Panic when using "over" in an expression context

[philrz]

Repro is with Zed commit f1be6a4.

@mattnibs and I were working toward responding to a community user's inquiry in a Slack thread. I'd gotten to the point where I had this working in a user-defined operator:

$ cat op.zed 
op MyFlatten(val): (
  yield (over val | over this | collect(this))

$ zq -version
Version: v1.14.0-17-gf1be6a4a

$ echo '[[1,2],[3,4]]' | zq -I op.zed 'MyFlatten(this)' -
[1,2,3,4]

So I thought to try the equivalent in a user-defined function, which triggered this panic.

$ cat func.zed 
func MyFlatten(val): (
  (over val | over this | collect(this))
)

$ echo '[[1,2],[3,4]]' | zq -I func.zed 'yield MyFlatten(this)' -
panic: runtime error: index out of range [0] with length 0

goroutine 53 [running]:
github.com/brimdata/zed/runtime/sam/expr.Var.Eval(...)
	/Users/phil/work/zed/runtime/sam/expr/var.go:14
github.com/brimdata/zed/runtime/sam/op/traverse.(*Over).over(0xc0003e3da0, {0x28d5478?, 0xc000789f50}, {{0x28c76d0?, 0x36de9e0?}, 0x0?, 0x0?})
	/Users/phil/work/zed/runtime/sam/op/traverse/over.go:72 +0x164
github.com/brimdata/zed/runtime/sam/op/traverse.(*Over).Pull(0xc0003e3da0, 0xc0?)
	/Users/phil/work/zed/runtime/sam/op/traverse/over.go:56 +0x1a9
github.com/brimdata/zed/runtime/sam/op/traverse.(*Scope).run(0xc0006d1c70)
	/Users/phil/work/zed/runtime/sam/op/traverse/scope.go:69 +0x37
created by github.com/brimdata/zed/runtime/sam/op/traverse.(*Scope).Pull.func1 in goroutine 52
	/Users/phil/work/zed/runtime/sam/op/traverse/scope.go:47 +0x56

Meanwhile @mattnibs was coming up with a more sophisticated variant that was closer to the user's original intent, and it looks like he hit a similar panic.

$ cat maybe_flatten.zed
func maybe_flatten_fn(a): (
   kind(a) == "array"  
     ? (over a | and(kind(this) == "array")) 
        ? "It's an array of arrays, so I would flatten"
        : "It's an array containing some non-array values, so I would not flatten"
     : "It's not even an array, so I ain't touchin' that"
)

yield [[1,2],[3,4]], "hello"
| yield maybe_flatten_fn(this)

$ echo null | zq -I maybe_flatten.zed -
panic: runtime error: index out of range [0] with length 0

goroutine 25 [running]:
github.com/brimdata/zed/runtime/sam/expr.Var.Eval(...)
	/Users/phil/work/zed/runtime/sam/expr/var.go:14
github.com/brimdata/zed/runtime/sam/op/traverse.(*Over).over(0xc00056e840, {0x28d5478?, 0xc000876e10}, {{0x28c76d0?, 0x36de9e0?}, 0x0?, 0x0?})
	/Users/phil/work/zed/runtime/sam/op/traverse/over.go:72 +0x164
github.com/brimdata/zed/runtime/sam/op/traverse.(*Over).Pull(0xc00056e840, 0x0?)
	/Users/phil/work/zed/runtime/sam/op/traverse/over.go:56 +0x1a9
github.com/brimdata/zed/runtime/sam/op/traverse.(*Scope).run(0xc0006788c0)
	/Users/phil/work/zed/runtime/sam/op/traverse/scope.go:69 +0x37
created by github.com/brimdata/zed/runtime/sam/op/traverse.(*Scope).Pull.func1 in goroutine 24
	/Users/phil/work/zed/runtime/sam/op/traverse/scope.go:47 +0x56

As a side-effect of working on this, we also both recognized that this functionality should be mentioned in the expressions docs, but we might as well fix the bug before calling more attention to it.

[philrz]

Verified in Zed commit 3e5febe.

Both of the programs shown above that previously caused a panic now run successfully.

$ zq -version
Version: v1.14.0-21-g3e5febe7

$ echo '[[1,2],[3,4]]' | zq -I func.zed 'yield MyFlatten(this)' -
[1,2,3,4]

$ echo null | zq -I maybe_flatten.zed -
"It's an array of arrays, so I would flatten"
"It's not even an array, so I ain't touchin' that"

Thanks @mattnibs!