[swift-evolution] Proposal: Introduce User-defined "Dynamic Member Lookup" Types

[Magnus Ahltorp]

> 4 Dec. 2017 02:40 Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> That’s a good principle.  However, a dynamic member lookup is just a member lookup.  By that principle, it should look like a member lookup :-)
> 
> Further, I incorporated some of the conversation with Matthew into the proposal, showing how adding even a single sigil to dynamic member lookup to distinguish it is problematic:
> https://gist.github.com/lattner/b016e1cf86c43732c8d82f90e5ae5438#increasing-visibility-of-dynamic-member-lookups
> 
> Further, adding something like .dynamic would completely undermind the proposal.  You can already write:
> 
> 	x.get(“foo”).get(“bar”)
> 
> having to write:
> 
> 	x.dynamic.foo.dynamic.bar
> 
> has no point.

This example shows what many on this list don't believe: that any Swift method or member access can fail. If the return value of this "get" method is an IUO, or not an Optional at all, and doesn't throw, then the expression would have to fail hard if "foo" didn't resolve to something meaningful.

The most common argument against this proposal is that someone could make an API using Dynamic Member Lookup that could fail even though it is not apparent to the caller. But, as we see in the example, this is just as possible today.

Another example where potential failure is not apparent to the caller:

struct Foo { 
    var bar: Int
} 

struct Example { 
    var n: Int 
    var foo: Foo { return Foo(bar: n + 1) } 
} 

let a = Example(n: Int.max) 
a.foo.bar

Swift doesn't save the caller in this case, so together with Chris' example, I don't really understand the "not apparent to the caller" argument. Crashability is totally in the hands of the implementor of a method or member access, now and with the proposal.

/Magnus