It's a function, not a method. So it can't be part of an interface.
I suppose you could make an argument for passing it as a function argument to a higher ordered function. But that is kind of an unusual way to use such a function. And isn't a common pattern in go.
> My point here was: if you know you’re creating a struct to pass to the mashaller at compile time, then you’re already going to use JSONAble fields. So it’s not an issue.
Maybe. Maybe not. For a simple struct where all the fields are primitive types sure. But if you have a complicated hierarchy, you might miss that some strict in the hierarchy has a field that can't be marshaled.
Say you are marshaling a struct that contains a struct from a third party library, and it works fine. Then you update to a new version of that library, and that added a new field for a type that doesn't work, say a map[A]string where A is a struct. Your program still compiles fine, but then you get an error at runtime when you try to serialize something that worked before. Hopefully you have tests that catch it.
The mashaller here is a function, yes. But having a consistent signature means you can interchange those marshallers. It’s a trick I use heavily in murex to enable typed pipelines.
And I do this via a pipe interface. So it’s not uncommon to reuse function signatures for interface types (fewer runtime abstractions) while retaining some level of dynamism.
> Say you are marshaling a struct that contains a struct from a third party library, and it works fine. Then you update to a new version of that library, and that added a new field for a type that doesn't work, say a map[A]string where A is a struct.
The first mistake there is to blindly forward an internal API struct that you don’t control, and depend on that business logic.
Which comes back to my point about if you want type safety then you’d create a struct to be marshelled rather than passing data you don’t control.
I do take your point that the compiler in smarter languages would catch this. But you’d equally be told not to do this sort of thing on those languages too. So it feels like a moot point.