: [Proposal] Change UnicodeScalar initializer to failable

Evolution Discussion
1

Hi,
I would like to propose changing unicodescalar initializer to failable.
Currently, when you pass an invalid value to the UnicodeScalar initializer the swift stdlib crashes the program by calling _precondition. This is bad if you construct a unicode scalar from unknown input.
As a result. I would like to propose to mark the initializer as failable and return nil in case of a failure.

Currently, in the code below, the stdlib crashes the program by calling _precondition if codepoint is not a valid unicode.
var string = “"
let codepoint: UInt32 = 55357 // this is invalid
let ucode = UnicodeScalar(codepoint) // Program crashes at this point.
string.append(code)

After marking the initializer as failable, users can write code like this. And the program will execute fine even codepoint is invalid.
var string = "" let codepoint: UInt32 = 55357 // this is invalid
let ucode = UnicodeScalar(codepoint)
if ucode != nil {
  string.append(code!)
} else {
  // do something else
}

As the initializer is now failable, it returns an optional, so optional unchaining or forced unwrapping needs to be used. Alternatively, its also possible to leave status quo and force the users to do input checks
before trying to construct a UnicodeScalar. But i feel having failable initializer helps user to write more robust code.
-Xin

2

+1, thank you for working on this improvement!

Dmitri

···

On Tue, Jul 19, 2016 at 10:14 AM, Xin Tong via swift-evolution <swift-evolution@swift.org> wrote:

Hi,

I would like to propose changing unicodescalar initializer to failable.

Currently, when you pass an invalid value to the UnicodeScalar initializer
the swift stdlib crashes the program by calling _precondition. This is bad
if you construct a unicode scalar from unknown input.

As a result. I would like to propose to mark the initializer as failable and
return nil in case of a failure.

--
main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr@gmail.com>*/

3

+1

I think this helps making Swift code more robust and should be included in
Swift 3.

The scenario described by Xin is a real world and not an academic one.

Also, the change he proposes is a very small one, so you get "much bang for
the bucks" in my point of view.

Björn

···

Am Dienstag, 19. Juli 2016 schrieb Xin Tong via swift-evolution :

Hi,

I would like to propose changing unicodescalar initializer to failable.

Currently, when you pass an invalid value to the UnicodeScalar initializer the swift stdlib crashes the program by calling _precondition. This is bad if you construct a unicode scalar from unknown input.

As a result. I would like to propose to mark the initializer as failable and return nil in case of a failure.

Currently, in the code below, the stdlib crashes the program by calling _precondition if codepoint is not a valid unicode.

var string = “"

let codepoint: UInt32 = 55357 // this is invalid

let ucode = UnicodeScalar(codepoint) // Program crashes at this point.

string.append(code)

After marking the initializer as failable, users can write code like this. And the program will execute fine even codepoint is invalid.

var string = "" let codepoint: UInt32 = 55357 // this is invalid

let ucode = UnicodeScalar(codepoint)

if ucode != nil {

  string.append(code!)

} else {

  // do something else

}

As the initializer is now failable, it returns an optional, so optional unchaining or forced unwrapping needs to be used. Alternatively, its also possible to leave status quo and force the users to do input checks

before trying to construct a UnicodeScalar. But i feel having failable initializer helps user to write more robust code.

-Xin

4

+1

"Explicit is better than implicit"

Daniel Duan

···

Sent from my iPhone

On Jul 21, 2016, at 9:34 AM, Björn Forster via swift-evolution <swift-evolution@swift.org> wrote:

+1

I think this helps making Swift code more robust and should be included in Swift 3.

The scenario described by Xin is a real world and not an academic one.

Also, the change he proposes is a very small one, so you get "much bang for the bucks" in my point of view.

Björn

Am Dienstag, 19. Juli 2016 schrieb Xin Tong via swift-evolution :

Hi,
I would like to propose changing unicodescalar initializer to failable.
Currently, when you pass an invalid value to the UnicodeScalar initializer the swift stdlib crashes the program by calling _precondition. This is bad if you construct a unicode scalar from unknown input.
As a result. I would like to propose to mark the initializer as failable and return nil in case of a failure.

Currently, in the code below, the stdlib crashes the program by calling _precondition if codepoint is not a valid unicode.
var string = “"
let codepoint: UInt32 = 55357 // this is invalid
let ucode = UnicodeScalar(codepoint) // Program crashes at this point.
string.append(code)

After marking the initializer as failable, users can write code like this. And the program will execute fine even codepoint is invalid.
var string = "" let codepoint: UInt32 = 55357 // this is invalid
let ucode = UnicodeScalar(codepoint)
if ucode != nil {
  string.append(code!)
} else {
  // do something else
}

As the initializer is now failable, it returns an optional, so optional unchaining or forced unwrapping needs to be used. Alternatively, its also possible to leave status quo and force the users to do input checks
before trying to construct a UnicodeScalar. But i feel having failable initializer helps user to write more robust code.
-Xin

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