There is not a phase between in-between, Any evaluation must happen in either of the two phases.
On Sunday, December 29, 2024 at 3:13:38 PM UTC+8 Kaiming Yang wrote: > An update from the bug: Seems the behavior is WAI because the execution > order of dereference and function calls are not defined. deference of > `foo.bar` may happen before or after (which is this case) the function call > of `compute_value()` > > Spec states following about order of evaluation: > > > when evaluating the operands <https://go.dev/ref/spec#Operands> of an > expression, assignment, or return statement > <https://go.dev/ref/spec#Return_statements>, > > all function calls, method calls, receive operations > <https://go.dev/ref/spec#Receive%20operator>, and binary logical > operations <https://go.dev/ref/spec#Logical_operators> are evaluated in > lexical > left-to-right order. > > Notably, there is no selector expression, pointer dereference expression > (explicit or implicit) and indexing expression (mentioned as example in > spec) in that list. > On Saturday, December 28, 2024 at 7:01:10 PM UTC-8 Kaiming Yang wrote: > >> Thanks for the confirmation, I created >> https://github.com/golang/go/issues/71054 for this. >> >> On Saturday, December 28, 2024 at 6:11:52 PM UTC-8 Ian Lance Taylor wrote: >> >>> On Sat, Dec 28, 2024 at 5:38 PM Kaiming Yang <yax...@gmail.com> wrote: >>> > >>> > I'm trying to understand execution order of assignment statement, as >>> spec mentioned: >>> > >>> > > The assignment proceeds in two phases. First, the operands of index >>> expressions and pointer indirections (including implicit pointer >>> indirections in selectors) on the left and the expressions on the right are >>> all evaluated in the usual order. Second, the assignments are carried out >>> in left-to-right order. >>> > >>> > I crafted a example about it: https://go.dev/play/p/6RulNQwIk4n, the >>> core of that example is this line: >>> > >>> > phase_2, foo.bar.value = "??????", compute_value() >>> > >>> > where `foo.bar.value` causes panic due to nil pointer. >>> > >>> > The execution result shows `compute_value()` is invoked, but `phase_2` >>> variable is not updated. Seems like the panic happened after phase 1 is >>> complete but before phase 2 started. >>> > >>> > I assume the panic should happen in one of two phases, but not >>> in-between. If panic happened in phase 1, `compute_value()` should not be >>> invoked due to it happen later in same phase; If panic happened in phase 2, >>> `phase_2` variable should have been changed already. >>> > >>> > Is the behavior working-as-indented? if so, what am I missing to >>> understand it? >>> >>> Thanks. This looks like a bug to me. I think the panic should occur >>> before compute_value is called. When I run your program with gccgo I >>> get >>> >>> original >>> panic: runtime error: invalid memory address or nil pointer dereference >>> [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4037fb] >>> >>> goroutine 1 [running]: >>> main.main >>> foo.go:22 >>> >>> Want to open a bug report at https://go.dev/issue? Thanks. >>> >>> Ian >>> >> -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/golang-nuts/744b3159-b922-4b0d-90ed-a62130e6479fn%40googlegroups.com.
