Recursively nested types fail

[Dave Thomas]

// This proto includes a recusively nested message.
message NestedTestAllTypes {
  NestedTestAllTypes child = 1;
  TestAllTypes payload = 2;
  repeated NestedTestAllTypes repeated_child = 3;
}

This results in:

falanx_test.fs(835, 13): [FS0001] This expression was expected to have type� 
ConcreteCodec<KeyValuePair<string,JToken> list,KeyValuePair<string,JToken> list,NestedTestAllTypes,NestedTestAllTypes>

but here has type�:
 
(IReadOnlyDictionary<string,JsonValue> -> Result<NestedTestAllTypes,DecodeError>) * (NestedTestAllTypes -> IReadOnlyDictionary<string,JsonValue>)

I think this is something that @gusty was expecting with Fleece?

[Dave Thomas]

As single self referencing type fails:

[<CLIMutable>]
type NestedTestAllTypes =
    { mutable child : NestedTestAllTypes option }

    static member JsonObjCodec =
        fun child  -> { child = child}
        |> Fleece.Newtonsoft.withFields
        |> Fleece.Newtonsoft.jfieldOpt ("child") (fun x -> x.child) 

[Dave Thomas]

falanx_test.fs(800, 12): [FS0001] Type mismatch. Expecting a:

(IReadOnlyDictionary<string,JsonValue> -> Result<(TestAllTypes option -> NestedTestAllTypes),DecodeError>) * (NestedTestAllTypes -> IReadOnlyDictionary<string,JsonValue>) -> ConcreteCodec<KeyValuePair<string,JToken> list,KeyValuePair<string,JToken> list,NestedTestAllTypes,NestedTestAllTypes>

but given a:

SplitCodec<IReadOnlyDictionary<string,JsonValue>,(TestAllTypes option -> NestedTestAllTypes),NestedTestAllTypes> -> (IReadOnlyDictionary<string,JsonValue> -> Result<NestedTestAllTypes,DecodeError>) * (NestedTestAllTypes -> IReadOnlyDictionary<string,JsonValue>)

The type:

ConcreteCodec<KeyValuePair<string,JToken> list,KeyValuePair<string,JToken> list,NestedTestAllTypes,NestedTestAllTypes>

does not match the type:

(IReadOnlyDictionary<string,JsonValue> -> Result<NestedTestAllTypes,DecodeError>) * (NestedTestAllTypes -> IReadOnlyDictionary<string,JsonValue>)

[Gustavo Leon]

Fleece should handle recursive types without any problem when using implicit codecs (overloads) which is the case here.

This failure seems to be due to a bug that was introduced recently.

I'll have a look and see how can it be solved.

[Dave Thomas]

@gusty The solution was to use the ConcreteCodec parts and depreciate the use of `jfieldOpt etc?

[Gustavo Leon]

Yes.

To add more context: the problem is that ConcreteCodecwas introduced as the way to define codecs, while the jfield family of functions were kept as backward compatible, but this caused a problem with recursive definitions as both overloads exists and the priority is given to the ConcreteCodec.

So, an easy fix is:

• in Fleece rename the JsonObjCodec method
• in Falanx also change that to that new name

A better but more complicated fix is what you say: use in Falanx ConcreteCodec which will be the supported way moving forward.

[Dave Thomas]

Would ConcreteCodec alleviate the issue with tuple encoding?

[Gustavo Leon]

Yes, ConcreteCodec does not suffer the infinite recursion, as it's the default overload.