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Adjusting inout Declarations for Type Decoration


The inout keyword indicates copy-in/copy-out argument behavior. In its current implementation the keyword prepends argument names. We propose to move the inout keyword to the right side of the colon to decorate the type instead of the parameter label.

The initial Swift-Evolution discussion of this topic took place in the "Replace 'inout' with &" thread.

Thread to Proposal, Review


In Swift 2, the inout parameter lives on the label side rather than the type side of the colon although the keyword isn't modifying the label but its type. Decorating types instead of labels offers identifiable advantages:

  • It enables the inout keyword to properly integrate into full type syntax, for example:

    (x: inout T) -> U // => (inout T) -> U
  • It avoids notational similarity with arguments labeled inout, for example:

    func foo(inOut x: T) // foo(inOut:), type (T) -> Void
    func foo(inout x: T) // foo(_:), type (inout T) -> Void
  • Moving it would allow inout to be used as a parameter label. While this isn't a particularly strong motivation by itself, currently inout is the only keyword not allowed as a parameter label in Swift 3. Removing this restriction would simplify the language.

  • It better matches similar patterns in other languages such as borrowing in Rust, that may be later introduced back to Swift

Detailed design

parameter → external-parameter-name optlocal-parameter-name : type-annotation
type-annotation → inout type-annotation

Alternatives Considered

Decorations using @inout (either @inout(T) or @inout T) were considered and discarded

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