proposal: Go 2: add a "scope lifting" operator ^ for short variable declarations in if and for statements

[markusheukelom]

Operator ^ lifts the declaration of a variable to the enclosing scope when used in if and for statements:

if ^f, err := os.Open("foo.bar"); err != nil {
	return fmt.Errorf("error opening file: %w", err)
}
f.Read() 	// ok: f is in scope now

Using ^f, the declaration of f escapes the if statement while keeping err local to the if statement.

The expression ^f, err := works just like f, err := except that if f is declared, it is declared in the enclosing block.

Other notes:

1. ^ can only be used with if and for; anywhere else it is illegal

   To be more exact:
   - in the ShortVarDecl of the SimpleStatement of the IfStmt
   - in the InitStmt of the ForClause of the ForStmt
   - in the RangeClause of the ForStmt

2. Single lifts are allowed only; ^^f is illegal, etc.

3. I know that ^ is used as bitwise XOR operator; this should be no problem as for example * is also used for both derefencing and multiplication operator

4. I chose ^ because it is a visual cue for the effect of the operation.

[D1CED]

^ is already an unary operator (bitwise complement; same as ~ in C)

Edit: Example

if *a /* dereference */ := *b /* dereference */; true { ... }
if ^a /* scope lift */ := ^b /* complement */; true { ... }

Edit2: Well the first line of the example would not work because a can not be a new variable but you get my point.

[markusheukelom]

@D1CED thanks for the example. According to language specification no expressions are allowed currently allowed on the left hand side of a short variable declaration:

https://golang.org/ref/spec#Short_variable_declarations

So I don't really understand the example. Both lines do not seem to be valid go?

[ianlancetaylor]

I believe there are similar ideas in #377.

[zigo101]

The other two similar ideas are using (id) and *&id to indicate an identifier that it has been declared in outer scopes.
The two notations don't introduce new notations, they are already supported now, though they are both a little more verbose.
Ref: #38388, #30318, #377

[ianlancetaylor]

Closing as a dup of ideas expressed in #377.