[swift-evolution] [proposal] Allow function argument type to be omitted when passing a default value from which it can be inferred

[Chris Lattner]

On May 10, 2016, at 3:02 AM, Sam Dods via swift-evolution <swift-evolution at swift.org> wrote:
> I propose that function argument types could be omitted in the same way as variable and property argument types are omitted when they are set at point of definition.
> 
> At present the type of properties and variables can be inferred as below:
> 
> class EvolutionManager {
>  let proposalManager = ProposalManager()           // type is inferred
> 
>  func proposeTopic(topic: String) {
>    let evolution = Evolution(topic: topic)         // type is inferred
>    proposalManager.proposeEvolution(evolution)
>  }
> }
> 
> However, function arguments cannot be inferred, as below:
> 
> class EvolutionManager {
>  func proposeEvolution(evolution: Evolution, proposalManager = ProposalManager()) {       // compiler error, cannot infer type

We have a pretty strict rule here: types are allowed to be inferred in implementations, but not interfaces.  This is important for efficient compilation, encourages people to think about their API interfaces, and somewhat reduces the damage when they don’t.

-Chris