---
title: "A Kotlin Style .copy Function for Swift Structs"
description: "Immutable state objects are used to keep rendering code from modifying shared values, supporting clearer value origins, fewer bugs, and more focused testing. Kotlin data classes provide a copy function that replaces selected fields while preserving the rest, allowing the new object to become the source of truth. Swift structs lack an equivalent convenience, and defining a complete initializer plus a parameter-based copy method becomes cumbersome, particularly with many or nested properties. Optional properties expose a further issue: nil must sometimes mean “set this field to nil,” not “use the existing value.” A functional builder closure solves that distinction by collecting overrides while retaining the original struct’s values for other fields, and a Sourcery stencil can generate the initializer and builder boilerplate."
---

# A Kotlin Style .copy Function for Swift Structs

[Shopify](https://yomu.fyi/company/shopify) · 2023-10-18 · Sep 2, 2021

**Type:** Tutorial

## Summary

Immutable state objects are used to keep rendering code from modifying shared values, supporting clearer value origins, fewer bugs, and more focused testing. Kotlin data classes provide a copy function that replaces selected fields while preserving the rest, allowing the new object to become the source of truth. Swift structs lack an equivalent convenience, and defining a complete initializer plus a parameter-based copy method becomes cumbersome, particularly with many or nested properties. Optional properties expose a further issue: nil must sometimes mean “set this field to nil,” not “use the existing value.” A functional builder closure solves that distinction by collecting overrides while retaining the original struct’s values for other fields, and a Sourcery stencil can generate the initializer and builder boilerplate.

## Context

The work addresses the difficulty of applying Kotlin’s immutable-object and copy-function pattern to Swift structs. Swift lacks a similar copy function, and optional properties make a copy method based on nil defaults unable to distinguish clearing a value from leaving the current value unchanged.

## Approach / What changed

The proposed implementation defines an initializer covering every struct field, then uses a functional builder closure to capture overrides for selected properties while taking the original struct’s values for the remaining fields. A Sourcery stencil template generates the initializer and builder boilerplate.

## Takeaways

- A copy method whose override parameters default to nil cannot set an optional property to nil when nil also means “retain the current value.”
- The functional builder pattern lets callers specify only selected property overrides while preserving all other values from the original struct.
- Sourcery is used with a stencil template to generate the copy initializer and builder code for Swift structs.

**Tags:** [Architecture](https://yomu.fyi/topic/architecture), [Developer Experience](https://yomu.fyi/topic/developer-experience), [Refactoring](https://yomu.fyi/topic/refactoring)

- Source: [Shopify](https://shopify.engineering/kotlin-style-copy-function-swift-structs)
- Source URL: https://shopify.engineering/kotlin-style-copy-function-swift-structs
- Ingested by Yomu: 2026-08-30T15:29:20.478Z

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