elm: decode json that contains a json array - elm

Question

So I need to decode a json that contains a json array in elm. Here is my model:
type alias ValidationResult =
{ parameter : String
, errorMessage : String
}
type alias ErrorResponse =
{ validationErrors : List ValidationResult }
And here is an example of the json:
{"ValidationErrors": [{"Parameter": "param1","ErrorMessage": "message 1"},{"Parameter": "param2","ErrorMessage": "error message 2"}]}
I've tried to create a ValidationResult decoder, like:
decodeValidationResults : Decoder ValidationResult
decodeValidationResults =
map2 ValidationResult
(at [ "Parameter" ] Json.Decode.string)
(at [ "ErrorMessage" ] Json.Decode.string)
But I don't know how to proceed further.
I am using elm 0.18

Answer 1

You are almost there! You just need a decoder that decodes the ErrorResponse type. To do so, create another decoder that uses a list of the decoder you've already created, assuming the field name is "ValidationErrors":
import Json.Decode exposing (..)
decodeErrorResponse : Decoder ErrorResponse
decodeErrorResponse =
map ErrorResponse
(field "ValidationErrors" (list decodeValidationResults))
One bit of advice: You can use Json.Decode.field instead of Json.Decode.at when there is only a single level. You can rewrite decodeValidationResults as this:
decodeValidationResults : Decoder ValidationResult
decodeValidationResults =
map2 ValidationResult
(field "Parameter" string)
(field "ErrorMessage" string)