carbon-lang/proposals/p6008.md

Replace impl fn with override fn

Pull request

Table of contents

• Abstract
• Problem
• Background
• Proposal
• Rationale

Abstract

This proposal renames the syntax used to mark an overriding definition of a virtual method from impl fn to override fn to avoid ambiguity.

Problem

The phrase impl fn was introduced to mark overriding virtual functions, however, it is now ambiguous: besides indicating an overriding virtual function, it can be parsed as an "impl" declaration when the construct following "impl" begins with a lambda introduced by "fn".

Background

The original syntax was adopted in proposal #777 to emphasize that implementing interfaces and overriding virtual functions are similar operations.

Proposal

This proposal is to replace the impl fn syntax with override fn for method overriding in class inheritance.

Note that impl is still a modifier keyword for library and package declarations (impl library ... and impl package ... respectively). The former is unambiguous and the latter is easy to disambiguate, so no change is needed for these two cases.

Rationale

This proposal is focused on the That code is easy to read, understand, and write Carbon goal. Changing the keyword from impl to override makes the intent clearer, resolves syntax ambiguity, and adheres to existing C++ conventions.