Dan Kortschak <d...@kortschak.io> wrote:
> The gopher operator is allowed to be used because the body of the for
> loop is a new scope. In the second example, the a, b := b, a is not in
> a new scope.
So the declaration of the variables in the for loop itself is in outer
scope compared to the body of the for loop? In that case, redeclaring them
in the inner scope (== the loop body) would not be allowed either, no?
It also wouldn't explain the k := k case since ':=' has to redeclare at
least one new variable according to the spec.
Cheers,
Silvan
> On Sun, 2020-01-12 at 09:09 +0100, 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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