Brian Gesiak modocache

Thoughts on Swift 2 Errors

When Swift was first announced, I was gratified to see that one of the (few) philosophies that it shared with Objective-C was that exceptions should not be used for control flow, only for highlighting fatal programming errors at development time.

So it came as a surprise to me when Swift 2 brought (What appeared to be) traditional exception handling to the language.

Similarly surprised were the functional Swift programmers, who had put their faith in the Haskell-style approach to error handling, where every function returns an enum (or monad, if you like) containing either a valid result or an error. This seemed like a natural fit for Swift, so why did Apple instead opt for a solution originally designed for clumsy imperative languages?

I'm going to cover three things in this post:

	- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender {
	if ([segue.identifier isEqualToString:@"user"]) {
	// Type casts? I thought we were using a static language here?!
	UserViewController userViewController = (UserViewController )segue.destinationViewController;
	// I have to expose a property in order to configure the view controller.
	// The view controller is in an invalid state prior to this assignment.
	userViewController.user = self.selectedUser;
	[self.navigationController pushViewController:userViewController animated:YES];
	} else if ([segue.identifier isEqualToString:@"settings"]) {
	// More of the same boilerplate...

	# https://drone.io/github.com/modocache/cargo/admin

	go get code.google.com/p/go.tools/cmd/cover
	go get github.com/onsi/ginkgo/ginkgo
	go get github.com/onsi/gomega
	go get ./...
	go build
	$GOPATH/bin/ginkgo -r --randomizeAllSpecs -cover

	package signatures

	import (
	"github.com/go-martini/martini"
	"github.com/martini-contrib/binding"
	"github.com/martini-contrib/render"
	"labix.org/v2/mgo"
	)

	type Server *martini.ClassicMartini

	Thread 1
	Queue : com.apple.main-thread (serial)
	#0 0x000000011062aeda in __mmap ()
	#1 0x0000000110628acc in mmap ()
	#2 0x0000000121178577 in ___lldb_unnamed_function567$$CoreSymbolication ()
	#3 0x00000001211789bf in ___lldb_unnamed_function572$$CoreSymbolication ()
	#4 0x00000001211777d2 in ___lldb_unnamed_function544$$CoreSymbolication ()
	#5 0x00000001211775a7 in ___lldb_unnamed_function541$$CoreSymbolication ()
	#6 0x00000001211767c0 in ___lldb_unnamed_function523$$CoreSymbolication ()
	#7 0x0000000121176eae in ___lldb_unnamed_function537$$CoreSymbolication ()

	NSMutableArray <NSString > arrayOfStrings = [NSMutableArray array];
	[arrayOfStrings addObject:@"hello"];
	[arrayOfStrings addObject:@1]; // compiler error
	[arrayOfStrings addObject:(NSString *)@1]; // OK by compiler

	NSString *string = [arrayOfStrings firstObject]; // OK by compiler
	NSNumber *number = [arrayOfStrings firstObject]; // compiler error
	NSNumber actuallyANumberThough = (NSNumber )[arrayOfStrings lastObject]; // OK by compiler, and will work at runtime

	Process: Xcode [59837]
	Path: /Applications/Xcode-beta.app/Contents/MacOS/Xcode
	Identifier: com.apple.dt.Xcode
	Version: 7.1 (9069)
	Build Info: IDEFrameworks-9069000000000000~10
	Code Type: X86-64 (Native)
	Parent Process: ??? [1]
	Responsible: Xcode [59837]
	User ID: 1199935289

	bgesiak@bgesiak-mbp @ ~/GitHub/modocache/Argo $ ./bin/setup
	==> Cloning https://github.com/Carthage/Carthage.git
	==> Checking out tag 0.7.5
	==> make prefix_install PREFIX=/usr/local/Cellar/carthage/0.7.5
	CompileSwiftSources normal x86_64 com.apple.xcode.tools.swift.compiler
	CompileSwift normal x86_64 /tmp/carthage20151014-30637-1ky1qwy/Carthage/Checkouts/Runes/Source/Optional.swift
	CompileSwift normal x86_64 /tmp/carthage20151014-30637-1ky1qwy/Carthage/Checkouts/Runes/Source/Array.swift
	(3 failures)
	make: *** [installables] Error 65

	#import <Foundation/Foundation.h>

	static NSString _Nullable mightReturnAString() { / ... */ }
	static void takesANonnullString(NSString _Nonnull string) { / ... */ }

	int main() {
	// The following raises a warning when
	// CLANG_WARN_NULLABLE_TO_NONNULL_CONVERSION is turned on.
	takesANonnullString(mightReturnAString()); // Warning: Implicit conversion from nullable pointer 'NSString * _Nullable' to non-nullable pointer type 'NSString * _Nonnull'

	char +[XCSymbolicationRecord softLinkCoreSymbolication](void * self, void * _cmd) {
	if (*_softLinkCoreSymbolication.onceToken != 0xffffffffffffffff) {
	dispatch_once(_softLinkCoreSymbolication.onceToken, void ^(void * _block) {
	rbx = [NSBundle bundleWithPath:@"/System/Library/PrivateFrameworks/CoreSymbolication.framework"];
	if (rbx != 0x0) {
	if ([rbx loadAndReturnError:0x0] != 0x0) {
	rax = dlsym(0xfffffffffffffffe, "CSIsNull");
	*__XCT_CSIsNull = rax;
	if (rax != 0x0) {
	rax = dlsym(0xfffffffffffffffe, "CSSymbolicatorCreateWithTask");