json decoding: undefined is not a function

[gpaul]

When parsing json into an algebraic datatype with Json.Decode.customDecoder and 'oneOf' I get the following error:
"expecting one of the following:\n undefined is not a function\n expecting a String but got {"aaa":"1"}"

The code follows:

import Graphics.Element exposing (show)
import Dict exposing (Dict)
import Json.Decode exposing ((:=), succeed, object2, list, string, bool, dict, int, maybe, decodeString, map, value, Decoder, customDecoder, oneOf)

jsonString = """{"aaa": "1"}"""

type V = Str String
            | Object (Dict String V)

stringer : String -> Result String V
stringer s = Ok (Str s)

stringer2 : Decoder V
stringer2 = customDecoder string stringer

objecter : Decoder (Dict String V)
objecter = dict decodeV

objecter2 : Decoder V
objecter2 = customDecoder objecter objecter3

objecter3 : (Dict String V) -> Result String V
objecter3 d = Ok (Object d)

decodeV = oneOf
    [
    objecter2
    , stringer2
    ]

main =
 case (decodeString decodeV jsonString) of
     Ok value -> show value
     Err msg -> show msg

[gpaul]

The generated code looks like this. Could it be an order of initialization issue, where decodeV references objecter2, which references objecter, which references decodeV again?

   var Object = function (a) {
      return {ctor: "Object"
             ,_0: a};
   };
   var objecter3 = function (d) {
      return $Result.Ok(Object(d));
   };
   var Str = function (a) {
      return {ctor: "Str",_0: a};
   };
   var stringer = function (s) {
      return $Result.Ok(Str(s));
   };
   var stringer2 = A2($Json$Decode.customDecoder,
   $Json$Decode.string,
   stringer);
   var objecter2 = A2($Json$Decode.customDecoder,
   objecter,
   objecter3);
   var objecter = $Json$Decode.dict(decodeV);
   var decodeV = $Json$Decode.oneOf(_L.fromArray([objecter2
                                                 ,stringer2]));

[gpaul]

The generated code looks like this. Could it be an order of initialization issue, where decodeV references objecter2, which references objecter, which references decodeV again?

   var Object = function (a) {
      return {ctor: "Object"
             ,_0: a};
   };
   var objecter3 = function (d) {
      return $Result.Ok(Object(d));
   };
   var Str = function (a) {
      return {ctor: "Str",_0: a};
   };
   var stringer = function (s) {
      return $Result.Ok(Str(s));
   };
   var stringer2 = A2($Json$Decode.customDecoder,
   $Json$Decode.string,
   stringer);
   var objecter2 = A2($Json$Decode.customDecoder,
   objecter,
   objecter3);
   var objecter = $Json$Decode.dict(decodeV);
   var decodeV = $Json$Decode.oneOf(_L.fromArray([objecter2
                                                 ,stringer2]));

[jvoigtlaender]

See elm/compiler#873 and the various issues occurring as links there that are related to Json.Decode specifically. For example, https://github.com/elm-lang/core/issues/361 and elm/compiler#1244.

So yes, the cause here is the recursive dependencies. And it is well known that it is an issue that should be fixed. If you need a workaround as long as it is not yet fixed, I think the discussion in https://github.com/elm-lang/core/issues/361 (about introducing dummy arguments to delay evaluation) might help you.

[jvoigtlaender]

See elm/compiler#873 and the various issues occurring as links there that are related to Json.Decode specifically. For example, https://github.com/elm-lang/core/issues/361 and elm/compiler#1244.

So yes, the cause here is the recursive dependencies. And it is well known that it is an issue that should be fixed. If you need a workaround as long as it is not yet fixed, I think the discussion in https://github.com/elm-lang/core/issues/361 (about introducing dummy arguments to delay evaluation) might help you.

[gpaul]

Thanks, that works perfectly. Feel free to close this ticket.

[gpaul]

Thanks, that works perfectly. Feel free to close this ticket.

[awalterschulze]

So for future reference:
objecter2 becomes

objecter2 : () -> Decoder V
objecter2 () = customDecoder objecter objecter3

And decodeV becomes

decodeV () = oneOf
    [
    objecter2 ()
    , stringer2
    ]

And main becomes

main =
 case (decodeString (decodeV ()) jsonString) of
     Ok value -> show value
     Err msg -> show msg

Is this correct?

[awalterschulze]

So for future reference:
objecter2 becomes

objecter2 : () -> Decoder V
objecter2 () = customDecoder objecter objecter3

And decodeV becomes

decodeV () = oneOf
    [
    objecter2 ()
    , stringer2
    ]

And main becomes

main =
 case (decodeString (decodeV ()) jsonString) of
     Ok value -> show value
     Err msg -> show msg

Is this correct?

[jvoigtlaender]

Yes, plus a corresponding change to the use of decodeV in objecter.

[jvoigtlaender]

Yes, plus a corresponding change to the use of decodeV in objecter.

[awalterschulze]

Ah yes good catch

objecter : Decoder (Dict String V)
objecter = dict (decodeV ())

[awalterschulze]

Ah yes good catch

objecter : Decoder (Dict String V)
objecter = dict (decodeV ())