[SR-491] Double#description is not roundtrip-safe

[swift-ci]

Previous ID	SR-491
Radar	None
Original Reporter	dankogai (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	Bug
Assignee	@tbkka
Priority	Medium

md5: 1c1d9ebdc5e0745501c97e00c87b6dc7

duplicates:

• [SR-491] Double#description is not roundtrip-safe #43108

Issue Description:

In order to stringily a double roundtrip-safe, we need up to 17 decimal digits. But Double#description
of current Swift (on all platforms) gives us one digit short.

import Foundation

let log2   =    log(2.0)
let log2Sd =    log2.description
let log2Sp17g = String(format:"%.17g",log2)
let log2Spa =   String(format:"%a",log2)

Double(log2Sd)    == log2   // false
Double(log2Sp17g) == log2   // true
Double(log2Spa)   == log2   // true

http://swiftlang.ng.bluemix.net/#/repl/937bf49dd2f110c6f4c0fd942de051d734b3e051887b1c6b762c39e2c7956146

As you see in the attached screenshot, Playground is okay. So is REPL.

Perl 5 and Python 2 have the same symptom:

• Perl 5 http://ideone.com/J7kVwv

• Python 2 https://repl.it/BcF7/0

While most other modern languages do not:

• Java http://ideone.com/OSeb5S

• JavaScript http://ideone.com/qonDEO

• Python 3 https://repl.it/BcFb/0

• Ruby http://ideone.com/BrfiBO

IMHO this is rather an undocumented feature than a bug but with JSON so ubiquitous and all of its numbers are doubles, we should make Double#description roundtrip-safe.

[tbkka]

This is the same issue reported in #43108. I'm working on a fix for that issue and will discuss it further there.

[tbkka]

Done with commit 97a934c