Swift s guard keyword

Question

Swift 2 introduced the guard keyword  which could be used to ensure that various data is configured ready to go  An example I saw on this website demonstrates an submitTapped function   func submitTapped         guard username text characters count  gt  0 else           return            print  All good       I am wondering if using guard is any different than doing it the old fashioned way  using an if condition  Does it give benefits  which you could not get by using a simple check

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Unlike if  guard creates the variable that can be accessed from outside its block  It is useful to unwrap a lot of Optionals

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Source  Guard in Swift  Let s see the example to understand it clearly   Example 1   func validate                  guard 3 gt 2 else                    print   False                    return                             print   True     True         validate     In the above example we see that 3 is greater than 2 and the statement inside the guard else clause are skipped and True is printed   Example 2   func validate                  guard 1 gt 2 else                    print   False                False      return                             print   True            validate     In the above example we see that 1 is smaller than 2 and the statement inside the guard else clause are executed and False is printed followed by return   Example 3  gaurd let  unwrapping optionals through guard let  func getName args myName  String          guard let name   myName   name isEmpty else        print   Condition is false               Condition is false            return                               print  Condition is met  name            getName args        In the above example we are using guard let to unwrap the optionals  In the function getName we   ve defined a variable of type string myName which is optional  We then use guard let to check whether the variable myName is nil or not  if not assign to name and check again  name is not empty  If both the conditions qualified i e  true the else block will be skipped and print    Conditions is met with name      Basically we are checking two things here separated by comma  first unwrapping and optional and checking whether that satisfies condition or not   Here we are passing nothing to the function i e  empty string and hence Condition is false is print   func getName args myName  String          guard let name   myName   name isEmpty else        print   Condition is false                  return                              print  Condition is met   name       Condition is met Hello       getName args   Hello     Here we are passing    Hello    to the function and you can see the output is printed    Condition is met Hello

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Reading this article I noticed great benefits using Guard  Here you can compare the use of guard with an example   This is the part without guard   func fooBinding x  Int         if let x   x where x  gt  0              Do stuff with x         x description               Value requirements not met  do something      Here you   re putting your desired code within all the conditions  You might not immediately see a problem with this  but you could imagine how confusing it could become if it was nested with numerous conditions that all needed to be met before running your statements   The way to clean this up is to do each of your checks first  and exit if any aren   t met  This allows easy understanding of what conditions will make this function exit   But now we can use guard and we can see that is possible to resolve some issues   func fooGuard x  Int         guard let x   x where x  gt  0 else              Value requirements not met  do something         return               Do stuff with x     x description           Checking for the condition you do want  not the one you don   t  This again is similar to an assert  If the condition is not met    guard   s else statement is run  which breaks out of the function    If the condition passes  the optional variable here is automatically unwrapped for you within the scope that the guard   statement was called     in this case  the fooGuard     function     You are checking for bad cases early  making your function more readable and easier to maintain      This same pattern holds true for non-optional values as well   func fooNonOptionalGood x  Int        guard x  gt  0 else              Value requirements not met  do something         return               Do stuff with x    func fooNonOptionalBad x  Int        if x  lt   0              Value requirements not met  do something         return               Do stuff with x     If you still have any questions you can read the entire article  Swift guard statement   Wrapping Up  And finally  reading and testing I found that if you use guard to unwrap any optionals       those unwrapped values stay around for you to use in the rest of your   code block      guard let unwrappedName   userName else       return    print  Your username is   unwrappedName      Here the unwrapped value would be available only inside the if block  if let unwrappedName   userName       print  Your username is   unwrappedName      else       return       this won t work     unwrappedName doesn t exist here  print  Your username is   unwrappedName

