[swift-evolution] [Proposal Draft] parameter forwarding

David Hart david at hartbit.com
Mon Jan 11 16:12:48 CST 2016 

I’m already not a huge fan of the complexity introduced with memberwise initializers, but see some usefulness. But I see much less use in parameter forwarding.
I’m very afraid of all this "syntactic sugar” and would prefer a simpler language even if it means a little bit more typing.

> On 11 Jan 2016, at 19:26, Félix Cloutier via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>>> On Jan 10, 2016, at 10:17 PM, Félix Cloutier <felixcca at yahoo.ca <mailto:felixcca at yahoo.ca>> wrote:
>>> 
>>> I was okay with memberwise initializers but this pushes me beyond my comfort zone.
>> 
>> Hi Felix, can you elaborate on why?  This feature is quite similar to features in other languages that are generally considered to be quite useful.
>> 
>> /* snip */
> 
> I don't find it similar to how dynamic languages do parameter packing, and not useful in the same conditions. In Python, you'll use parameter packing and unpacking mostly with functions that receive variable parameters. Swift's variadic functions don't work like that. It's strictly less powerful and less needed than C++'s perfect forwarding because Swift doesn't have lvalue (or rvalue) references. Besides, perfect forwarding is mostly used when you don't know the call target, and the parameter packing proposal requires you to know it.
> 
> The fact that there are forwarding solutions in other languages doesn't mean that Swift has the problems that these solutions are trying to address.
> 
>>> I'm not sold on the usefulness of the feature. Memberwise initializers save you from typing out the init parameters and assignments to each field. Argument forwarding saves you from spelling out the parameters *more than once* (because you still need to type them out for the receiving function) and from *one call*. While I've been annoyed at initializers, I don't think I've ever been particularly annoyed at forwarding functions.
>> 
>> Both features save approximately the same amount of code.  They save explicit declaration of parameters as well as a single action with the provided argument.
> 
> Here's a 4-field example for memberwise initializers:
> 
>> struct Foo {
>> 	var bar: Int
>> 	var baz: Int
>> 	var frob: Int
>> 	var nicate: Int
>> }
> 
> it's either gonna be "memberwise init(...) {}" (23 chars) or:
> 
>> 	init(bar: Int, baz: Int, frob: Int, nicate: Int) {
>> 		self.bar = bar
>> 		self.baz = baz
>> 		self.frob = frob
>> 		self.nicate = nicate
>> 	}
> 
> 
> 120 characters excluding tabs, and with very short insignificant names. That's like six times less code for the same thing.
> 
> Let's have a 4-parameter method:
> 
>> class Foo {
>> 	func foo(bar: Int, baz: Int, frob: Int, nicate: Int) { /* snip */ }
>> }
> 
> 
> Now this is either going to be "func bar(...fooPack) { foo(bar: 1, ...fooPack) }" (48 chars) or "func bar(baz: Int, frob: Int, nicate: Int) { foo(bar: 1, baz: baz, frob: frob, nicate: nicate) }" (96 chars).
> 
>> More importantly, forwarding is a general purpose feature that when combined with partial initializers and property lists can support much more expressive memberwise initialization than contained in the initial proposal.  There was quite a bit of discussion about both the limitations of the memberwise initialization proposal as well as the specificity of it to exactly one use case (memberwise initialization).  Forwarding plays a role in removing the limitations while building on a more general foundation.
> 
> I disagree with the critics and I don't support these other proposals. In my opinion, the objections you're referring to were theoretical-style "this doesn't solve the General Problem". The vast majority of users don't need the General Problem to be solved, and if a specific solution gets us 95% of the way there with little disturbance, it's probably best to take it and leave behind the major changes required for the remaining 5%.
> 
> The question here is whether memberwise initialization gets us 95% of the way there. I think it does, and I'm gonna need real code and user feedback to change my mind. This is unlikely to happen before memberwise initializers ship.
> 
> I really think that we should wait to see how memberwise initialization plays out before laying out plans to extend it.
> 
>>> At this point, I feel that a competent macro system is a better investment than adding distinct bits of automation wherever there appears to be repetition.
>> 
>> I agree that a macro system would be great, but it is explicitly not in scope for Swift 3.  It would also not be capable of implementing parameter forwarding as described in this proposal.
