Rethinking commands as Kleisli arrows with a bit more stuff. This show the basics of how to start with Kleisli arrows (of signals) which are interesting and useful in their own right. I often need to compose monadic pipelines `a -> Signal a`, and it's nice to be able to reify these rather than thread everything through (very pointy) flatmaps. Ne…

gistfile1.m

typedef RACSignal *(^RACSignalKleisliBlock)(id input);

@interface RACSignalKleisli : NSObject
- (id)initWithSignalBlock:(RACSignalKleisliBlock)block;
- (instancetype)compose:(RACSignalKleisli *)kleisli;
- (RACSignal *)execute:(id)input;
@end

@implementation RACSignalKleisli {
	RACSignalBlock _signalBlock;
}

- (id)initWithSignalBlock:(RACSignalBlock)block {
	if (self = [super init]) {
		_signalBlock = [block copy];
	}

	return self;
}

- (instancetype)pullback:(RACSignalKleisli *)kleisli {
	return [[self.class alloc] initWithSignalBlock:^RACSignal * (id input) {
		return [[kleisli execute:input] flattenMap:^(id mappedInput) {
			return [self execute:mappedInput];
		}];
	}];
}

- (RACSignal *)execute:(id)input {
	return [_signalBlock(input) replay];
}

@end

@interface RACCommand : NSObject
- (id)initWithEnabled:(RACSignal *)enabled kleisli:(RACSignalKleisli *)kleisli;
@property (nonatomic, strong, readonly) RACSignal *executing;
@property (nonatomic, strong, readonly) RACSignal *enabled;
- (RACSignal *)execute:(id)input;
@end

@implementation RACCommand {
	RACSignalKleisli *_decoratedKleisli;
}

- (id)initWithEnabled:(RACSignal *)enabled kleisli:(RACSignalKleisli *)kleisli {
	if (self = [super init]) {
		_enabled = [enabledSignal replayLast];

		RACSignalKleisli *disable = [RACSignalKleisli kleisli:^(id input) {
			return [[self.enabled take:1] flattenMap:^(NSNumber *enabled) {
				if (enabled.boolValue) {
					return [RACSignal return:input];
				}
				else {
					NSError *error = [NSError errorWithDomain:RACCommandErrorDomain code:RACCommandErrorNotEnabled userInfo:@{ NSLocalizedDescriptionKey: NSLocalizedString(@"The command is disabled and cannot be executed", nil), RACUnderlyingCommandErrorKey: self }];
					return [RACSignal error:error];
				}
			}];
		}];

		_decoratedKleisli = [kleisli pullback:disable];
	}

	return self;
}

- (RACSignal *)execute:(id)input {
	return [_decoratedKleisli execute:input];
}


@end