This can consider discussion, just sharing my idea, to replace 2 keywords we can attempt to address decade old problem: if err != nil and switch x:

1. Focus on less typing
   When there is something in err variable, it will fall under the same condition as default case in switch/when statement. It should sound natural.

when err {
    ... handle error
    return
}

2. This is an inverted of Ruby case...when statement.
   Expand the scope in place rather than replacing if err != nil with switch.

when err := i.(type) {
case T:
    ...
default:
    ...
}