Elm : How to Encode Nested objects - encode

Question

Model
type alias Model {
name : String
, poi_coordinates : Coordinates
}
type alias Coordinates =
{
coord_type : String
, coordinates : List Float
}
poiFormEncoder : Model -> Encode.Value
poiFormEncoder model =
Encode.object
[
( "name", Encode.string model.name )
, ( "type", Encode.string model.poi_coordinates.coord_type)
, ( "poi_coordinates", Encode.array Encode.float (Array.fromList model.poi_coordinates.coordinates) )
]
Can i ask how to encode for this data type? I have no idea , and the encoder i did gives no coordinates fill. Any help is really appreciate. The Json file format is at below
[
{
"name": "Mcd",
"coordinates": {
"type": "Point",
"coordinates": [
101.856603,
2.924
]
}
},
.
.
.
]

Answer 1

You can nest calls to Json.Encode.object. Each time you want a new object in the output, you need another one, e.g:
poiFormEncoder : Model -> Encode.Value
poiFormEncoder model =
Encode.object
[ ( "name", Encode.string model.name )
, ( "coordinates"
, Encode.object
[ ( "type", Encode.string model.poi_coordinates.coord_type )
, ( "coordinates", Encode.list Encode.float model.poi_coordinates.coordinates )
]
)
]
This should make sense: it is a list of (key, value) pairs, and the value should be another object.
On a side note, it will depend on your use case, but your Coordinates type looks like a prime candidate for a custom Elm type, e.g:
type Coordinates
= Point { x : Float, y : Float }
| Polar { r : Float, t : Float }
| ...
If you find you are doing a lot of checking the string type value and then dealing with the coordinates accordingly, something like this might be a much nicer structure to use internally. Of course, the best representation will depend on how you are using the type.