Merge pull request #221 from tjw/SR-842

First draft of proposal for <https://bugs.swift.org/browse/SR-842>.

Author: gribozavr

proposals/NNNN-autoreleasepool-signature.md

@@ -0,0 +1,129 @@
+# Add Generic Result and Error Handling to autoreleasepool()
+
+* Proposal: [SE-NNNN](https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-autoreleasepool-signature.md)
+* Author(s): [Timothy J. Wood](https://github.com/tjw)
+* Status: **Awaiting review**
+* Review manager: TBD
+
+## Introduction
+
+The `autoreleasepool` function in the standard library does not currently
+support a return value or error handling, making it difficult and error-prone
+to pass results or errors from the body to the calling context.
+
+Swift-evolution thread: A first call for discussion was
+[made here](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160314/013054.html).
+Dmitri Gribenko pointed out that adding a generic return type would be useful
+(first in my premature pull request) and then also [here](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160321/013059.html).
+Jordan Rose [pointed out](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160321/013077.html)
+that care was needed to avoid inferring an incorrect return type for the body
+block, but after testing we confirmed that this is handled correctly by
+the compiler.
+
+## Motivation
+
+The current signature for `autoreleasepool` forces the creation of temporary
+variables to capture any results of the inner computation, as well as any error
+to eventually throw, in the case they are needed in the calling code. This extra
+boilerplate clutters up the intent, as well as introduces the risk of
+accidentally unwrapping a `nil` value.
+
+For example:
+
+```swift
+func doWork() throws -> Result {
+    var result: Result? = nil
+    var error: ErrorProtocol? = nil
+    autoreleasepool {
+        do {
+            ... actual computation which hopefully assigns to result but might not ...
+        } catch let e {
+            error = e
+        }
+    }
+    guard let result = result else {
+        throw error!
+    }
+    return result!
+}
+```
+
+## Proposed solution
+
+I'd like to propose altering the signature of the standard library
+`autoreleasepool` function to allow for a generic return type, as well as
+allowing a `throw` of an error:
+
+```swift
+public func autoreleasepool<Result>(@noescape body: () throws -> Result) rethrows -> Result
+```
+
+The case above becomes much more clear and less error-prone since the compiler
+can enforce that exactly one of the error and result are used:
+
+```swift
+func doWork() throws -> Result {
+    return try autoreleasepool {
+        ... actual computation which either returns or throws ...
+    }
+}
+```
+
+As an aside, since this proposes changing the signature already, I would like
+to further propose changing the argument label from `code` to `body`. This seems
+more in line with the parameter name used in the rest of the standard library,
+but isn't central to this proposal.
+
+## Detailed design
+
+The updated standard library function would read:
+
+```swift
+public func autoreleasepool<Result>(@noescape body: () throws -> Result) rethrows -> Result {
+    let pool = __pushAutoreleasePool()
+    defer {
+        __popAutoreleasePool(pool)
+    }
+    return try body()
+}
+```
+
+## Impact on existing code
+
+No impact expected.
+
+## Alternatives considered
+
+The [original request, SR-842](https://bugs.swift.org/browse/SR-842) only
+suggested adding `throws`, but Dmitri Gribenko pointed out that adding a generic
+return type would be better.
+
+I also explored whether third-party code could wrap `autoreleasepool` themselves
+with something like:
+
+```swift
+func autoreleasepool_generic<ResultType>(@noescape code: Void throws -> ResultType) rethrows -> ResultType {
+    var result:ResultType?
+    var error:ErrorProtocol?
+
+    autoreleasepool {
+        do {
+            result = try code()
+        } catch let e {
+            error = e
+        }
+    }
+
+    if let result = result {
+        return result
+    }
+
+    throw error! // Doesn't compile.
+}
+```
+  
+but this doesn't compile, since in a function with `rethrows`, only the call to
+the passed in function that is marked as `throws` is allowed to throw.
+Even if it was possible to create a `rethrows` wrapper from the non-throwing
+function, it is better to add the safety to the standard library in the
+first place.