JSON Decode question

[Eelco Hoekema]

Hi,

I'm a bit stuck in decoding some json. In http://package.elm-lang.org/packages/elm-lang/core/4.0.0/Json-Decode#andThen there is an example of decoding a tagged union for a shape. 

type Shape
    = Rectangle Float Float
    | Circle Float

shape : Decoder Shape
shape =
  ("tag" := string) `andThen` shapeInfo

shapeInfo : String -> Decoder Shape
shapeInfo tag =
  case tag of
    "rectangle" ->
        object2 Rectangle
          ("width" := float)
          ("height" := float)

    "circle" ->
        object1 Circle
          ("radius" := float)

    _ ->
        fail (tag ++ " is not a recognized tag for shapes")




How would the corresponding JSON look like? Should that be 

{"rectangle" : {"width" : "2", "height": "3"}}

eelco

[Mario Sangiorgio]

No, the json explicitly has a tag attribute to let your code know what is the shape type it encodes followed by the attributes for that shape.

Something like:

{ "tag": "rectangle", "width": "2", "height": "3"}

{ "tag": "circle", "radius": "2"}

Note that there is no nesting.

Mario

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[Mario Sangiorgio]

Actually you need width, height and radius to be numbers not strings. Otherwise you'll get parsing errors.

{ "tag": "rectangle", "width": 2, "height": 3}

{ "tag": "circle", "radius": 2}

[Eelco Hoekema]

Thanks! I was making things too complex, apparently. 

Am i correct that the assumption here is that "tag" MUST be the first string the decoder runs into? So if my backend returns the JSON with alphabetically sorted keys { "height": 3, "tag": "rectangle", "width": 2}, the decoder will run into an error?

eelco

[Janis Voigtländer]

No, the order in the input is irrelevant.