On Friday, November 17, 2017 at 3:19:55 PM UTC, Thiago Temple wrote:
>
> I have situation where I have to a jso object similar to this
>
> { "level": 1, "displayValue": "some text", "dataValue": "a string with
> the value" }
>
> { "level": 1, "displayValue": "some text", "dataValue": { "name": "xxx",
> "scope": "xxxx" } }
>
> Both cases for dataValue are valid, it can either have a string or an
> object.
>
> I have created types as such:
>
> type DataValue
> = Val String
> | Key KeyType
>
> type alias KeyType =
> { name: String, source: String }
>
> type alias MyObj =
> { level: Int, displayValue: String, dataValue: DataValue}
>
> How can I decode dataValue for MyObj?
>
> Thanks
>
I think the Decode.oneOf suggestion from Aaron VonderHaar is most likely to
be useful to you.
Just to give another way, here is some decoder code for a JSON structure
where I also added an explicit @type field to indicate which type it is to
be decoded to. This is a little more complex, and in my case required the
back-end to play nicely and insert the @type fields too. It does avoid the
trial and error approach of 'oneOf' too:
{-|
Describes the ContentModel view type.
-}
type ContentModel =
TitledAsContentModel Titled
| MdContentAsContentModel MdContent
| PanelAsContentModel Panel
{-|
A JSON encoder for the ContentModel type.
-}
contentModelEncoder : ContentModel -> Encode.Value
contentModelEncoder model =
case model of
TitledAsContentModel titled -> titledEncoder titled
MdContentAsContentModel mdContent -> mdContentEncoder mdContent
PanelAsContentModel panel -> panelEncoder panel
{-|
A JSON decoder for the ContentModel type.
-}
contentModelDecoder : Decoder ContentModel
contentModelDecoder =
let
toContentModel typeName =
case typeName of
"Titled" -> map TitledAsContentModel titledDecoder
"MdContent" -> map MdContentAsContentModel mdContentDecoder
"Panel" -> map PanelAsContentModel panelDecoder
_ -> Decode.fail ("unknown type: " ++ typeName)
in
field "@type" Decode.string
|> andThen toContentModel
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