Constraining type definition to enable Fleece (de-)serialization

General
1

Say I have a record type…

type Message<^a ...> =
    { id: Guid
      causation: Guid
      correlation: Guid
      name: string
      body: ^a }

I believe I need to use some duck typing to constrain the generic to implement OfJson and ToJson but I am struggling with the syntax.

Right now, I am using…

type Message< ^a when ^a : (static member OfJson: JsonValue -> ^a ParseResult)
                                 and ^a : (static member ToJson : ^a -> JsonValue)>

but when I define Message.JsonObjCode as

with
    static member JsonObjCodec = ...

I get an error ‘The code is not sufficiently generic. The type variable ^a (…) could not be generalized because it would escape its scope.’

Hmm, what am I missing?

2

I think you’re missing the inline keyword. Try “member inline”. See https://github.com/dotnet/fsharp/issues/3302 and https://stackoverflow.com/a/5904947/5652483

3

This is old but still for the record, here’s the answer.

Never declare the same parameter at type level declaration and in static member with constraints.Just use a different parameter name in the type declaration, without any constraint.

It’s probably not necessary to declare the method inline but I’m not sure as I can’t see its body.

1 Like
4

Thank you. Old but relevant and helpful!