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0128-unicodescalar-failable-initializer.md

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Change failable UnicodeScalar initializers to failable

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:

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.

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:

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.