[swift-corelibs-dev] Adding type conversion capabilities to JSON encode/decode

Youming Lin ylin at us.ibm.com
Wed Aug 30 16:35:33 CDT 2017


Brandon

I cooked up a simple example and it works as expected.

ylin at youming-mbpr:~/Swift/Configuration$ swift
Welcome to Apple Swift version 4.0-dev (LLVM 2dedb62a0b, Clang b9d76a314c,
Swift 0899bd328a). Type :help for assistance.
  1> import Foundation
  2> struct A: Codable {
  3.     var integer: Int
  4.
  5.     public init(integer: Int) {
  6.         self.integer = integer
  7.     }
  8.
  9.     public init(from decoder: Decoder) throws {
 10.         print("Custom decoder")
 11.         let container = try decoder.container(keyedBy:
CodingKeys.self)
 12.         integer = try container.decode(Int.self, forKey: .integer)
 13.     }
 14. }
 15.
 16. try! JSONDecoder().decode(A.self, from: JSONEncoder().encode(A
(integer: 42)))
Custom decoder
$R0: A = {
  integer = 42
}

You should be able to implement your custom init to convert the number into
a string and JSONDecoder should use that automatically.

Thanks,

Youming Lin
IBM Cloud, Swift at IBM, Kitura developer
Austin, TX
GitHub: @youming-lin



From:	"Sneed, Brandon" <brsneed at ebay.com>
To:	Youming Lin <ylin at us.ibm.com>
Cc:	Tony Parker <anthony.parker at apple.com>,
            "swift-corelibs-dev at swift.org" <swift-corelibs-dev at swift.org>,
            "swift-corelibs-dev-bounces at swift.org"
            <swift-corelibs-dev-bounces at swift.org>
Date:	08/30/2017 04:14 PM
Subject:	Re: [swift-corelibs-dev] Adding type conversion capabilities to
            JSON encode/decode



Thanks Youming,

Ok, thanks!  I did try that, but I can’t seem to figure out how to make it
actually get used in the decoding process.  That being my preferred way, I
tried it first.  I chalked it not working up to Swift not knowing which of
the 2 init(from decoder:) functions to call, mine or theirs.  But, maybe
there’s something I’m missing here.

Any insight is appreciated.

Thanks!



Brandon Sneed

From: Youming Lin <ylin at us.ibm.com>
Date: Wednesday, August 30, 2017 at 1:18 PM
To: "Sneed, Brandon" <brsneed at ebay.com>
Cc: Tony Parker <anthony.parker at apple.com>, "swift-corelibs-dev at swift.org"
<swift-corelibs-dev at swift.org>, "swift-corelibs-dev-bounces at swift.org"
<swift-corelibs-dev-bounces at swift.org>
Subject: Re: [swift-corelibs-dev] Adding type conversion capabilities to
JSON encode/decode



Brandon

>JSON’s types effectively end up matching specifically to primitives, of
which there is no mechanism to override the behavior of how a String gets
decoded for instance.

You can override the default behavior with your own custom init(from:)
implementation for your Codable struct:
https://developer.apple.com/documentation/swift/decodable/2894081-init

You can check Foundation source code (i.e., the URL struct) on how this can
be implemented.

Thanks,

Youming Lin
IBM Cloud, Swift at IBM, Kitura developer
Austin, TX
GitHub: @youming-lin

nactive hide details for "Sneed, Brandon via swift-corelibs-dev" ---08/30
"Sneed, Brandon via swift-corelibs-dev" ---08/30/2017 03:07:05 PM---Hi
Tony, I like the idea that the type itself is responsible for the
conversion. My own json encode

From: "Sneed, Brandon via swift-corelibs-dev"
<swift-corelibs-dev at swift.org>
To: Tony Parker <anthony.parker at apple.com>
Cc: "swift-corelibs-dev at swift.org" <swift-corelibs-dev at swift.org>
Date: 08/30/2017 03:07 PM
Subject: Re: [swift-corelibs-dev] Adding type conversion capabilities to
JSON encode/decode
Sent by: swift-corelibs-dev-bounces at swift.org




Hi Tony,

I like the idea that the type itself is responsible for the conversion. My
own json encode/decode library worked this way and it was really great,
however in trying to leverage Swift4 into it, or to replace it, I just
don’t see how that’s possible given how it’s currently structured.

JSON’s types effectively end up matching specifically to primitives, of
which there is no mechanism to override the behavior of how a String gets
decoded for instance. The only way I can think of to accomplish that is to
create *another* type, JSONString for example, but since String is a
struct, I can’t subclass it, and instead need to have the real value buried
inside of it … it seems to start getting messy very quickly. It also adds
the obfuscation of dealing with yet another type, which I’m not against,
but just feels less than ideal.


