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

[Félix Cloutier]

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> 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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