[WIP] Compile time safe keypath

[robrix]

⚠️ Not yet ready for merge due to problems with class keypaths, e.g. @keypath(NSObject, version).

This allows it to be used to initialize static variables with the same type-safety and autocompletion benefits, at the cost of complicating the macro’s definition.

Note that this can only apply to the two-argument variant, since there is no way to split the string at compile-time.

Further, an additional change allows types to be passed as the first argument of @keypath, removing the need for workarounds like @keypath(Foo.new, …) which can be jarring, and which may be inconvenient in cases where e.g. +new is unavailable or you don’t even have a concrete class (e.g. with protocols). However, as @lldong points out, this currently breaks generation of class keypaths (e.g. @keypath(NSObject, version)).

The trick this uses to take the type of a comma express compiler-visible mechanism might be useful elsewhere; is something like that already available elsewhere? I ended up not requiring that trick at all; using the comma operator simplified this greatly. Seems like only multistatement things would need the block trick, and they’re probably already in one.

• Can we pass classes directly for class key paths but also allow types?

[robrix]

As an unanticipated benefit of this process, you can now use Xcode’s ⌃⌘E feature with @keypath(x, y) invocations to rename any instances of that method/keypath within a scope.

[lldong]

@robrix @keypath(NSObject, version) failed with:

error: used type 'typeof(NSObject)' (aka 'NSObject') where arithmetic or pointer type is required

[jakebromberg]

@lldong use
@keypath(NSObject *, version)

[robrix]

@lldong That’s not a valid type name; you want NSObject *.

[robrix]

Cheers @jakebromberg 😄

[lldong]

@robrix @jakebromberg + (NSInteger)version is a class method of NSObject. The implementation on master branch allows @keypath(NSObject, version)

libextobjc/extobjc/EXTKeyPathCoding.h

Lines 22 to 23 in 6a669e5

	NSString *versionPath = @keypath(NSObject, version);
	// => @"version"

[robrix]

Ahh, I gotcha—good catch @lldong, cheers.

[robrix]

@jspahrsummers I don’t know how you feel about any of this, but I’m inclined to kick it to the curb given Swift.

[jspahrsummers]

[robrix]

that is a surprisingly disturbing gif

(we could always revert to non-typename keypaths so that the bare class name can still be passed)