What would be the Go structs for this terse JSON document?

The document has minimal key/value pair metadata on purpose. In a list of people, we could have something like "name":"joe" or we could have a key of joe. It errs on having fewer keys.

Perhaps this document is just too dynamic for Go structs? I've tried YAML as well, it's the structs that's the problem. The structs remain empty because it's not mapping correctly.

Playground link: https://play.golang.org/p/PGSjoKvNja

Or do I need to roll my own UnmarshalJSON and have conditionals (or switch) for "actions"? That's fine if so. I could do coercion and validation in there, loop through the doc and detect what the problematic generic action bit is and then create a struct of the right type.

If the data is NOT consistent, as @Adrian stated, you should not go for what I'm showing.

Otherwise, you should be able to unmarshal your string with the following struct generated with json-to-go, a very useful tool to get a struct out of a json

type Custom struct {
	Ball []struct {
		Throw struct {
			Strength string `json:"strength"`
		} `json:"throw"`
	} `json:"ball"`
	Frisbee []struct {
		Fling struct {
			Curve string `json:"curve"`
		} `json:"fling"`
		Catch struct {
			Trick string `json:"trick"`
			Jump string `json:"jump"`
		} `json:"catch"`
	} `json:"frisbee"`
}

And then

func main() {

    var c Custom
    err := json.Unmarshal([]byte(input), &c )
    if err != nil {
        panic(err)
    }

     fmt.Println(input)

}

Which prints out

{
	"ball": [{
		"throw": {
			"strength": "60%"
		}
	}, {
		"throw": {
			"strength": "20%"
		}
	}],
	"frisbee": [{
		"fling": {
			"curve": "left"
		}
	}, {
		"catch": {
			"trick": "behind back",
			"jump": "sure"
		}
	}]
}

Have a look at this Playground I set up.

It would be:

type AutoGenerated struct {
	Ball []struct {
		Throw struct {
			Strength string `json:"strength"`
		} `json:"throw"`
	} `json:"ball"`
	Frisbee []struct {
		Fling struct {
			Curve string `json:"curve"`
		} `json:"fling,omitempty"`
		Catch struct {
			Trick string `json:"trick"`
			Jump string `json:"jump"`
		} `json:"catch,omitempty"`
	} `json:"frisbee"`
}

Ofcourse you can define a separate type for each inline struct definition. You can use this online tool (which is used for generating the above data structure).