This macro was built to try out a potential syntax for multi-statement if/switch expressions in Swift before the pitch for multi-statement if/switch expressions lands.
When a function is annotated with @BareLastExprs, the last expression in a multi-statement
if/switch expression branch is used as the result. For fullness, the last expression in
a function or closure body is implicitly returned. Together these allow for quite neat code in
my opinion.
The following code sample shows off a variety of complex expressions with implicit returns
powered by the @BareLastExprs function body macro.
@BareLastExprs
func fortune(_ number: Int) -> String {
print("Requesting fortune for \(number)")
let fortune = switch number {
case 1, 3, 5:
print("Warning: Support for odd numbers is unstable")
if number == 3 {
"You have a long and prosperous future"
} else {
"Your future looks bleak"
}
case 2, 4, 6:
if number == 6 {
"You must watch your back tomorrow (good luck...)"
} else {
"Your shoes will develop an untimely hole"
}
default:
print("Warning: I've never encountered \(number) before")
"Spaghetti will fall, meatballs will rise"
}
print("Processing...")
if Int.random(in: 0..<10) == 0 {
print("Warning: Quantum interference detected in RAM")
"Fortune got corrupted, please try again"
} else {
fortune
}
}Since function body macros haven't landed yet, you'll have to download the latest toolchain
snapshot and enable the experimental BodyMacros features. I've been using
the 2023-12-07 toolchain (macOS),
but any newer toolchains should also work. I haven't tested the macro on Linux yet, let me know
if it works!
/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-XXXX-XX-XX-x.xctoolchain/usr/bin/swift run -Xswiftc -enable-experimental-feature -Xswiftc BodyMacrosBe aware that you may get weird compiler crashes if you try to run the example with Xcode, I'm not really sure what's causing it, but I don't think it's the macro's fault. Copying and pasting the expanded code and compiling that runs fine.
The macro wraps code blocks in closures to implement bare last expressions. Which means that assignments to uninitialized variables won't compile (sorry definite initialization).
let opposite: Int
let value = if condition {
opposite = 0 // cannot assign to value: 'opposite' is a 'let' constant
1
} else {
opposite = 1 // cannot assign to value: 'opposite' is a 'let' constant
0
}