Brandon Sneed

From: <anthony.parker at apple.com> on behalf of Tony Parker
<anthony.parker at apple.com>
Date: Wednesday, August 30, 2017 at 11:30 AM
To: "Sneed, Brandon" <brsneed at ebay.com>
Cc: Itai Ferber <iferber at apple.com>, "swift-corelibs-dev at swift.org"
<swift-corelibs-dev at swift.org>
Subject: Re: [swift-corelibs-dev] Adding type conversion capabilities to
JSON encode/decode

I’m still not convinced that we should actually provide such a strategy.

Conversions like those below seem like the domain of each type that is
being decoded. If, in a particular type, the “number” can be either a true
number or a string, then that type can try decoding it as one or the other
and fall back as required. That puts the responsibility of doing that kind
of conversion in the type itself.

JSON has very few types already. I’m not sure we want to blur the line
between numbers and strings automatically…

- Tony
            On Aug 30, 2017, at 11:24 AM, Sneed, Brandon via
            swift-corelibs-dev <swift-corelibs-dev at swift.org> wrote:

            Hi Itai,

            No problem! Thanks for the heads up. Is there any way I could
            be involved? Happy to do the work to whatever guidance your
            team might have. I’m mostly just interested in it being there
            soon, hence volunteering.

            Thanks!


            Brandon Sneed

            From: <iferber at apple.com> on behalf of Itai Ferber <
            iferber at apple.com>
            Date: Wednesday, August 30, 2017 at 11:22 AM
            To: "Sneed, Brandon" <brsneed at ebay.com>
            Cc: "swift-corelibs-dev at swift.org" <
            swift-corelibs-dev at swift.org>
            Subject: Re: [swift-corelibs-dev] Adding type conversion
            capabilities to JSON encode/decode

            Hi Brandon,
            Thanks for looking at this! We’ve got plans internally to
            potentially add a strategy to JSONEncoder/JSONDecoder to allow
            lenient conversions like this — i.e. implicitly stringify
            numbers (or parse them from string input), among some others.
            This would be opt-in for consumers of JSONDecoder while not
            requiring any special annotations on Codable types.
            — Itai
            On 30 Aug 2017, at 10:59, Sneed, Brandon via swift-corelibs-dev
            wrote:
            Hi everyone,

            Just throwing this out to see if anyone else is working on
            this, or has opinions/suggestions on how it’s implemented. I’d
            like to add this to the Codable/JSONDecoder/JSONEncoder system
            if no one else is working on it.

            Type type conversion, I mean given this JSON payload:

            {
            "name": "Endeavor”,
            "abv": 8.9,
            "brewery": "Saint Arnold”,
            "style": "ipa"
            }

            and a struct defined as:

            struct Beer: Codable {
            let name: String
            let abv: String
            let brewery: String
            let style: BeerStyle
            }

            Notice that “abv” is a number in the JSON, but a String in the
            struct. I’d like to make it such that I can let the system know
            it’s ok to convert it from a number to a string as opposed to
            throwing an exception. The benefits are:

            1. It’s defensive; service types can change without causing my
            application to crash.
            2. It allows a developer to work with the types they want to
            work with as opposed to what the server provides, thus saving
            them time of writing a custom encode/decode code for all
            members.

            The argument against it that I’ve heard is generally “it’s a
            service bug, make them fix it”, which is valid but the reality
            is we’re not all in control of the services we injest. The same
            type of logic could be applied to a member name changing,
            though I haven’t seen this happen often in practice. I do see
            types in a json payload change with some frequency though. I
            think much of the reason stems from the fact that type
            conversion in javascript is effectively free, ie: you ask for a
            String, you get a String if possible.

            To implement this type conversion in practice, looking at it
            from the point of view using Codable/JSON(en/de)coder, one way
            would be to make it opt-in:

            struct Beer: Codable, CodingConvertible {
            let name: String
            let abv: String
            let brewery: String
            let style: BeerStyle
            }

            I like this because looking at the struct, the members still
            remain clear and relatively unambiguous. The downside is it’s
            unknown which member is likely to get converted. And since it’s
            opt-in, conversion doesn’t happen if the CodingConvertible
            conformance isn’t adhered to.

            Another option would be to box each type, like so:

            struct Beer: Codable {
            let name: String
            let abv: Convertible<String>
            let brewery: String
            let style: BeerStyle
            }

            This seems tedious for developers, but would show which types
            are being converted. It does however seriously weaken benefit
            #1 above.

            Those example usages above aside, I do think it’d be best if
            this conversion behavior was the default and no end-developer
            changes required. I think that could be done without impact to
            code that’s been already been written against the JSON
            en/decode bits.

            I’m very open to alternatives, other ideas, or anything else
            you might have to say on the subject. Thanks for reading!



            Brandon Sneed




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