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When a condition is met using guard it exposes variables declared within the guard block to the rest of the code-block  bringing them into its scope  Which  as previously stated  will certainly come in handy with nested if let statements   Note that guard requires a return or a throw in its else statement   Parsing JSON with Guard  Below is an example of how one might parse a JSON object using guard rather than if-let  This is an excerpt from a blog entry that includes a playground file which you can find here    How to use Guard in Swift 2 to parse JSON  func parseJSONWithGuard data    String   AnyObject   throws - gt  Developer        guard let firstname   data  First   as  String  else           return Developer      we could return a nil Developer              guard let lastname   data  Last   as  String else           throw ParseError BadName    or we could throw a custom exception and handle the error            guard let website   data  WebSite   as  String else           throw ParseError BadName            guard let iosDev   data  iosDeveloper   as  Bool else           throw ParseError BadName              return Developer first  firstname  last  lastname  site  website  ios  iosDev       download playground  guard playground  More info   Here s an excerpt from the The Swift Programming Language Guide      If the guard statement   s condition is met  code execution continues   after the guard statement   s closing brace  Any variables or constants   that were assigned values using an optional binding as part of the   condition are available for the rest of the code block that the guard   statement appears in       If that condition is not met  the code inside the else branch is   executed  That branch must transfer control to exit the code block   that that guard statement appears in  It can do this with a control   transfer statement such as return  break  or continue  or it can call   a function or method that doesn   t return  such as fatalError

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Guard statement going to do   it is couple of different   1  it is allow me to reduce  nested if statement 2  it is increase my scope which my variable accessible  if Statement   func doTatal num1   Int   num2  Int          nested if statement     if let fistNum   num1 where num1  gt  0           if let lastNum   num2 where num2  lt  50              let total   fistNum   lastNum                     don t allow me to access out of the scope     total   fistNum   lastNum      Guard statement  func doTatal num1   Int   num2  Int          reduce  nested if statement and check positive way not negative way      guard let fistNum   num1 where num1  gt  0 else        return           guard  let lastNum   num2 where num2  lt  50 else        return              increase my scope which my variable accessible     let total   fistNum   lastNum

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Simply put  it provides a way to validate fields prior to execution  This is a good programming style as it enhances readability  In other languages  it may look like this   func doSomething         if something    nil              return  break  throw error  etc                    But because Swift provides you with optionals  we can t check if it s nil and assign its value to a variable  In contrast  if let checks that it s not nil and assigns a variable to hold the actual value  This is where guard comes into play  It gives you a more concise way of exiting early using optionals

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One benefit is elimination a lot of nested if let statements  See the WWDC  What s New in Swift  video around 15 30  the section titled  Pyramid of Doom

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From Apple documentation    Guard Statement  A guard statement is used to transfer program control out of a scope if one or more conditions aren   t met   Synatx   guard condition else       statements     Advantage   1  By using guard statement we can get rid of deeply nested conditionals whose sole purpose is validating a set of requirements   2  It was designed specifically for exiting a method or function early   if you use if let below is the code how it looks     let task   URLSession shared dataTask with  request     data  response  error  in          if error    nil               if let  statusCode    response as  HTTPURLResponse   statusCode  statusCode  gt   200  amp  amp  statusCode  lt   299                   if let data   data                          Process Data Here                      print  Data    data                       else                       print  No data was returned by the request                                    else                   print  Your request returned a status code other than 2XX                            else               print  Error Info    error debugDescription                        task resume     Using guard you can transfer control out of a scope if one or more conditions aren t met   let task   URLSession shared dataTask with  request     data  response  error  in                 GUARD  was there an error                 guard  error    nil  else                   print  There was an error with your request    error                    return                               GUARD  Did we get a successful 2XX response                 guard let statusCode    response as  HTTPURLResponse   statusCode  statusCode  gt   200  amp  amp  statusCode  lt   299 else                   print  Your request returned a status code other than 2XX                    return                               GUARD  was there any data returned                 guard let data   data else                   print  No data was returned by the request                    return                              Process Data Here              print  Data    data      task resume     Reference    1  Swift 2  Exit Early With guard 2  Udacity 3  Guard Statement

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With using guard our intension is clear  we do not want to execute rest of the code if that particular condition is not satisfied  here we are able to extending chain too  please have a look at below code   guard let value1   number1  let value2   number2 else   return       do stuff here

