---
title: "SE-0128: Change failable UnicodeScalar initializers to failable"
framework: swift-evolution
role: article
path: swift-evolution/0128-unicodescalar-failable-initializer
---

# SE-0128: Change failable UnicodeScalar initializers to failable

* Proposal: [SE-0128](0128-unicodescalar-failable-initializer.md)
* Author: [Xin Tong](https://github.com/trentxintong)
* Review Manager: [Chris Lattner](https://github.com/lattner)
* Status: **Implemented (Swift 3.0)**
* Decision Notes: [Rationale](https://forums.swift.org/t/accepted-se-0128-change-failable-unicodescalar-initializers-to-failable/3546)
* Implementation: [apple/swift#3662](https://github.com/apple/swift/pull/3662)

## Introduction

This proposal aims to change some `UnicodeScalar` initializers (ones that are non-failable) from non-failable to failable. i.e., in case a `UnicodeScalar` can not be constructed, nil is returned.

## Motivation

Currently, when one passes an invalid value to the non-failable `UnicodeScalar` `UInt32` initializer, it crashes the program by calling `_precondition`. This is undesirable if one wishes to initialize a Unicode scalar from unknown input.

## Proposed solution

Mark the non-failable `UnicodeScalar` initializers as failable and return nil when the integer is not a valid Unicode codepoint.

Currently, the stdlib crashes the program by calling `_precondition` if the integer is not a valid Unicode codepoint. For example:

```swift var string = "" let codepoint: UInt32 = 55357 // this is invalid let ucode = UnicodeScalar(codepoint) // Program crashes at this point. string.append(ucode) ```

After marking the initializer as failable, users can write code like this and the program will execute fine even if the codepoint isn't valid.

```swift var string = "" let codepoint: UInt32 = 55357 // this is invalid if let ucode = UnicodeScalar(codepoint) {    string.append(ucode) } else {    // do something else } ```

## Impact on existing code

The initializers are now failable, returning an optional, so optional unwrapping is necessary.

The API changes include:

```swift public struct UnicodeScalar { -  public init(_ value: UInt32) +  public init?(_ value: UInt32) -  public init(_ value: UInt16) +  public init?(_ value: UInt16) -  public init(_ value: Int) +  public init?(_ value: Int) } ```

## Alternatives considered

Leave status quo and force the users to do input checks before trying to initialize a `UnicodeScalar`.