0096-dynamictype.md

Converting dynamicType from a property to an operator

• Proposal: SE-0096
• Author: Erica Sadun
• Review Manager: Chris Lattner
• Status: Implemented (Swift 3.0)
• Decision Notes: Rationale
• Bug: SR-2218

Introduction

This proposal establishes dynamicType as a named operator rather than a property.

Swift-evolution thread: RFC: didset and willset

Motivation

In Swift, dynamicType is a property. Because of that, it shows up as an "appropriate" code completion for all values regardless of whether it makes sense to do so or not. For example, Swift offers 4.dynamicType and myFunction().dynamicType, etc.

Unlike most properties, it does not express a logical attribute of a specific type. Instead, it can be applied to any expression. Since dynamicType behaves more like a operator (like sizeof), its user-facing calling syntax should follow suit.

Detailed Design

Upon adoption of this proposal, Swift resyntaxes dynamicType as an operator instead of a member. This proposal puts forth dynamicType:

dynamicType(value) // returns the dynamicType of value

Once the Swift language has sufficient capabilities, the goal is to migrate this operation to the standard library. At this time, this operation cannot be written as a stdlib feature and it will be implemented as a compiler feature.

Impact on Existing Code

Adopting this proposal will break code and require migration support. The postfix property syntax must change to an operator call.

Alternatives Considered

The core team may also consider typeof(x) instead of dynamicType(x) as the call is syntactically closer to sizeof(x). Note that this may introduce confusion. Unlike Swift, identically-named C++ and C# terms return static types. Javascript also includes typeof(x) but Javascript does not support static types.

Acknowledgements

Thank you, Nate Cook