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[SR-10237] Codable: Unexpected behavior supplying default value for nullable field #52637

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swift-ci opened this issue Mar 30, 2019 · 2 comments
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@swift-ci
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@swift-ci swift-ci commented Mar 30, 2019

Previous ID SR-10237
Radar rdar://problem/33758682
Original Reporter joey (JIRA User)
Type Bug
Environment

Swift 5

Additional Detail from JIRA
Votes 0
Component/s Compiler
Labels Bug, Codable
Assignee None
Priority Medium

md5: e4633977df6db435fe2813d574985f12

Issue Description:

Given a JSON object with a nullable field, such as

{
  "count": null //may be string or null
}

If you try to decode it into this struct

struct Sample: Decodable {
  var count: String = "0" //let's default to 0 if null
}

You'll get the error

valueNotFound(Swift.String, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "count", intValue: nil)], debugDescription: "Expected String value but found null instead.", underlyingError: nil))

I expected no error, and count would be the value if non-null, otherwise "0".

I actually first tried defining count as a let, which worked without error, but this introduced a bug because it will always be that "default" value even when count is non-null.

@belkadan
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@belkadan belkadan commented Apr 1, 2019

I'm afraid that's just not the behavior of the default init(coder:), and it would be a breaking change to make it so. cc bendjones (JIRA User), @itaiferber

@itaiferber
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@itaiferber itaiferber commented Apr 1, 2019

Indeed, this was discussed waaaaaayyy back when in https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170710/037977.html, and although the behavior is surprising to some, the opposite behavior would be equally surprising to others.

We can't read into the intent of why default values are supplied. Sometimes, you'd really want the default value to be used in case decoding fails, but sometimes, you just define a default value to give your type a synthesized init without even considering what that might do for Codable synthesis. There's no way for us to guess correctly, so we do the conservative thing of always trying to decode and not squashing the error silently.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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