[swift-evolution] Infer types of default function parameters

[Haravikk]

> On 10 Mar 2017, at 21:40, Kilian Koeltzsch via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hi all,
> 
> I sent the message below to swift-users@ ~a day ago, but this might be a better place to ask and gather some discussion. It is a rather minor suggestion and I'm just looking for some opinions.
> 
> Declaring a function that has default parameters currently looks like this:
> 
> func foo(bar: String = "baz") {
>     print(bar)
> }
> 
> Now I'm wondering if there would be any problems if it were possible to omit the type annotation for default params and let Swift's type inference handle that. 
> 
> func foo(bar = "baz") {
>     print(bar)
> }
> 
> It feels to be equivalent to omitting type annotations with variable declarations. Obviously more complex types would still require annotations being specified. Off the top of my head I can't think of any negative ramifications this might bring, be it in simple function/method declarations or protocol extensions and elsewhere. 
> Any further input or examples for situations where this might cause issues would be much appreciated :)

I like the idea but I'm afraid I don't think I can support it.

I think it is more important for function/method declarations to have as explicit a signature as possible; I mean, I'm not even that comfortable with the ability to omit -> Void on non-returning functions (I always include it just to be consistent).

As others point out, while this makes sense for types where there's only one obvious choice to infer, it's not quite so clear on things like ints where a function really needs to be absolutely clear on what type/width of int it expects, since it's not something you want to have to change in future.

One alternative I thought of was an operator for this purpose, e.g- := (chosen since the colon kind of suits the omitted type declaration); this would allow a developer to be explicit about wanting Swift to infer the type, but it would be inconsistent with regular variables where it's always inferred, so I'm not sure if it'd be a good option anyway.

Sorry, I do agree that it feels inconsistent that a function default doesn't behave more like a variable's initialisation, but at the same time they are two slightly different concepts so that's not necessarily a bad thing.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20170311/fbb16d2f/attachment.html>