Go structs with pointers to maps return unexpected type errors

[rorydbain]

If I have a Go struct containing a pointer to a map, I'm unable to use that structure in Otto.

Here's an example test case:

func TestGoStructMapPointerField(t *testing.T) {
	type S struct {
		A *map[string]any `json:"a"`
	}

	tt(t, func() {
		test, vm := test()
		vm.Set("s", S{A: &map[string]any{
			"one": 1,
		}})

		test(`'a' in s`, true)
		test(`s.a.one`, 1)
	})
}

And the resulting error:

--- FAIL: TestGoStructMapPointerField (0.00s)
panic: TypeError: invalid value (map): missing runtime: map[one:1] (map[string]interface {}) [recovered]
	panic: TypeError: invalid value (map): missing runtime: map[one:1] (map[string]interface {}) [recovered]
	panic: TypeError: invalid value (map): missing runtime: map[one:1] (map[string]interface {}) [recovered]
	panic: TypeError: invalid value (map): missing runtime: map[one:1] (map[string]interface {})

[stevenh]

While this should work, is there a specific reason why you're using a pointer to a map as its a reference type anyway?

[rorydbain]

While this should work, is there a specific reason why you're using a pointer to a map as its a reference type anyway?

I had a usecase where *map[string]any was being used to indicate that the field was optional and might not be set - which I realise is niche but felt like a bit of a footgun.