We've just have a discussion with a colleague about the case of what
exactly variable would a function literal close over, if it calls a
method on a field of a struct-typed variable available in the scope of
the enclosing function.
Here's the code:
-------------------------------8<--------------------------------
package main
import (
"fmt"
)
type Foo struct {
val int
}
func (foo *Foo) Show() {
fmt.Println(foo.val)
}
type Bar struct {
foo *Foo
}
func main() {
foo := &Foo{123}
bar := &Bar{foo}
fn := func() {
bar.foo.Show()
}
fn()
bar.foo = &Foo{456}
fn()
}
-------------------------------8<--------------------------------
(Playground link: [1]).
and the dispute was about whan variable would fn() close over.
I proposed that it will close over bar.foo since it's the "narrowest"
variable needed to be able to call Show().
As it turned out, I was wrong, and fn() closes over bar.
My line of reasoning stems from the following passage from the spec [2]:
| Structured variables of array, slice, and struct types have elements and
| fields that may be addressed individually. Each such element acts like a
| variable.
which seem to suggest in the case above bar.foo is a variable, and since
bar itself is not needed to call the Show() method provided by the
bar.foo's type, the function literal would close over the field alone
(presumably internally storing a pointer to it).
The part of the spec which discusses function literals [3] states:
| Function literals are closures: they may refer to variables defined in a
| surrounding function. Those variables are then shared between the
| surrounding function and the function literal, and they survive as long
| as they are accessible.
So there is no clearly defined rule of how the compiler decides what to
capture.
I've got hunch that may be [2] really has concurrent access in mind when
it states "Each such element acts like a variable", and this does not
apply to other cases.
Can anyone please explain where there is a flaw in my line of reasoning
which led me to a wrong conclusion?
1. https://play.golang.org/p/_VfdzQDNZR
2. https://golang.org/ref/spec#Variables
3. https://golang.org/ref/spec#Function_literals
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