The range clause introduces a new scope. See this version of your second
example: https://play.golang.org/p/UXI_w6B4DW5
-- Ian
On Sun, 12 Jan 2020, at 8:09 AM, Silvan Jegen wrote:
> Hi fellow gophers
>
> The following code compiles
>
>
> package main
>
> import (
> "fmt"
> )
>
> func main() {
> mymap := map[string]int{"a": 1, "b": 2}
> for k, v := range mymap {
> k, v := k, v
> fmt.Printf("k %s, v: %d\n", k, v)
> }
> }
>
>
> while this one doesn't.
>
>
> package main
>
> import (
> "fmt"
> )
>
> func main() {
> a := 1
> b := 2
> a, b := a, b
> fmt.Printf("a %d, b: %d\n", a, b)
> }
>
>
> I looked at the language spec and for the ':=' (the "walrus operator") in
> the "Short variable declarations"[0] section the spec says the following.
>
> "Unlike regular variable declarations, a short variable declaration
> may redeclare variables provided they were originally declared earlier
> in the same block (or the parameter lists if the block is the function
> body) with the same type, and at least one of the non-blank variables is
> new. As a consequence, redeclaration can only appear in a multi-variable
> short declaration."
>
> This explains why the second example doesn't compile since none of the
> variables in the second short variable declaration is new.
>
> The same is true for the first example however and this one compiles
> just fine (even when using a short variable declaration on only one
> variable like in 'k := k' which shouldn't be allowed either according
> to this quote).
>
> The only explanation for this that I could come up with after being asked
> about this case is that the variables declared in the range clause are
> a special case, the so called "iteration variables" according to the
> "For statements" section[1]. The spec does not mention this but for
> these "iteration variables" the rules for the regular short variable
> declaration don't seem to uply. If that is indeed the case, I wonder if
> the spec shouldn't be updated to reflect this.
>
> Is this really the case or am I missing something?
>
>
> Cheers,
>
> Silvan
>
>
> [0] https://golang.org/ref/spec#Short_variable_declarations
> [1] https://golang.org/ref/spec#For_statements
>
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