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[NSWindow ex_patchInstanceSelector: @selector(sendEvent:) withReplacementBlock: ^(EXPatchIMP *patch) {
    return ^(NSEvent *event) {
        NSWindow *window = (__bridge NSWindow *) patch->self;
        if (window != self.window && [event type] == NSLeftMouseUp && [window isKeyWindow]) {
            /* The last view seen */
            static NSView *lastView = nil;
                            
            NSPoint point = [window.contentView convertPoint: [event locationInWindow] fromView: nil];
            NSView *hitView = [window.contentView hitTest: point];

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// Asynchronously loads UIImages from the given URLs, but throttles the loading
// to keep memory and CPU usage sane.
- (RACSignal *)loadImagesAtURLs:(NSArray *)imageURLs {
    // Map each URL to a signal of work. The result is a signal of work signals.
    return [[imageURLs.rac_signal
        map:^(NSURL *imageURL) {
            return [[[[NSURLConnection
                // Load the URL asynchronously.
                rac_sendAsynchronousRequest:[NSURLRequest requestWithURL:imageURL]]
                reduceEach:^(NSURLResponse *response, NSData *data) {

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// Reads until the given stream is exhausted, creating and subscribing to a new
// signal using `writer` for each chunk of data that is read.
//
// Sends completed or error.
- (RACSignal *)readStream:(Stream *)stream writingWithGenerator:(RACSignalGenerator *)writer {
    return [[[RACSignal
        create:^(id<RACSubscriber> subscriber) {
            while (!subscriber.disposable.disposed && stream.hasData) {
                NSError *error = nil;
                NSData *data = [stream readData:&error];

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MVVM example

For a command that initiates a network request when:

• The network connection is up (precondition)
• The network request is not already in progress (enforced serialization)

The lifecycle proceeds as follows:

1. The view model exposes the command and describes its behavior

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- (id)initWithEnabled:(RACSignal *)enabled provider:(RACSignalProvider *)provider {
    if (self = [super init]) {
        _enabled = [enabledSignal replayLast];
 
        _decoratedProvider = [[[RACSignalProvider alloc]
            initWithBlock:^(id input) {
                return [[self.enabled take:1] flattenMap:^(NSNumber *enabled) {
                    if (enabled.boolValue) {
                        return [RACSignal return:input];
                    } else {

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Use cases

1. Turning an eager, pure computation into a lazy one: RACSequence, RACSignal
2. Side-effecting work that is safe to perform multiple times: RACSignal, RACCommand
3. Side-effecting work that is safe to perform multiple times serially, but not concurrently: RACCommand, RefCount()
   • RACScheduler would seem to apply here, but execution could become interleaved and thus not serial
4. Side-effecting work that should only be performed once, then memoized: RACMulticastConnection with a RACReplaySubject, RACSequence
5. Enabling/disabling a UI based on whether work can be performed: RACCommand, RACReplaySubject
   • Replaying is a Good Idea™ because this information may arrive before the UI is ready for it
6. Updating a UI with information about work in progress: RACCommand, RACSubject

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__block void (^myRecursiveBlock)(int);
id myBlock = ^(int x) {
    if (x > 5) {
        // Release the block from its own implicit retain cycle.
        myRecursiveBlock = nil;
    } else {
        myRecursiveBlock(x + 1);
    }
};

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Control/Monad/Trans/Resource.hs:613:24:
    Could not deduce (m ~ IO)
    from the context (MonadBaseControl IO m)
      bound by the type signature for
                 resourceForkIO :: MonadBaseControl IO m =>
                                   ResourceT m () -> ResourceT m ThreadId
      at Control/Monad/Trans/Resource.hs:601:19-81
      `m' is a rigid type variable bound by
          the type signature for
            resourceForkIO :: MonadBaseControl IO m =>

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#define self \
	( \
		_Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wunused-value\"") self, \
		/* Depends on _cmd being a const copy, like other captured variables. */ \
		_Pragma("clang diagnostic pop") (*(_cmd = _cmd, &self)) \
	)

// Replace libextobjc's @weakify with one that's aware of the "self" macro.
#undef ext_weakify_
#define ext_weakify_(INDEX, CONTEXT, VAR) \

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#define $(...) \
    ({ \
        __weak __typeof__(self) weakSelf = self; \
        \
        ^(__VA_ARGS__) { \
            __strong __typeof__(weakSelf) self = weakSelf; \
            $_body_

#define $_body_(...) \
            __VA_ARGS__ \