I want to decode JSON, but my type alias needs more information than the JSON provides. I use this type alias: type alias Location = { id : Int , name : String , checked : Bool } The JSON only contains the fields id and name. The field checked should be True by default. I have no idea wher…

D.field "locations" <| D.list <| D.map3 Location ( D.field "id" D.int ) ( D.field "name" D.string ) ( D.succeed True ) You use the .succeed decoder. It does not evaluate anything, but gives a sucessful decoding of whatever value you pass to it :slight_smile:

It’s also possible to stick with map2 by replacing Location with a lambda function: decodeLocations : Decoder (List Location) decodeLocations = Dec.field "locations" (Dec.list (Dec.map2 (\id name -> { id = id, name = name, checked = True }) (Dec.field "id" De…

Thank you both. Both approaches help me to discover possibilities I would never think of. Thanks Atlewee to use the <| operator as well, showing me how to use it in this situation. I think I will go with the Dec.succeed, because it keeps the name of the type in the function. Any suggestions on a …

I think I will go with the Dec.succeed, because it keeps the name of the type in the function Just for full understanding, you can keep “the name of the type” in the function in the map2 situation too: decodeLocations : Decoder (List Location) decodeLocations = Dec.field "locations" …

Thanks a lot, lydell, for elaborating on the subject. It is helping me a lot!

Set default values while decoding JSON

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Set default values while decoding JSON

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