>> 
>> I hope you will consider discussing this further.
>> 
>> Matthew
>> 
>>> 
>>> Félix
>>> 
>>>> Le 10 janv. 2016 à 22:44:36, Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>>> 
>>>> I have always considered the Flexible Memberwise Initialization proposal to be just a first step (as evidenced by the many future enhancements it discussed).  Its review has inspired new ideas and helped to shape my vision of the best long-term solution.  My final thoughts about the review can be found here: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160104/006176.html <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160104/006176.html>
>>>> 
>>>> Parameter forwarding is the first in a series of three proposals describing general features that can work together to form a complete solution.
>>>> 
>>>> The proposal drafts can be found at the following links:
>>>> 
>>>> * Parameter forwarding: https://github.com/anandabits/swift-evolution/blob/parameter-forwarding/proposals/NNNN-parameter-forwarding.md <https://github.com/anandabits/swift-evolution/blob/parameter-forwarding/proposals/NNNN-parameter-forwarding.md>
>>>> * Partial initializers: https://github.com/anandabits/swift-evolution/blob/partial-initializers/proposals/NNNN-partial-initializers.md <https://github.com/anandabits/swift-evolution/blob/partial-initializers/proposals/NNNN-partial-initializers.md>
>>>> * Property lists: https://github.com/anandabits/swift-evolution/blob/property-lists/proposals/NNNN-property-lists.md <https://github.com/anandabits/swift-evolution/blob/property-lists/proposals/NNNN-property-lists.md>
>>>> 
>>>> Matthew
>>>> Parameter Forwarding
>>>> 
>>>> Proposal: SE-NNNN <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-parameter-forwarding.md>
>>>> Author(s): Matthew Johnson <https://github.com/anandabits>
>>>> Status: Awaiting review
>>>> Review manager: TBD
>>>> Introduction
>>>> 
>>>> This feature introduces an automatic parameter forwarding mechanism.
>>>> 
>>>> Swift-evolution thread: Proposal Draft: parameter forwarding <https://lists.swift.org/pipermail/swift-evolution>
>>>> Motivation
>>>> 
>>>> There are many cases where a function declares parameters simply for the purpose of forwarding the provided arguments to another function. This results in reduntant parameter specifications that make code less clear and less readable by obscuring the simple forwarding that is actually happening.
>>>> 
>>>> This feature will be especially useful in initializers such as:
>>>> 
>>>> Convenience initializers that foward parameters directly to a designated initializer
>>>> Designated initializers that foward parameters directly to a super initializer
>>>> Designated initializers that foward parameters directly to a member initializer, perhaps in a composition-based design
>>>> If the partial initilaizer proposal is accepted, designated initializers that forward parameters to one or more partial initializers
>>>> NOTE: I haven’t had time to think too much aboue use cases beyond initialization. Please share examples and I will add them to this proposal.
>>>> 
>>>> Proposed solution
>>>> 
>>>> The proposed solution is to introduce an automatic parameter forwarding mechansim. It allows users to provide direct arguments for some parameters while forwarding others.
>>>> 
>>>> The basic mechanism looks like this:
>>>> 
>>>> func foo(i i: Int, s: String, f: Float = 42, d: Double = 43, b: Bool = false) { }
>>>> 
>>>> // user writes:
>>>> func bar(...fooParams) {
>>>>     foo(i: 32, ...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar(s: String, f: Float = 42, d: Double = 43, b: Bool = false) {
>>>>     foo(i: 32, s: s, f: f, d: d, b: b)
>>>> }
>>>> Some things to note about the syntax:
>>>> 
>>>> ...fooParams is a placeholder introduced with ... and followed by an identifier.
>>>> In the signature it can be placed anywhere in the parameter list.
>>>> At the call site, it must appear at the end of the argument list.
>>>> The placeholder matches the parameters not directly provided including their external label and default value if those exist.
>>>> Parameters corresponding to the matched parameters are synthesized by the compiler where the placeholder exists in the parameter list, including the default argument if one exists.
>>>> The identifier portion of the placeholder may be omitted if only one set of forwarded parameters exist within the function.
>>>> Additional details will be introduced with a corresponding example.
