partial JSON decoding with defaults

[Peter Damoc]

Hello kind people, 

Is there a way to decode a piece of JSON to a value with some defaults provided?

e.g. let's say we have the following type and JSON decoder: 

type alias UserData = 

  {name: String

  , avatar_url: String

  , repos_url: String

  , languages: List String

  }

decodeUser : Json.Decoder (UserData)

decodeUser = Json.object3 UserData

    ("name" := Json.string) 

    ("avatar_url" := Json.string)

    ("repos_url" := Json.string)

I want to decode the JSON data into the type BUT the languages will be extracted at a later stage and I want to be able to just provide an empty list to the decoder in order to be able to provide a valid value. 

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[Joseph Hager]

You should be able to use http://package.elm-lang.org/packages/elm-lang/core/2.1.0/Json-Decode#succeed

I haven't tested this, but perhaps:

decodeUser = Json.object3 UserData

    ("name" := Json.string) 

    ("avatar_url" := Json.string)

    ("repos_url" := Json.string)

    (succeed [])

[Joseph Hager]

Sorry, needs to be object4...

decodeUser = Json.object4 UserData

    ("name" := Json.string) 

    ("avatar_url" := Json.string)

    ("repos_url" := Json.string)

    (succeed [])

[Peter Damoc]

Hmm... it does not work for me. 

The code I'm using is here:
https://github.com/pdamoc/elmChallenges/blob/master/challenge4.elm
I've tried replacing UserData with User and add succeed but it just gives a type error related to succeed.

Maybe I'm missing something.  

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[Max Goldstein]

Try this:

decodeUser : Json.Decoder User

decodeUser = Json.object4 User

    ("name" := Json.string)

    ("avatar_url" := Json.string)

    ("repos_url" := Json.string)

    (Json.oneOf

        [ "languages" := Json.list Json.string

        , Json.succeed []

        ])

[Peter Damoc]

Hi Max, 

I tried it at first as Joseph suggested and it failed because I was using just succeed and it was the Task.succeed :) 

Now it works. 

Thank you both, 

Peter

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