Why are null checks bad / why would I want an optional to succeed if it's null?

Optionals provide clarity of type. An Int stores an actual value - always, whereas an Optional Int (i.e. Int?) stores either the value of an Int or a nil. This explicit "dual" type, so to speak, allows you to craft a simple function that can clearly declare what it will accept and return. If your function is to simply accept an actual Int and return an actual Int, then great.

func foo(x: Int) -> Int

But if your function wants to allow the return value to be nil, and the parameter to be nil, it must do so by explicitly making them optional:

func foo(x: Int?) -> Int?

In other languages such as Objective-C, objects can always be nil instead. Pointers in C++ can be nil, too. And so any object you receive in Obj-C or any pointer you receive in C++ ought to be checked for nil, just in case it's not what your code was expecting (a real object or pointer).

In Swift, the point is that you can declare object types that are non-optional, and thus whatever code you hand those objects to don't need to do any checks. They can just safely just use those objects and know they are non-null. That's part of the power of Swift optionals. And if you receive an optional, you must explicitly unpack it to its value when you need to access its value. Those who code in Swift try to always make their functions and properties non-optional whenever they can, unless they truly have a reason for making them optional.

The other beautiful thing about Swift optionals is all the built-in language constructs for dealing with optionals to make the code faster to write, cleaner to read, more compact... taking a lot of the hassle out of having to check and unpack an optional and the equivalent of that you'd have to do in other languages.

The nil-coalescing operator (??) is a great example, as are if-let and guard and many others.

In summary, optionals encourage and enforce more explicit type-checking in your code - type-checking that's done by by the compiler rather than at runtime. Sure you can write "clean" code in any language, but it's just a lot simpler and more automatic to do so in Swift, thanks in big part to its optionals (and its non-optionals too!).

1. Avoids error at compile time. So that you don't pass unintentionally nulls.

2. In Java, any variable can be null. So it becomes a ritual to check for null before using it. While in swift, only optional can be null. So you have to check only optional for a possible null value.

3. You don't always have to check an optional. You can work equally well on optionals without unwrapping them. Sending a method to optional with null value does not break the code.

There can be more but those are the ones that help a lot.

TL/DR: The null checks that you say can be avoided with clean programming can also be avoided in a much more rigorous way by the compiler. And the null checks that you say are necessary can be enforced in a much more rigorous way by the compiler. Optionals are the type construct that make that possible.

var length = text?.length

This is actually a good example of one way that optionals are useful. If text doesn't have a value, then it can't have a length either. In Objective-C, if text is nil, then any message you send it does nothing and returns 0. That fact was sometimes useful and it made it possible to skip a lot of nil checking, but it could also lead to subtle errors.

On the other hand, many other languages point out the fact that you've sent a message to a nil pointer by helpfully crashing immediately when that code executes. That makes it a little easier to pinpoint the problem during development, but run time errors aren't so great when they happen to users.

Swift takes a different approach: if text doesn't point to something that has a length, then there is no length. An optional isn't a pointer, it's a kind of type that either has a value or doesn't have a value. You might assume that the variable length is an Int, but it's actually an Int?, which is a completely different type.

If I call the length function, I expect it to contain a length. If it doesn't, the code should deal with it right there, not continue on.

If text is nil then there is no object to send the length message to, so length never even gets called and the result is nil. Sometimes that's fine — it makes sense that if there's no text, there can't be a length either. You may not care about that — if you were preparing to draw the characters in text, then the fact that there's no length won't bother you because there's nothing to draw anyway. The optional status of both text and length forces you to deal with the fact that those variables don't have values at the point where you need the values.

Let's look at a slightly more concrete version:

var text : String? = "foo"
var length : Int? = text?.count

Here, text has a value, so length also gets a value, but length is still an optional, so at some point in the future you'll have to check that a value exists before you use it.

var text : String? = nil
var length : Int? = text?.count

In the example above, text is nil, so length also gets nil. Again, you have to deal with the fact that both text and length might not have values before you try to use those values.

var text : String? = "foo"
var length : Int = text.count

Guess what happens here? The compiler says Oh no you don't! because text is an optional, which means that any value you get from it must also be optional. But the code specifies length as a non-optional Int. Having the compiler point out this mistake at compile time is so much nicer than having a user point it out much later.

var text : String? = "foo"
var length : Int = text!.count

Here, the ! tells the compiler that you think you know what you're doing. After all, you just assigned an actual value to text, so it's pretty safe to assume that text is not nil. You might write code like this because you want to allow for the fact that text might later become nil. Don't force-unwrap optionals if you don't know for certain, because...

var text : String? = nil
var length : Int = text!.count

...if text is nil, then you've betrayed the compiler's trust, and you deserve the run time error that you (and your users) get:

error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

Now, if text is not optional, then life is pretty simple:

var text : String = "foo"
var length : Int = text.count

In this case, you know that text and length are both safe to use without any checking because they cannot possibly be nil. You don't have to be careful to be "clean" -- you literally can't assign anything that's not a valid String to text, and every String has a count, so length will get a value.

Why would I ever want to use optionals instead of nulls?

Back in the old days of Objective-C, we used to manage memory manually. There was a small number of simple rules, and if you followed the rules rigorously, then Objective-C's retain counting system worked very well. But even the best of us would occasionally slip up, and sometimes complex situations arose in which it was hard to know exactly what to do. A huge portion of Objective-C questions on StackOverflow and other forums related to the memory management rules. Then Apple introduced ARC (automatic retain counting), in which the compiler took over responsibility for retaining and releasing objects, and memory management became much simpler. I'll bet fewer than 1% of Objective-C and Swift questions here on SO relate to memory management now.

Optionals are like that: they shift responsibility for keeping track of whether a variable has, doesn't have, or can't possibly not have a value from the programmer to the compiler.