soroush.md

Introducing Guard-Catch

• Proposal: SE-TBD
• Author(s): Soroush Khanlou
• Status: tbd
• Review manager: tbd

Introduction

This proposal introduces a guard-catch statement to Swift, that is congruent to the existing guard-else statement with error catching support.

This proposal was first discussed on the Swift Evolution list in the TBD thread.

Motivation

There is no natural way to incorporate a throwing request into a guard without applying try? and discarding the error. Introducing guard-catch allows you to retrieve a successful value and handle a failure without losing error information.

In the current Swift language design, you can only achieve this approach by:

• discarding the thrown error; or
• embedding the guard statement into a do-catch scope.

Neither produces a satisfactory or elegant solution for using throwing calls within a guard statement.

Design

The statement derives from the guard-else grammar, substituting a catch clause for else, where the error is pre-defined and ready to use.

guard let value = try throwingFunction() 
    catch { 
        // handle error here, then leave scope
        print (error); return 
    }

As with guard-else, the catch clause must leave scope after handling the error.

Impact on Existing Code

This change is purely additive.

Alternatives and Future Directions

Not incorporating this statement.