'm late to this bandwagon. However I had a heavy investment in an Objective C code base and a lot of distractions. Enough with the excuses, better late than never.I know others have held off for many of the same reasons I did. So the question is useful. Was transitioning to swift worth it? The answer is a definite yes.
The Pros
No pointers and no nil-able values or objects unless they are wrapped in an option. This cuts down on a large class of errors.Named parameters. This language is targeted for Objective C programmers so of course it has name parameters, however I still count it as a plus because named parameters increase readability and are a good idea, even C# has named parameters theses days.
No lisp-like nested square brackets everywhere (yes I know lisp uses round brackets, however I am talking about the nesting behaviour)
Immutable values using the let keyword. This reminds me of Scala's val and var keywords and of F#'s let keyword, although F# makes it harder to make values and objects mutable. Immutable values can make an application easier to reason about and make it easier to do concurrent programming (less need for locking)
No more header files.
Memory management that doesn't feel like a tacked on after thought .
Only one form of closure instead of the two different flavours in Objective C (selectors and blocks)
The 'if let' statements ability to chain together multiple let statements capped with a where cause reduces nesting and as nesting plays havoc with readability this is all to the good. There is a 'if var' version but I never see it used.
Swift has its own REPL which in Swift's case is called a playground. REPLs are common in many interpreted languages, they are ideal for learning and experimentation. REPL is pronounced 'reppel' and stands for Read Evaluate Print Loop and at its heart in a very simple, effective and old piece of technology.
No more need to box and unbox value types just to use collections. I remember when C# had this problem (versions 1.0 and 1.1 fixed in 2.0), but at least C# 1.1 had the __box and __unbox keywords, in Objective-C you have to wrap and unwrap value types by hand.
Swift has generics, type safety and even type inference. Plugging that hole (lack of generics) in the type system pays off massive benefits in reduction of errors and increased ease of use. There is no way to have general purposes containers like lists and dictionaries without either having a generic type system, using dynamic types or breaking the type system by casting away type.
Less use of special characters in syntax, no more semi-colons at the end of your lines and a more modern cleaner looking syntax.
Initial Annoyances / Gotchas
You will need to lose the habit of surrounding if conditions with brackets as this can break things. This is initially is frustrating, but when you get used to it, it actually looks cleaner.
Significant white space between operators and operands or lack there of will turn binary operators into prefix unary operators.
Xcode habit of warning you that you have not used a declared variable or constant straight after you have declared it and before you have had the chance to type the line that will use it, can initially be distracting.
The Cons
I am used to uninformative and obscure error messages, but Xcode has reached a new low in this lost art of sowing confusion. After a while you get an idea about what it could be complaining about, but even when you know what is causing the problem, drawing a meaningful relationship between the error message and the cause of the problem can be bewildering.
The navigation and intellisense tends to break when you need them most.
No refactoring support for swift yet compared to Xcode refactoring support for its other supported languages, not that Xcode Objective-C refactoring support is terribly good, but at least its there.
Lost Opportunities
There is a lot I could say about this, but Objective-C does worse in features I want to complain about so I will just point you to a post that Rob Napier wrote about how swift is not functional and save the rant I was going to write here for another blog post.
The Experience
The first one or two weeks was frustrating. Most of the frustration was centred around the compiler. However the learning curve was surprisingly easy. After two weeks I felt more comfortable with Swift than Objective-C even though I have been using Objective-C for years. There is very little in Swift that isn't a standard expected feature in a modern programming language. Even the syntax and the keywords are pretty stock standard. Given the blandness of Swift the main reason for its existence is obviously to have built-in compatibility with the existing libraries and APIs and to make migration from Objective-C and interoperability with Objective-C easy. In achieving these goals Swift is successful.
Conclusion
I am not going to rewrite all my Objective-C code base in Swift. However all new work is going to be in Swift and even major modifications to old projects will probably be in Swift as you can mix the two languages in the same project.
