *)deferred;
- (OMPromise *)promise;
- (void)fulﬁl:(id)result;
- (void)fail:(NSError *)error;
- (void)progress:(ﬂoat)progress;
@end

Example
{
OMDeferred *deferred = [OMDeferred deferred];
dispatch_async(dispatch_get_global_queue(0, 0), ^
{
NSError *error = nil;
NSNumber *sum = [self addFirstNumber:number1
toSecondNumber:number2
withError:&error];
if(error){
[deferred fail:error];
} else {
[deferred fulﬁll:sum];
}
});
return deferred.promise;
}

Recap
— Promise is an immutable data structure
— Deferred is a subclass of Promise which makes it
mutable

To add up confusion I
implemented promises in
Swift

First attempt was just to
port it

Using Enums for state representation feels right
enum PromiseState{
case Unfulﬁlled
case Fulﬁlled
case Failed
}

I need inheritance so let's
use classes

Using constants as read only property is not a good
idea as you have to set them directly in the
initializer
class Promise<T> {
let state : PromiseState
let result : T
let error : NSError
}

Switching to var makes it work but Promise is not
immutable any more
class Promise<T> {
var state : PromiseState
var result : T?
var error : NSError?
init(){
state = .Unfulfilled
}
func fulfill(value : T){
result = value
state = .Fulfilled
}
}

Trying to make a read only property
class Promise<T> {
var state : PromiseState
var result : T? {
get {
return self.result
}
}
var error : NSError?
init(){
state = .Unfulﬁlled
}
}

Sadly this was an inﬁnite loop
— There are no Read-Only properties in Swift
— Only Read-Only Computed Properties

Schock number 2
— There are no ivars in Swift
— And no visibility constrains (but they promised
to introduced those)

After initial shock pass, I
decided to go functional

func future<T>(execution : ()->FutureResult<T>)(handler: FutureHandler<T>) {
var result : FutureResult<T>?
var done = false
dispatch_async(dispatch_get_global_queue(0, 0)) {
result = execution()
done = true
}
dispatch_async(dispatch_get_global_queue(0, 0)) {
while !done {}
dispatch_async(dispatch_get_main_queue()) {
switch handler {
case .FulFilled(let fulﬁlled):
switch result! {
case .Result(let value):
fulﬁlled(value())
case .Error : println("can't process error")
}
case .Failed(let failed):
switch result! {
case .Result: println("can't process result")
case .Error(let error) : failed(error())
}
}
}
}
}

Main concept = currying
func add(n1:Int)(n2:Int) -> Int {
return n1 + n2
}
let increaseByTwo = add(2)
let sum = increaseByTwo(3)

Using enums with Associated Values
enum FutureHandler<T>{
case FulFilled((T)->())
case Failed((NSError)->())
}
enum FutureResult<T>{
case Result(@auto_closure ()->T)
case Error(@auto_closure ()->NSError)
}

Here is how you can use it:
var f = future {
FutureResult.Result(2 + 3)
}
f (handler: FutureHandler.FulFilled {
result in
println("Promise fulﬁlled : (result)")
})

It's not a beauty but it
works

However you can't concatenate futures in this
implementation
— typealias F = (FutureHandler<Int>) -> F

For the tough ones, who survived this talk:
— https://github.com/b52/OMPromises
— https://github.com/mzaks/siwft-future

Thank you!
Maxim Zaks (@iceX33)

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