[swift-evolution] [Proposal]: Escaping another (unused) scope pyramide with 'guard try catch'

[David Hart]

The only reason to favour guard is to have the compiler force you to exit the containing scope.

> On 05 Feb 2016, at 19:05, Félix Cloutier via swift-evolution <swift-evolution at swift.org> wrote:
> 
> We could do it without a new guard syntax if `do` didn't need a block statement.
> 
> Félix
> 
>> Le 5 févr. 2016 à 12:54:47, Adrian Zubarev via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>> 
>> Hello dear Swift community,
>> 
>> this proposal might seem like some new syntax sugar, but it also aims to escape the 'do { }' scope from the 'do try catch‘ mechanism while making the existing error handling more powerful.
>> 
>> Lets assume we have some sort of network type, which can throw a ton of different errors:
>> 
>> struct TCPListener {
>> 	
>> 	init(address: String) throws { /* implement */ }
>> 
>> 	func accept() throws -> TCPConn { /* implement */ }
>> 
>> 	/* ... */
>> }
>> 
>> A way of implimentation might look like this:
>> 
>> let listener: TCPListener
>> do {
>> 	listener = try TCPListener("some valid address")
>> 
>> 	// we could do more work here, but if we need to catch more
>> 	// errors we will result in a new PYRAMIDE OF DOOM
>> } catch {
>> 	fatalError()
>> }
>> 
>> At this point think about the comment inside the 'do { }' scope. Such an application might result in a new pyramide of doom as we know from optional unwrapping before 'guard else' mechanism was introduced.
>> 
>> let clientConn: TCPConn
>> do {
>> 	clientConn = try listener.accept() // save to call accept method
>> } catch {
>> 	fatalError()
>> } 
>> 
>> As you can see this application might not need the extra 'do' scope at all, and if it does, the 'do try catch' is still there.
>> 
>> I propose a new error handling mechanism that mimics the solution for optional pyramide of doom, which adds a slightly better syntax and removes the unneeded/unused 'do { }' scope (as for the example from above). Not only can this mechanism guarantee the execution of a throwing function without any errors (like a true guard condition) it also can assign returned values to a new constant/variable.
>> 
>> Introducing the 'guard try catch' mechanism:
>> 
>> guard try throwingFunc() catch { 
>> 	/* handle error */ 
>> }
>> 
>> guard try throwingFunc() catch _ { 
>> 	/* handle error */ 
>> }
>> 
>> guard try throwingFunc() catch pattern { 
>> 	/* handle error */ 
>> }
>> 
>> guard try throwingFunc() catch pattern where condition { 
>> 	/* handle error */ 
>> }
>> 
>> guard let newInstance = try throwingFuncReturns() catch ... { 
>> 	/* handle error */ 
>> }
>> 
>> Where '...' represents the different combinations of possible patterns already showed in the first 4 examples.
>> 
>> We also might want the return type to be mutable.
>> 
>> guard var newMutableInstance = try throwingFuncReturns() catch ... { 
>> 	/* handle error */ 
>> }
>> 
>> This mechanism also makes the error handling more powerful, since it can catch more specific errors defined with 'where condition'.
>> 
>> Lets rebuild the example from above with the new mechanism:
>> 
>> guard let listener = try TCPListener("some valid address") catch {
>> 	fatalError()
>> }
>> 
>> guard let clientConn = try listener.accept() catch {
>> 	fatalError()
>> } 
>> 
>> One think that should be mentioned here is that the method call from the second 'guard' is safe, because a 'guard' body may not fall through.
>> 
>> Impact on existing codebase: None, because the mechanism is new and does not break any existing code.
>> 
>> I'm really curious about your opinions.
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