More complex RAC form validation

gistfile1.m

// Check name validity
RACSignal *nameValid = [RACAbleWithStart(self.name) map:^(NSString *name) {
    return @(name.length > 0);
}];

RACSignal *nameValidationMessage = [nameValid map:^ id (NSNumber *valid) {
    return valid.boolValue ? nil : @"Please enter a name that matches our bullshit rules";
}];

// Check email validity
RACSignal *emailMatches = [RACSignal combineLatest:@[ RACAbleWithStart(self.email), RACAbleWithStart(self.confirmEmail) ] reduce:^(NSString *email, NSString *confirmEmail) {
    return [email isEqual:confirmEmail];
}];

RACSignal *emailValidationMessage = [emailMatches map:^ id (NSNumber *matches) {
    return matches.boolValue ? nil : @"Both email addresses must match";
}];

// Populate the error message field
RAC(self.errorString) = [[RACSignal
    combineLatest:@[ nameValidationMessage, emailValidationMessage ]]
    // -map: is used here instead of a reduce block so that the logic extends
    // to an arbitrary number of combined signals.
    map:^(RACTuple *messages) {
        return [messages.rac_sequence objectPassingTest:^ BOOL (NSString *message) {
            return message != NSNull.null;
        }];
    }];

// Determine whether the form is valid
RAC(self.valid) = [[RACSignal
    combineLatest:@[ nameValid, emailMatches ]]
    map:^(RACTuple *validChecks) {
        BOOL allValid = [validChecks.rac_sequence all:^(NSNumber *isValid) {
            return isValid.boolValue;
        }];

        return @(allValid);
    }];