[swift-evolution] [Proposal] Remove force unwrapping in function signature.

Saagar Jha saagarjha28 at gmail.com
Mon Jun 27 18:42:56 CDT 2016 

Alright, I’ve written it up a proposal; you can find it here
<https://gist.github.com/saagarjha/f33fecd4576f40133b6469da942ef453>. This
is my first proposal (and anyways I’ve been told that I can be unclear), so
if you guys see anything that should be changed feel free to let me know.
Here it is inline:
Remove implicitly unwrapped optionals as function parameters

   - Proposal: SE-NNNN
   <NNNN-remove-implicitly-unwrapped-function-parameters.md>
   - Author: Swift Developer <https://github.com/swiftdev>
   - Status: *Awaiting review*
   - Review manager: TBD

Introduction

Swift, in contrast with Objective-C, makes a distinction between values
that may be nil and values that can never be nil through its use of
Optionals. Due to the fact that Objective-C does not make this distinction,
Objective-C functions that do not use the Nullability
<https://developer.apple.com/swift/blog/?id=25> annotations are imported
with parameters of the implicitly unwrapped optional type. Unfortunately,
this allows users to write their own Swift code that looks like this:

func foo(bar: Int!) {
    //…
}

Due to the confusion this may cause, we would like to propose the *removal
of implicitly unwrapped optionals as function parameters*. Discussion on
this topic may be found here
<http://article.gmane.org/gmane.comp.lang.swift.evolution/21730/>.
Motivation

Implicitly unwrapped optionals are currently allowed in function
declarations. Consider the following function:

func triple(forceUnwrapping aNumber: Int) -> Int {
    return aNumber * 3
}

let possiblyNil = Int("foo")
triple(forceUnwrapping: possiblyNil)

possiblyNil is an Int?; thus, this example will not compile due to
triple(forceUnwrapping:) expecting an Int. It is easy to imagine a Swift
beginner writing code that looks like this to "fix" the problem:

func triple(forceUnwrapping aNumber: Int!) -> Int {
    return aNumber * 3
}

let possiblyNil = Int("foo")
triple(forceUnwrapping: possiblyNil)

While this version compiles, it crashes due to the force unwrapping of a nil
value. Unfortunately, the compiler "hides" this fact by making it seem like
it's acceptable to pass in nil–it doesn't make the forced unwrapping
*explicit*.
Proposed solution

The safest solution, in this case, is to prevent the use of implicitly
unrwapped optionals in function signatures. By forcing users to write

func triple(forceUnwrapping aNumber: Int) -> Int {
    return aNumber * 3
}

or

func triple(forceUnwrapping aNumber: Int?) -> Int {
    return aNumber * 3
}

the compiler will complain, reminding users that they should probably
attempt to safely unwrap the optional before using it.
Detailed design

The proposal will prevent the use of implicitly unwrapped optionals in
function signatures for both Swift code as well as imported Objective-C
code. As non-annotated Objective-C functions are currently imported as
implicitly unwrapped, they will be converted to optionals as a preliminary
step. Non-audited frameworks can be audited in the future so that they can
be tagged with _Nonnull if necessary.
Impact on existing code

This is a proposal is a source breaking change, but it should be easily
mitigated using a migrator. Existing functions with implicitly unwrapped
optionals can be changed to optional; users can easily shadow variables
with a guard or change their function to non-optional.
Alternatives considered Importing Objective-C functions as-is, but
disallowing implictly unwrapped optionals in Swift code

This reduces the burden on existing frameworks and adding Nullability
annotations, but creates a sort of disconnect between Objective-C and Swift
in that it prevents Swift developers from writing functions with implicitly
unwrapped optionals.
Doing nothing

Obviously, this has the benefit of keeping the current behavior and not
requiring a migrator. However, I believe that the unsafe behavior that this
encourages is not worth keeping.


