If I have a certain tagged union type, like Shape
here, how would I construct a JSON decoder for it in Elm?
type alias Rectangle = { width : Int, height : Int }
type alias Circle = { radius: Int }
type Shape
= ShapeRectangle Rectangle
| ShapeCircle Circle
Given your JSON looks like
{ "radius" : 10 }
or
{ "width" : 20, "height" : 15}
Then this will do the trick
import Json.Decode as Json exposing ((:=))
decodeShape : Json.Decoder Shape
decodeShape =
Json.oneOf
[ decodeShapeRectangle
, decodeShapeCircle
]
decodeShapeRectangle : Json.Decoder Shape
decodeShapeRectangle =
Json.map ShapeRectangle <|
Json.object2 Rectangle
("width" := Json.int)
("height" := Json.int)
decodeShapeCircle : Json.Decoder Shape
decodeShapeCircle =
Json.object1 (ShapeCircle << Circle)
("radius" := Json.int)
A couple of additional things: I often add a 'type' and 'tag' field to help disambiguate when I have data types with common field names. The JSON then looks like
{ "type":"shape", "tag":"circle", "radius":10 }
Also, I think :=
will be replaced by field
in the upcoming 0.18 release.
Regards,
Michael
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