Decode a JSON tuple to Elm tuple

Question

My JSON looks like the following

{ "resp":
    [ [1, "things"]
    , [2, "more things"]
    , [3, "even more things"]
    ]
}

the problem is that I can't parse the JSON tuples into Elm tuples:

decodeThings : Decoder (List (Int, String))
decodeThings = field "resp" <| list <| map2 (,) int string

It compiles, but when ran, it throws

BadPayload "Expecting an Int at _.resp[2] but instead got [3, \"even more things\"]

For some reason it reads [3, "even more things"] as only one thing and not as tuple in JSON format.
How can I parse my JSON into a List (Int, String)?

Answer 1

The accepted answer is more complicated than it needs to be. Try:

import Json.Decode as Decode

decodeTuple : Decode.Decoder (Int, String)
decodeTuple = 
    Decode.map2 Tuple.pair 
        (Decode.index 0 Decode.int) 
        (Decode.index 1 Decode.string)

and then, as you note, for the list

Decode.list decodeTuple

Answer 2

You need a decoder which turns a javascript array of size two into an Elm tuple of size two. Here is an example decoder:

arrayAsTuple2 : Decoder a -> Decoder b -> Decoder (a, b)
arrayAsTuple2 a b =
    index 0 a
        |> andThen (\aVal -> index 1 b
        |> andThen (\bVal -> Json.Decode.succeed (aVal, bVal)))

You can then amend your original example as follows:

decodeThings : Decoder (List (Int, String))
decodeThings = field "resp" <| list <| arrayAsTuple2 int string

(Note that my example decoder does not fail if there are more than two elements, but it should get you pointed in the right direction)

Answer 3

I could not get either Chad Gilbert's or Simon H's solution to work with Elm 0.19. I am quite new to Elm, but this is what I could get to work:

import Json.Decode as Decode
import Json.Decode.Extra as Decode

{-| Decodes two fields into a tuple.
-}
decodeAsTuple2 : String -> Decode.Decoder a -> String -> Decode.Decoder b -> Decode.Decoder (a, b)
decodeAsTuple2 fieldA decoderA fieldB decoderB =
    let
        result : a -> b -> (a, b)
        result valueA valueB =
            (valueA, valueB)
    in
        Decode.succeed result
            |> Decode.andMap (Decode.field fieldA decoderA)
            |> Decode.andMap (Decode.field fieldB decoderB)