> On Jan 11, 2018, at 4:21 PM, Paul Cantrell <cantr...@pobox.com> wrote:
>
> This raises a question related to Chris’s: what is the utility of having Limb
> conform to a protocol instead of just providing allValues ad hoc? Does
> CaseEnumerable / ValueEnumerable serve any purpose other than triggering
> special behavior in the compiler? Would the protocol ever be used as the type
> of something in code?
>
> My answers, admittedly weak ones, are: (1) conventions are nice and
> consistency is nice, and (2) you never know how an abstraction might be used,
> but you do know that people will be angry when it should fit but doesn’t. I
> can’t come up with a more compelling or specific argument than those.
Here's a place where you might want to use the protocol: Suppose you're writing
a table view data source that displays editable controls for a form. You
support several different types of controls, one of which is a list of choices
for a picker controller.
enum Control<Value> {
case textField
case picker(choices: [Value])
…
func makeView() -> UIView { … }
subscript(valueOf view: UIView) -> Value { get { … } set { … } }
}
Presumably you end up writing a schema which looks something like:
formDataSource = FormDataSource(value: person)
formDataSource.fields = [
Section(title: nil, fields: [
Field(title: "Name", keyPath: \.name, control:
.textField),
Field(title: "Gender", keyPath: \.gender, control: .
picker(choices: Array(Gender.allValues))),
…
])
]
tableView.dataSource = formDataSource
The `Array(Gender.allValues)` here is clutter; it'd be nice if we didn't have
to write it explicitly. After all, if you're choosing a value of something you
know is an enum, it's sensible to assume that you want to choose from all
values if it doesn't specify anything more specific. If `ValueEnumerable` is a
formalized protocol, you can do that with a constrained extension:
extension Control where Value: ValueEnumerable {
static var picker: Control {
return .picker(choices: Array(Value.allValues))
}
}
And now you just need to write:
Field(title: "Gender", keyPath: \.gender, control: .
picker)
Basically, having `ValueEnumerable` be a formal protocol means we can extend
this small automatic behavior into larger automatic behaviors. I can imagine,
for instance, building a fuzzer which, given a list of `WritableKeyPath`s whose
types are all `ValueEnumerable`, automatically generates random instances of a
type and feeds them into a test function. If we get read-write reflection, we
might be able to do this automatically and recursively. That'd be pretty cool.
--
Brent Royal-Gordon
Architechies
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