>>>> 
>>>> Omitting the placeholder identifier
>>>> 
>>>> The above example can be written more concisely by omitting the placeholder identifier.
>>>> 
>>>> func foo(i i: Int, s: String, f: Float = 42, d: Double = 43, b: Bool = false) { }
>>>> 
>>>> // user writes:
>>>> func bar(...) {
>>>>     foo(i: 32, ...)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar(s: String, f: Float = 42, d: Double = 43, b: Bool = false) {
>>>>     foo(i: 32, s: s, f: f, d: d, b: b)
>>>> }
>>>> NOTE: If the community feels strongly that the identifier should be required I am willing to do so.
>>>> 
>>>> Multiple forwarded parameter sets
>>>> 
>>>> It is possible for a single function to forward more than one set of parameters:
>>>> 
>>>> func foo(i i: Int, s: String, f: Float = 42) { }
>>>> func foo2(d: Double = 43, b: Bool = false) { }
>>>> 
>>>> // user writes:
>>>> func bar(...fooParams, ...foo2Params) {
>>>>     foo2(...foo2Params)
>>>>     foo(i: 32, ...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar(s: String, f: Float = 42, d: Double = 43, b: Bool = false) {
>>>>     foo(i: 32, s: s, f: f, d: d, b: b)
>>>> }
>>>> Direct arguments
>>>> 
>>>> Any direct arguments provided in the forwarding call must follow the usual argument ordering rules, with the only exception being that it is allowed to omit some arguments that would normally be required. When the compiler performs forwarding it will insert forwarded arguments in the correct location.
>>>> 
>>>> func foo(i i: Int, s: String, f: Float = 42, d: Double = 43, b: Bool = false) { }
>>>> 
>>>> func bar(...fooParams) {
>>>>     // error: `i` must precede `s` in the argument list
>>>>     foo(s: "hello", i: 32, ...fooParams)
>>>> }
>>>> 
>>>> // user writes:
>>>> func bar(...fooParams) {
>>>>     foo(i: 32, f: 0, ...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar(s s: String, d: Double = 43, b: Bool = false) {
>>>>     foo(i: 32, s: s, f: 0, d: d, b: b)
>>>> }
>>>> Multi-forwarding the same parameters
>>>> 
>>>> It is allowed to use the same identifier in multiple forwarding calls as long as the signature of the matched parameters matches exactly, including any default values. 
>>>> 
>>>> func foo(i i: Int, s: String, d: Double = 43) { }
>>>> func bar(i i: Int, s: String, d: Double = 43) { }
>>>> 
>>>> // user writes:
>>>> func baz(...fooBarParams) {
>>>>     foo(...fooBarParams)
>>>>     bar(...fooBarParams)
>>>> }
>>>> 
>>>> // compiler synthesizes: 
>>>> func baz(i i: Int, s: String, d: Double = 43) {
>>>>     foo(i: i, s: s, d: d)
>>>>     bar(i: i, s: s, d: d)
>>>> }
>>>> NOTE: This provision might be controversial. If the community doesn’t like it or the implementation is too complex I will remove it.
>>>> 
>>>> Unambiguous call
>>>> 
>>>> When forwarding parameters to a function that is overloaded the caller must provide enough direct arguments to make the call unambiguous.
>>>> 
>>>> func foo(i i: Int, s: String, d: Double = 43, b: Bool = false) { }
>>>> func foo(i i: Int, s: String, d: Double = 43, f: Float = 42) { }
>>>> 
>>>> // user writes:
>>>> func bar(...fooParams) {
>>>>     // error: ambiguous use of foo
>>>>     // foo(i: 32, ...fooParams)
>>>>     
>>>>     // ok: `b` makes the call to foo unambiguous
>>>>     foo(b: true, ...fooParams)
>>>>     // ok: `f` makes the call to foo unambiguous
>>>>     foo(f: 24, ...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes: 
>>>> func bar(i i: Int, s: String, d: Double = 43) {
>>>>     foo(i: i, s: s, d: d, b: true)
>>>>     foo(i: i, s: s, d: d, f: 24)
>>>> }
>>>> Default values
>>>> 
>>>> When forwarding to a function that accepts default values it is possible to explicitly request the default value. This allows for disambiguation and also allows the forwarding function to suppress a defaulted parameter from participating in forwarding without needing to supply a specific value. The default keyword is used to do this.
