[swift-users] Should I be using more catchless do blocks?

[Michael Ilseman]

> On Jun 19, 2017, at 11:47 AM, Michael Savich <savichmichael at icloud.com> wrote:
> 
> Yeah, it's all about balance to be sure. Though one benefit of do blocks is in functions that are tied to a sense of time. It seems to me that the in case of something like viewDidLoad separating code into too many functions can obscure the fact that the code is meant to be executed at that time.

I was referring to defining a local function inside your function that you’re refactoring. As in, rather than say:

func foo(...) -> … {
	// some initialization
	do {
		… some local variables, some not local...
	}

	code1...

	// some more scoped work
	do {
		… some local variables, some not local...
	}

	code2...

	// some finalization
 	do {
		… some local variables, some not local...
	}
}

You have:

func foo(…) -> … {
	func doLocalSetup(…inputs...)  {
		… use explicit inputs and local variables
	}
	func performScopedWork(…inputs…) {
		… use explicit inputs and local variables
	}
	func doFinalTearDown(…inputs…) {
		… use explicit inputs and local variables
	}

	doLocalSetup(…)
	defer { doFinalTearDown(…) }
	
	code1...

	performScopedWork(…)

	code2...	
}


That’s just one option. You also mentioned using closures, which can be less clear if you’re relying on implicit captures rather than explicit parameters (which can have labels/names). It all depends on the details.

> Closures can provide much of the same functionality but I'm pretty sure inline closures have to have names and sometimes risking a bad name is worse than no name at all.
> 

That might be the case. However, often such a do block is worthy of a comment before it, and good names make really good comments.

> Anyway, do you think that most Swift users are even aware that do can be used in this fashion?
> 

I wouldn’t think it would be obvious to new Swift programmers, but might be familiar to programmers coming from other languages that use scopes heavily. 

It probably depends on your team specifics. As you mentioned, you only recently learned of this behavior, so your experience might be a useful proxy for whether others are or are not familiar.

> Sent from my iPad
> 
> On Jun 19, 2017, at 2:33 PM, Michael Ilseman <milseman at apple.com <mailto:milseman at apple.com>> wrote:
> 
>> Introducing scope to manage lifetimes of local variables is a useful and valuable practice. Note that it might also be an opportunity to refactor the code. Any do block you want to introduce could also be a local function definition that you call later. Alternatively, it could be generalized and extracted into a utility component. Long function bodies with many do blocks could be a code smell.
>> 
>> 
>>> On Jun 18, 2017, at 7:07 PM, Michael Savich via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>> 
>>> So, something I did not know until recently is that do blocks in Swift are for more than just error handling, they can also be used to tighten scope. 
>>> 
>>> I'm wondering, why not use a ton of do blocks? Like, if I have a ViewController lifecycle method like viewDidLoad, I could segment it into out a do block for creating subviews, a do block for loading data into them, and a do block for adding them to the view itself. This seems like it would enforce grouping code tightly together.
>>> 
>>> Yes I could adopt a functional style of programming, but that has its downsides too, namely reading any functional code involves trawling through a long sequence of function calls. What I'm saying is, do blocks seem like a way to get many of the benefits of functional programming while maintaining the readability of imperative code. (Sorry functional programmers, I promise I love Haskell too!)
>>> 
>>> So I guess what I'm saying is… somebody talk me down from this ledge. Is there a reason I shouldn't refactor my projects to be full of do blocks? And can this usage of do really be considered idiomatic Swift? Or will most people reading my code be left wondering where all the try and catch statements are?
>>> 
>>> Sent from my iPad
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>> 

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