How to decode JSON to record built by a data constructor? - elm

Question

There is the type
type User
= Anonymous
| Named {name : String, email : String}
Json.Decode.object2 doesn't fit here because its first arguments type is (a -> b -> c) but Named has { email : String, name : String } -> User type.
How to decode to User?

Answer 1

Another way of doing this could be to define a function that accepts a name and email and returns your Named constructor:
userDecoder : Decoder User
userDecoder =
let
named =
object2
(\n e -> Named { name = n, email = e })
("name" := string)
("email" := string)
in
oneOf [ null Anonymous, named ]

Answer 2

Since your Named constructor takes a record as a parameter, it might be a little cleaner to create a type alias for the name and email record.
type alias NamedUserInfo =
{ name : String
, email : String
}
You could then redefine User to use the alias:
type User
= Anonymous
| Named NamedUserInfo
While the above isn't strictly necessary, I find that aliasing record types proves useful in many ways down the road. Here it is useful because it gives us a constructor NamedUserInfo that lets us clearly define a decoder:
import Json.Decode exposing (..)
namedUserInfoDecoder : Decoder NamedUserInfo
namedUserInfoDecoder =
object2
NamedUserInfo
("name" := string)
("email" := string)
And finally, your user decoder could be constructed like this:
userDecoder : Decoder User
userDecoder =
oneOf
[ null Anonymous
, object1 Named namedUserInfoDecoder
]
You can run your example through a quick test like this:
exampleJson =
"""
[{"user":null}, {"user": {"name": "John Doe", "email": "j.doe#mailg.com"}}]
"""
main =
text <| toString <| decodeString (list ("user" := userDecoder)) exampleJson
-- Ouputs:
-- Ok ([Anonymous,Named { name = "John Doe", email = "j.doe#mailg.com" }])