Looking at the ES6 spec draft, I've drawn the following conclusions
about Completion Records. Can someone confirm?
If [[type]] is 'normal', 'return', or 'throw',
[[target]] must be 'empty' (can't be an identifier).
Moreover, if [[type]] is 'return' or 'throw',
[[value]] must be a language value (can't be 'empty').
Looks reasonable but I haven't verified it myself.
If [[type]] is 'continue' or 'break', no restrictions:
[[target]] can be 'empty' or an Identifier,
[[value]] can be 'empty' or a language value,
and all combinations can occur.
...
The only ones whose existence is less than obvious are those where:
[[type]] is 'continue' or 'break', and [[value]] is a language value
I can see in the pseudo-code the points where they could be created
(in 12.1.1.2, 12.11.1.2, and 14.1.2), but I haven't tried to reverse-
engineer a program that would cause it to happen. Could someone explain
why it's useful to allow such Completion Records, and post a snippet of
code that would cause one to be created?
12.1.1.2 is the important one here. For example take this test case
which is expected to return "value":
---
L1:
do
{
"value"; Completion {[[type]]: normal, [[value]]: "value",
[[target]]: empty}
break L1; Completion {[[type]]: break, [[value]]: empty,
[[target]]: "L1"}
} Completion {[[type]]: break, [[value]]: "value",
[[target]]: "L1"}
while(false); Completion {[[type]]: normal, [[value]]: "value",
[[target]]: empty}
---
'continue' works the same and just ignore for now that 12.6.1.1 step 2.c
in the current draft is invalid (it needs to add an additional step to
convert a break completion value to a normal completion value, cf.
12.6.1 in the ES5.1 spec).
14.1.2 is only needed to inspect the completion record value:
eval('L1: do { "value"; break L1; } while(false);') === "value"
Thanks,
-Michael
- André
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