Better error on mismatched binary operator

[sh0rez]

Consider this invalid snippet:

{
  abc: null
} + {
  abc+: {
    foo: "bar"
  }
}

First, surprised me that it is invalid after all, because I assumed I could add object and null (which is represents "nothing", right?).

More importantly, the error message printed by go-jsonnet is pretty useless:

RUNTIME ERROR: Unexpected type null
	During manifestation	

While the one of c++-jsonnet is much better:

RUNTIME ERROR: binary operator + requires matching types, got null and object.
	test.jsonnet:(3:5)-(7:2)	object <anonymous>
	During manifestation

[sbarzowski]

I assumed I could add object and null

Well, "nothing" is a pretty ambiguous thing :-). You can often have different expectations for it depending on the context. Jsonnet takes the safe approach here – null is a value which has its own type and cannot be used implicitly as some other thing (this is similar to Python's None).

More importantly, the error message printed by go-jsonnet is pretty useless

I agree. We should figure out a useful convention for reporting type errors in stdlib functions. Currently the builtins do the simplest thing – they expect a specific type and error out immediately when they get a wrong one.