> 6 Dec. 2017 04:13 Thorsten Seitz <[email protected]> wrote:
>
> I just wanted to add that the single purpose of a static type system is to
> ensure that the methods being called on a receiver are present at runtime and
> take arguments of the types known at compile time. Of course the type system
> does not guarantee that those calls to not fail. Even in Haskell there is no
> guarantee that a function call does not fail as it allows partial patterns
> (therefore `head []` crashes). Or a function call might enter an infinite
> loop. But, to repeat, the type system guarantees that the method or member
> itself will be present.
>
> Therefore I cannot follow the argument that it follows from the possible
> failure of a Swift method or member access that it is ok or even a comparable
> type of failure if the method or member itself is not present. This would
> imply that Swift’s static type system is unnecessary. Why check for the
> presence of a method or member if calling it or accessing it might fail
> anyway? No, I do not buy that argument. And I do not think that I have to
> provide examples where a static type system is of help.
SOURCE MUSIC: "Soothe me"
Program crashes.
PROGRAMMER
What? What did I do?
RUNTIME
You failed to call a method correctly.
PROGRAMMER
The method was present, sir.
By your reasoning, this code would be ok:
class Example {
var a: Int { return values["a"]! }
var b: Int { return values["b"]! }
var c: Int { return values["c"]! }
var d: Int { return values["d"]! }
…
}
(which I believe is more or less equivalent to what has been suggested by some,
but by enumerating all the dynamic members that seem to exist)
but not
class Example: DynamicMemberLookupProtocol {
subscript(dynamicMember: String) -> Int {
return values[dynamicMember]!
}
}
> The remaining question therefore is whether it is ok to remove the static
> type system for certain isolated use cases and that might be the case. But I
> like others would prefer if those use cases would be isolated somehow
> visually. Someone proposed to require `dynamic` before expressions containing
> dynamic member lookups similar to `try`, with the additional option to
> enclose a whole block into `dynamic { … }` to express the same as prefixing
> each expression with `dynamic`. I still think this has merit:
>
> ````
> 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
>
> // will crash if foo or bar are not present
> let result = dynamic {
> let y = x.foo.bar
> let z = y.baz(42)
> return z.zork()
> }
>
> // will return nil if foo or bar are not present
> let result = dynamic? {
> let y = x.foo.bar
> let z = y.baz(42)
> return z.zork()
> }
> ````
Which is already possible with the proposal, using the existing "throw"
mechanism in Swift.
/Magnus
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