On Mon, Jun 27, 2016 at 1:35 PM Dennis Lysenko <dennis.s.lysenko at gmail.com>
wrote:

> +1. This is sort of how Kotlin does it. In Kotlin, IUOs are strictly a
> carryover from Java. They show up in method signatures from
> non-nullable-annotated Java, but you can't define a new method that takes
> e.g. an Int!.
>
> The limited scope of this proposal is ideal in my opinion since we see
> areas where IUOs are clearly useful (ViewControllers for instance) but
> defining new functions that take implicitly unwrapped optionals makes no
> sense. If you need to pass a IUO at the call site, you can define the
> function taking a non-optional value and pass the IUO to that. There is no
> use case I can think of for having it in method/function signatures.
>
> RE: language inconsistencies, there is no such issue in practice in Kotlin
> where there is also inconsistency in the same vein. I see it simply as a
> compromise that achieves the goal of keeping a useful feature but
> discouraging its overuse by forbidding its use in places where its use
> could confuse and snowball down the line into teaching developers worse
> code quality.
>
> On Mon, Jun 27, 2016 at 12:04 PM Charlie Monroe via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> Ok, I see - though I find myself using occasionally IUOs in Swift as well
>> - e.g. when you can't use the default values because they depend on self,
>> etc.
>>
>> Eliminating it just from method signatures IMHO brings an incosistency
>> into the language. Why would you eliminate it only from method signatures -
>> this proposal mentioned importing ObjC API in the beginning - why not then
>> mark those properties all as optional as well? IUOs are scheduled to be
>> removed completely once the language reaches a point where it can handle
>> most scenarios otherwise...
>>
>> Try to imagine some APIs brought to Swift with default being nullable:
>>
>> /// Imported from
>> public class NSOrderedSet : NSObject, NSCopying, NSMutableCopying,
>> NSSecureCoding, NSFastEnumeration {
>>
>>
>>     public var count: Int { get }
>>     public func objectAtIndex(idx: Int) -> AnyObject?
>>     public func indexOfObject(object: AnyObject?) -> Int
>>     public init()
>>     public init(objects: UnsafePointer<AnyObject?>, count cnt: Int)
>>     public init?(coder aDecoder: NSCoder?)
>> }
>>
>> This doesn't make much sense - mostly objectAtIndex(_:).
>>
>> On Jun 27, 2016, at 8:35 PM, Saagar Jha <saagarjha28 at gmail.com> wrote:
>>
>> I think you’re mistaking the scope of the proposal. It’s simply removing
>> IUOs in *function signatures*, not throughout the language.
>>
>> On Mon, Jun 27, 2016 at 11:31 AM Charlie Monroe via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> There are many useful cases for IUO in Swift - mostly when you have
>>> variables that cannot be calculated at the point of calling super.init(),
>>> but are guaranteed to be filled during initialization - i.e. during the
>>> lifetime of the object, the value is nonnil, but may be nil for a short
>>> period of time.
>>>
>>> Or @IBOutlets. Making all @IBOutlets optionals would make the code
>>> either riddled with ! or shadowed locally re-declared instance members.
>>>
>>>
>>> > On Jun 27, 2016, at 8:12 PM, Jean-Daniel Dupas <mailing at xenonium.com>
>>> wrote:
>>> >
>>> > Maybe we can prohibit it in Swift function declaration, and allow it
>>> only when importing native code.
>>> >
>>> > As David, I don’t see any compelling reason to allow such construct in
>>> Swift.
>>> >
>>> >> Le 27 juin 2016 à 10:39, Charlie Monroe via swift-evolution <
>>> swift-evolution at swift.org> a écrit :
>>> >>
>>> >> When you import ObjC code that has no nullability annotation, IUO
>>> make sense since:
>>> >>
>>> >> - they can be checked against nil
>>> >> - typically, most values in APIs are nonnull (looking at Foundation,
>>> for example, which is why Apple has the NS_ASSUME_NONNULL_BEGIN to mark
>>> entire regions as nonnull, yet there is no NS_ASSUME_NULL_BEGIN)
>>> >>
>>> >> Importing them as optionals would make it really hard to work with
>>> the code - whenever you get a value, it's an optional, even in cases where
>>> it makes no sense and adding ! to unwrap the optional is not a great
>>> solution. And the other solution is to use guards everywhere.
>>> >>
>>> >> IMHO the IUO is a nice (temporary) solution for using un-annotated
