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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