Error/Assertion wrapping

[ghostsquad]

I'd like to be able to do something like this, similar to how Go lets you wrap errors.

local assertFoo():: 
  assert false : "nope!";
  true;

assert assertFoo() : "higher level operation failed";

This is related to google/jsonnet#415

[sbarzowski]

An interesting idea. I imagine we would show the whole "stack" of errors. How would a more realistic example look like?

If it was limited to assertions, like in the example, we could get away with this without changing the semantics. It would just be a nice way to present errors (the intermediate assertions are already a part of the stack trace btw).

[ghostsquad]

Well, the problem right now, is that this doesn't actually seem to trigger the outer assert

❯ echo 'local foo() = (assert false : "inner"; true); assert foo() : "outer"; {}' | jsonnet -
RUNTIME ERROR: inner

	<stdin>:1:54-59	$
	During evaluation

[sbarzowski]

Well, the outer assert shows up in the stack trace (the 1:54-59 part). It doesn't "trigger" per se, because there is an error while checking the condition.

BTW you can use -e option instead of echo sth | jsonnet -.

[sparkprime]

There are 3 use cases to consider:

1. Testing that a library raises errors when it should do
2. Augmenting errors from libraries with application-level context
3. Switching to a fallback if an expression raises an error (dubious, but useful since we use errors to represent "missing" fields sometimes)

With that in mind, I've thought for a while that a proper catch construct is probably better than addressing each case separately.

[ghostsquad]

Actually, since it's related. What's the difference between assert and error? Is assert just short hand for if <cond> then error?

[sbarzowski]

Yes:

Object assertions are more than that and it causes a lot of trouble (e.g. google/jsonnet#451).