>>>> 
>>>> We can modify the previous example to use the defualt values:
>>>> 
>>>> func foo(i i: Int, s: String, d: Double = 43, b: Bool = false) { }
>>>> func foo(i i: Int, s: String, d: Double = 43, f: Float = 42) { }
>>>> 
>>>> // user writes:
>>>> func bar(...fooParams) {
>>>>     // ok: `b` makes the call to foo unambiguous, still uses default value
>>>>     foo(b: default, ...fooParams)
>>>>     // ok: `f` makes the call to foo unambiguous, still uses default value
>>>>     foo(f: default, ...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar(i i: Int, s: String, d: Double = 43) {
>>>>     foo(i: i, s: s, d: d, b: false)
>>>>     foo(i: i, s: s, d: d, f: 42)
>>>> }
>>>> It is also possible to explicitly request all defaults at once using default.... In this example, foois not overloaded:
>>>> 
>>>> func foo(i i: Int, s: String, d: Double = 43, b: Bool = false) { }
>>>> 
>>>> // user writes:
>>>> func bar(...fooParams) {
>>>>     foo(default..., ...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar(i i: Int, s: String) {
>>>>     foo(i: i, s: s, d: 43, b: false)
>>>> }
>>>> NOTE: The actual implementation of default arguments looks somewhat different. These examples are intended to communicate the behavior, not the exact details of implementation.
>>>> 
>>>> Generic parameters
>>>> 
>>>> If the types of any matched parameters reference any generic type parameters of the forwardee the generic type parameters must also be forwarded, along with any constraints on those generic parameters. 
>>>> 
>>>> func foo<T>(i i: Int, s: String, t: T, d: Double = 43, b: Bool = false) { }
>>>> 
>>>> // user writes:
>>>> func bar(...fooParams) {
>>>>     foo(...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar<T>(i i: Int, s: String, t: T, d: Double = 43, b: Bool = false) {
>>>>     foo(i: i, s: s, t: t, d: d, b: b)
>>>> }
>>>> If a generic parameter is referenced in a constraint that also references a generic parameter that will not be forwarded the constraint is resolved to a concrete type when possible. This may not be possible in all cases. When it is not possible a compiler error will be necessary.
>>>> 
>>>> func foo<S: SequenceType, T: SequenceType where S.Generator.Element == T.Generator.Element>
>>>>     (s: S, t: T) { }
>>>> 
>>>> // user writes:
>>>> func bar(...fooParams) {
>>>>     foo(t: [42], ...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar<S: SequenceType where S.Generator.Element == Int>(s: S) {
>>>>     foo(s: s, t: [42])
>>>> }
>>>> Syntheszied internal names
>>>> 
>>>> The compiler must ensure that all synthesized parameters have internal names that do not conflict with the internal names of any manually declared parameters. This applies to both generic type parameter names as well as value arguments in the parameter list of the function.
>>>> 
>>>> func foo<T>(i i: Int, s: String, t: T, d: Double = 43, b: Bool = false) { }
>>>> 
>>>> // user writes:
>>>> func bar<T>(t: T, ...fooParams) {
>>>>     // do something with t
>>>>     foo(...fooParams)
>>>> }
>>>> 
>>>> // compiler synthesizes:
>>>> func bar<T, InternalCompilerIdentifier>(t: T, i i: Int, s: String, t internalCompilerIdentifier: InternalCompilerIdentifier, d: Double = 43, b: Bool = false) {
>>>>     foo(t: t, i: i, s: s, t: internalCompilerIdentifier, d: d, b: b)
>>>> }
>>>> Detailed design
>>>> 
>>>> TODO but should fall out pretty clearly from the proposed solution
>>>> 
>>>> Impact on existing code
>>>> 
>>>> This is a strictly additive change. It has no impact on existing code.
>>>> 
>>>> Alternatives considered
>>>> 
>>>> I believe the forwarding mechanism itself is pretty straightforward and any alternatives would be lose functionality without good reason. 
>>>> 
>>>> The placeholder syntax is of course fair game for bikeshedding. I consider anything reasonably clear and concise to be acceptable.
>>>> 
>>>> 
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>> 
>> 
> 
> 
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