> On Dec 5, 2017, at 12:13 PM, Thorsten Seitz via swift-evolution
> <swift-evolution@swift.org> wrote:
>
> let result = dynamic x.foo.bar // will crash if foo or bar are not present
>
> let result = dynamic? x.foo.bar // will return nil if foo or bar are not
> present
Under the proposal given here, the compiler doesn’t know what will happen when
“foo” or “bar” are not present. This proposal does not even require the
implementation to report if a member is present or not, and there is certainly
no guarantee of a crash in such a case. The only thing the compiler knows is
that subscript(dynamicMember:) will be called.
Consider the following implementation:
struct PyVal: DynamicMemberLookupProtocol {
subscript(dynamicMember: String) -> PyVal? {
if let pythonMember = python_c_api.get(dynamicMember) {
return PyVal(pythonMember)
}
else {
return nil
}
}
}
let result = x.foo?.bar
There is no crashing here; result will be an optional type, and will be nil if
the requested property does not exist. Some other use of
DynamicMemberLookupProtocol might accept any value, and just log it to a file!
In that case, neither crashing nor returning an optional is required.
-BJ_______________________________________________
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