Swift: guard let vs if let

if let and guard let serve similar, but distinct purposes.

The "else" case of guard must exit the current scope. Generally that means it must call return or abort the program. guard is used to provide early return without requiring nesting of the rest of the function.

if let nests its scope, and does not require anything special of it. It can return or not.

In general, if the if-let block was going to be the rest of the function, or its else clause would have a return or abort in it, then you should be using guard instead. This often means (at least in my experience), when in doubt, guard is usually the better answer. But there are plenty of situations where if let still is appropriate.

Guard can improve clarity

When you use guard you have a much higher expectancy for the guard to succeed and it's somewhat important that if it doesn't succeed, then you just want to exit scope early. Like you guard to see if a file/image exists, if an array isEmpty or not.

func icon() -> UIImage {
    guard let image = UIImage(named: "Photo") else {
        return UIImage(named: "Default")! //This is your fallback
    }
    return image //-----------------you're always expecting/hoping this to happen
}

If you write the above code with if-let it conveys to the reading developer that it's more of a 50-50. But if you use guard you add clarity to your code and it implies I expect this to work 95% of the time...if it ever failed, I don't know why it would; it's very unlikely...but then just use this default image instead or perhaps just assert with a meaningful message describing what went wrong!

• Avoid guards when they create side effects, guards are to be used as a natural flow. Avoid guards when else clauses introduce side effects. Guards establish required conditions for code to execute properly, offering early exit

• When you perform significant computation in the positive branch, refactor from if to a guard statement and returns the fallback value in the else clause

_{From: Erica Sadun's Swift Style book}

Also as a result of the above suggestions and clean code, it's more likely you will want/need to add assertions into failed guard statements, it just improves readability and makes it clear to other developers what you were expecting.

guard​ ​let​ image = ​UIImage​(named: selectedImageName) else { // YESSSSSS
     assertionFailure(​"Missing ​​\(​selectedImageName​)​​ asset"​) 
     return
} 

guard​ ​let​ image = ​UIImage​(named: selectedImageName) else { // NOOOOOOO
​     ​return 
}

_{From: Erica Sadun's Swift Style book + some modifications}

_{(you won't use asserts/preconditions for if-lets. It just doesn't seem right)}

Using guards also help you improve clarity by avoiding pyramid of doom. See Nitin's answer.

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Guard avoids nesting by creating a new variable in the current scope

There is one important difference that I believe no one has explained well.

Both guard let and if let unwrap the variable however

With guard let you are creating a new variable that will exist in the current scope.

With if let you’re only creating a new variable inside the code block.

guard let:

func someFunc(blog: String?) {
    
    guard let blogName = blog else {
        print("some ErrorMessage")
        print(blogName) // will create an error Because blogName isn't defined yet
        return
    }
    print(blogName) // You can access it here ie AFTER the guard statement!!
    
    //And if I decided to do 'another' guard let with the same name ie 'blogName' then I would create an error!
    guard let blogName = blog else { // errorLine: Definition Conflicts with previous value.
        print(" Some errorMessage")
        return
    }
    print(blogName)
}

if-let:

func someFunc(blog: String?) {
    
    
    if let blogName1 = blog {
        print(blogName1) // You can only access it inside the code block. Outside code block it doesn't exist!
    }
    if let blogName1 = blog { // No Error at this line! Because blogName only exists inside the code block ie {}
        print(blogName1)
    }
}

For more info on if let do see: Why redeclaration of optional binding doesn't create an error

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Guard requires scope exiting

(Also mentioned in Rob Napier's answer) :

You MUST have guard defined inside a func. It's major purpose is to abort/return/exit scope, if a condition isn't met:

var str : String?

guard let blogName1 = str else {
    print("some error")
    return // Error: Return invalid outside of a func
}
print (blogName1)

For if let you don't need to have it inside any func:

var str : String?    
if let blogName1 = str {
   print(blogName1) // You don't get any errors!
}

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guard vs if

It's worth noting that it's more appropriate to see this question as guard let vs if let and guard vs if.

A standalone if doesn't do any unwrapping, neither does a standalone guard. See example below. It doesn't exit early if a value is nil. There are NO optional values. It just exits early if a condition isn't met.

let array = ["a", "b", "c"]
func subscript(at index: Int) -> String?{
   guard index > 0, index < array.count  else { return nil} // exit early with bad index
   return array[index]
}