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Like an if statement  guard executes statements based on a Boolean value of an expression  Unlike an if statement  guard statements only run if the conditions are not met  You can think of guard more like an Assert  but rather than crashing  you can gracefully exit   refer to  http   ericcerney com swift-guard-statement

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It really really does make the flow of a sequence with several lookups and optionals much more concise and clear and reduces lots of if nesting  See Erica Sadun post on replacing Ifs       Could get carried away  an example below       let filteredLinks   locationsLinkedToList filter   0 actionVerb    movementCommand       guard let foundLink   filteredLinks first else  return    lt Person gt  cannot go in that direction    nil  nil       guard filteredLinks count    1 else  return    lt Person gt  cannot decide which route to take    nil  nil       guard let nextLocation   foundLink toLocation else  return    lt Person gt  cannot go in that direction    nil  nil     See if that sticks

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When to use guards    If you   ve got a view controller with a few UITextField elements or some other type of user input  you   ll immediately notice that you must unwrap the textField text optional to get to the text inside  if any    isEmpty won   t do you any good here  without any input the text field will simply return nil   So you have a few of these which you unwrap and eventually pass to a function that posts them to a server endpoint  We don   t want the server code to have to deal with nil values or mistakenly send invalid values to the server so we   ll unwrap those input values with guard first   func submit         guard let name   nameField text else           show  No name to submit           return            guard let address   addressField text else           show  No address to submit           return            guard let phone   phoneField text else           show  No phone to submit           return            sendToServer name  address  address  phone  phone     func sendToServer name  String  address  String  phone  String              You   ll notice that our server communication function takes non-optional String values as parameters  hence the guard unwrapping beforehand  The unwrapping is a little unintuitive because we   re used to unwrapping with if let which unwraps values for use inside a block  Here the guard statement has an associated block but it   s actually an else block - i e  the thing you do if the unwrapping fails - the values are unwrapped straight into the same context as the statement itself      separation of concerns  Without guard  Without using guard  we   d end up with a big pile of code that resembles a pyramid of doom  This doesn   t scale well for adding new fields to our form or make for very readable code  Indentation can be difficult to follow  particularly with so many else statements at each fork   func nonguardSubmit         if let name   nameField text           if let address   addressField text               if let phone   phoneField text                   sendToServer name  address  address  phone  phone                else                   show  no phone to submit                           else               show  no address to submit                   else           show  no name to submit             Yes  we could even combine all these if let statements into a single statement separated with commas but we would loose the ability to figure out which statement failed and present a message to the user   https   thatthinginswift com guard-statement-swift

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There are really two big benefits to guard   One is avoiding the pyramid of doom  as others have mentioned     lots of annoying if let statements nested inside each other moving further and further to the right   The other benefit is often the logic you want to implement is more  if not let    than  if let     else    Here   s an example  suppose you want to implement accumulate     a cross between map and reduce where it gives you back an array of running reduces   Here it is with guard   extension Sliceable where SubSlice Generator Element    Generator Element        func accumulate combine   Generator Element Generator Element - gt Generator Element  - gt   Generator Element               if there are no elements  I just want to bail out and            return an empty array         guard var running   self first else   return                  running will now be an unwrapped non-optional         var result    running              dropFirst is safe because the collection            must have at least one element at this point         for x in dropFirst self                running   combine running  x              result append running                    return result            let a    1 2 3  accumulate         1 3 6  let b    Int    accumulate             How would you write it without guard  but still using first that returns an optional   Something like this   extension Sliceable where SubSlice Generator Element    Generator Element        func accumulate combine   Generator Element Generator Element - gt Generator Element  - gt   Generator Element             if var running   self first                var result    running               for x in dropFirst self                    running   combine running  x                  result append running                            return result                   else               return                         The extra nesting is annoying  but also  it   s not as logical to have the if and the else so far apart   It   s much more readable to have the early exit for the empty case  and then continue with the rest of the function as if that wasn   t a possibility

[swift2] Swift's guard keyword

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