brent.md

Adding Conditional Binding with Tuples

• Proposal: TBD
• Author: Brent Royal-Gordon
• Status: TBD
• Review manager: TBD

Introduction

This proposal introduces tuple-based if-let assignments to the Swift programming language.

Swift Evolution Discussion: [Accepted with Revision] SE-0099 Restructuring Condition Clauses

Motivation

SE-0099 removed cascaded if-let statements that allowed groups of conditional binding to omit the let keyword. This proposal re-introduces the convenience of multiple simultaneous conditional assignments.

Instead of using cascades like:

guard let a = opt1, b = opt2, c = opt3 else { ... leave scope ... }

You must now use the let keyword for each binding:

guard let a = opt1, let b = opt2, let c = opt3 else { ... leave scope ... }

Given that you can still use tuples with case, e.g.

guard case (.none, .none, .none) = (foo(), bar(), blort()) else { ... }
guard case (.Some(let a), .Some(let b), .Some(let c)) = (foo(), bar(), blort()) else { ... }

It seems only fair that if-let, guard-let, and while let should permit an analogous construct without using the extra verbiage required for case:

guard let (x, y, z) = (foo(), bar(), blort()) else { ... } // illegal

Unfortunately, they do not:

// error: initializer for conditional binding must have 
// Optional type, not '(Int?, Int?, Int?)' (aka 
// '(Optional<Int>, Optional<Int>, Optional<Int>)')

Detailed Design

This proposal extends Swift's public facing API to support conditional tuple bindings with simple let patterns, just as it does for case:

optional_binding_condition : 'let' pattern initializer | 'var' pattern initializer

Impact on Existing Code

This proposal is additive

Alternatives Considered

Not accepting this proposal