Some data formats are extensible, in addition to a fixed set of fields they may also allow additional, not known in advance fields to be included in the JSON. The case I am looking at just now is the Open API spec, which allows extensions in the form of fields starting with x-. So if you had some r…

With decoders, I find that it’s impossible to give good advice without knowing both the format coming in and what you want to do with the result in Elm. Ultimately a decoder is about mapping a concept in JSON to a concept in your Elm program’s data structures, and it depends on both of those two end…

Thanks - I need to specify the problem a bit more clearly. Suppose this is the JSON I was trying to decode: { name: "rupert", age: 42, x-superpower: "teleportation" } I want to decode the { name : String, age : Int } part which will always be the same and known in advance. I also want to…

In that case I think I’d try using the decodeDict decoder I wrote above and filtering the result so that the only keys are the x-* fields. Out of the box Elm doesn’t have good support for formatting JSON values as strings, but it looks like you can use https://package.elm-lang.org/packages/ThinkAlex…

Thanks, that gives me an idea. Use a generic Dict decoder that lifts out the x- fields, and also a specific Decoder for the fixed format fields that are expected and ignores any x- fields. I can then combine those together to get what I am after using Decode.map2 - gives me a direction to get start…

What you can do is lift all the fields to strings. module Main exposing (main) import Html exposing (text) import Json.Decode as Json import Dict fromBool val = if val then "True" else "False" asString = Json.oneOf [ Json.string , Json.map String.fromInt Json.int …

This looks good to me! I’d maybe use this filter function instead: (\key _ -> String.startsWith "x-" key)

[image] thought2: I’d maybe use this filter function instead: (\key _ -> String.startsWith "x-" key) This would work too if all the other fields have a predictable prefix. The solution I proposed has the advantage that it would capture all fields other that a known set (name, age).

[image] jacob: Out of the box Elm doesn’t have good support for formatting JSON values as strings, but it looks like you can use https://package.elm-lang.org/packages/ThinkAlexandria/elm-pretty-print-json/latest/ for that. I was intrigued to take a look at the pretty printer, as it also must…

Interestingly, that article was a result of me doing this same exercise, and writing just about the same code as you did, while trying to figure out if I could make a better decoder API. Doing that work and then trying to use the resulting Json values really drove home for me the idea that a decoder…

How to write a generic JSON decoder?

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How to write a generic JSON decoder?

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