Swift: `guard` 対 `if`　と、　`guard let` 対 `if let` (日本語)

It would be more appropriate to see this question as guard vs if and guard let vs if let.

`guard` vs `if`

Standalone guard and if are both simple flow control statements and do not do any unwrapping.

guard just exits early if its condition isn't met.

Comparison table

	`guard`	`if`
Function	Early exit	Flow control
Focus on ...	... the "early exit path": checking conditions are correct before going further.	... the “happy path”: the behavior of our function when everything has gone according to plan.
Readability, Clarity	Improved	Can lead to pyramids of doom
Nested Scope (adds levels of depth)	❌	◯
Enclosing Scope	Necessary	Not necessary
Must ...	return from the scope if the check fails
Return / Throw	Necessary	Not necessary
Think of it as ...	... validation.	... simple flow control
Preferred location	At the start of the scope

Examples

guard

let array = ["a", "b", "c"]
func at(at index: Int) -> String?{
   // exit early when index is out of bound
   guard index >= 0, index < array.count  else { return nil }
   return array[index]
}
print(at(1))

if

let array = ["a", "b", "c"]
let index = 1
if index >= 0, index < array.count {
    print(array[index])
} else {
    print(nil)
}

`guard let` vs `if let`

guard let and if let both unwrap optionals.

guard let, like guard, exits early if its condition isn't met.

Comparison table

	`guard let`	`if let`
Function	Unwraps optionals	Unwraps optionals
Designed to ...	... exit the current scope if the check fails.	... just unwrap some optionals.
Focus on ...	... the "early exit path": checking conditions are correct before going further.	... the “happy path”: the behavior of our function when everything has gone according to plan.
Enclosing Scope	Necessary	Not necessary
Must ...	return from the scope if the check fails
Return / Throw	Necessary	Not necessary
Readability, Clarity	Improved	Can lead to pyramids of doom
Nested Scope (adds levels of depth)	❌	◯
Visibility	The unwrapped optional is visible in the scope after the `guard let` statement.	The unwrapped optional is only visible in the `if let` statement's code block scope
Think of it as ...	... validating then unwrapping some optionals.	... just unwrapping some optionals.
Preferred location	At the start of the scope
Unwrap multiple optionals at once	◯	◯
Side Effects	Should be avoided	Ok

Examples

`guard let`

All the strings (firstNameString, lastNameString, emailString, passwordString) are accessible in the scope after the guard statements, if all the fields are valid:

guard-let

func validateThenRegisterUser() {
    // validation
    guard let firstNameString = firstName.text where firstNameString.characters.count > 0 else {
            firstName.becomeFirstResponder()
            return
        }
    guard let lastNameString = lastName.text where lastNameString.characters.count > 0 else {
            lastName.becomeFirstResponder()
            return
        }
    guard let emailString = email.text where 
        emailString.characters.count > 3 &&
        emailString.containsString("@") && 
        emailString.containsString(".") else {
            email.becomeFirstResponder()
            return
        }
    guard let passwordString = password.text where passwordString.characters.count > 7 else {
            password.becomeFirstResponder()
            return
        }

    // The validated information can now be used for registration
    let newUser = User()
    newUser.firstName = firstNameString
    newUser.lastName = lastNameString
    newUser.email = emailString
    newUser.password = passwordString
    APIHandler.sharedInstance.registerUser(newUser)
}

`if let`

All the strings (firstNameString, lastNameString, emailString, passwordString) are accessible only within the scope of the if statement code block.

This creates a "pyramid of doom" which presents many issues like:

readability,
ease of reordering/refactoring the code. For example, if the fields' validation order was altered, you would have to rewrite most of the code.

if-let

func validateThenRegisterUser() {
    // validation: starts the pyramid of doom
    if let firstNameString = firstName.text where firstNameString.characters.count > 0{
        if let lastNameString = lastName.text where lastNameString.characters.count > 0{
            if let emailString = email.text where emailString.characters.count > 3 && emailString.containsString("@") && emailString.containsString(".") {
                if let passwordString = password.text where passwordString.characters.count > 7{
                    // The validated information can now be used for registration
                    let newUser = User()
                    newUser.firstName = firstNameString
                    newUser.lastName = lastNameString
                    newUser.email = emailString
                    newUser.password = passwordString
                    APIHandler.sharedInstance.registerUser(newUser)
                } else {
                    password.becomeFirstResponder()
                }
            } else {
                email.becomeFirstResponder()
            }
        } else {
            lastName.becomeFirstResponder()
        }
    } else {
        firstName.becomeFirstResponder()
    }
}