>>> code until it is. But the "pressure" should be applied on the ObjC code.
>>> >>
>>> >>> On Jun 27, 2016, at 10:03 AM, David Rönnqvist <
>>> david.ronnqvist at gmail.com> wrote:
>>> >>>
>>> >>> I don’t know about the chances of getting approved, but I think this
>>> is something worth discussing.
>>> >>>
>>> >>> It might just be my ignorance, but I can’t think of a good reason
>>> why a function argument would be force unwrapped. Either it’s non-null and
>>> the caller is expected to unwrap it or it’s nullable and the method is
>>> expected to handle the nil value. So I’m positive to that part of the
>>> proposal.
>>> >>>
>>> >>> As to what we should do with the generated interfaces of Objective-C
>>> code that hasn’t been annotated with nullability, I think that needs input
>>> from more people to find the preferred solution.
>>> >>>
>>> >>> Once that’s been discussed some more, I’d be willing to write up a
>>> formal proposal if you don’t feel like it (assuming the discussion leads
>>> somewhere).
>>> >>>
>>> >>> - David
>>> >>>
>>> >>>
>>> >>>> On 27 Jun 2016, at 06:28, Charlie Monroe via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>> >>>>
>>> >>>> See https://github.com/apple/swift-evolution/blob/master/process.md
>>> - you would need to make an official proposal and submit it as pull
>>> request. But given the reaction here, it's unlikely to get approved.
>>> >>>>
>>> >>>> Also, the ObjC code without nullability is getting fairly rare -
>>> all Apple's frameworks are with nullability information (as far as I've
>>> checked) in macOS 10.12, iOS 10. Third party libraries should be updated to
>>> use nullability (and most libraries that are maintained already do).
>>> >>>>
>>> >>>>
>>> >>>>> On Jun 25, 2016, at 5:13 PM, Spromicky via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>> >>>>>
>>> >>>>> So, its proposal is dead, or what we must to do to force it to
>>> swift-evolution repo on GitHub?
>>> >>>>>
>>> >>>>>> Hello, everyone!
>>> >>>>>>
>>> >>>>>> I wanna propose to you to remove force unwrapping in fuction
>>> signature for swift code. That no sense in clear swift code. If we wanna
>>> use some optional value as function param, that is not optional, we must
>>> unwrap it before function call.
>>> >>>>>> People who new in swift look at how they old Obj-C code (without
>>> nullability modifiers) translate in to swift:
>>> >>>>>>
>>> >>>>>> Obj-C:
>>> >>>>>> - (void)foo:(NSInteger)bar {
>>> >>>>>> //...
>>> >>>>>> }
>>> >>>>>>
>>> >>>>>> Swift transaliton:
>>> >>>>>> func foo(bar: Int!) {
>>> >>>>>> //...
>>> >>>>>> }
>>> >>>>>>
>>> >>>>>> And think that force unwrapping in signature is good practice.
>>> And start write functions in clear swift code like this:
>>> >>>>>>
>>> >>>>>> func newFoo(bar: Int!) {
>>> >>>>>> //...
>>> >>>>>> }
>>> >>>>>>
>>> >>>>>> and use it like this:
>>> >>>>>>
>>> >>>>>> let bar: Int? = 1
>>> >>>>>> newFoo(bar)
>>> >>>>>>
>>> >>>>>> And it really work, and they does not think that this can crash
>>> in case if `bar` will be `nil`.
>>> >>>>>> But in clear swift we wanna work with parametrs in function that
>>> clearly or optional, or not.
>>> >>>>>>
>>> >>>>>> func newFoo(bar: Int) {
>>> >>>>>> //...
>>> >>>>>> }
>>> >>>>>>
>>> >>>>>> or
>>> >>>>>>
>>> >>>>>> func newFoo(bar: Int?) {
>>> >>>>>> //...
>>> >>>>>> }
>>> >>>>>>
>>> >>>>>> When we write a new function we know what we need in this case
>>> and use optional params or not.
>>> >>>>>>
>>> >>>>>> So my proposal is remove force unwrapping(`!`) from function
>>> signatures, cause it have no sense, and that confuse new users.
>>> >>>>>>
>>> >>>>>>
>>> >>>>>>
>>> >>>>> _______________________________________________
>>> >>>>> swift-evolution mailing list
>>> >>>>> swift-evolution at swift.org
>>> >>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> >>>>
>>> >>>> _______________________________________________
>>> >>>> swift-evolution mailing list
>>> >>>> swift-evolution at swift.org
>>> >>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> >>>
>>> >>
>>> >> _______________________________________________
>>> >> swift-evolution mailing list
>>> >> swift-evolution at swift.org
>>> >> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> >
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>> --
>> -Saagar Jha
>>
>>
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
> --
-Saagar Jha
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