Go: proposal: spec: add init statement to switch cases

[navytux]

Hello up there. Effective Go says:

It's therefore possible—and idiomatic—to write an if-else-if-else chain as a switch.

for if statement the specification allows to have simple statements inside if for declaring/initializing variables, e.g. this way:

if x := f(); x < 0 {
    ...
}

and it works for if-else-if-else too.

However given a long if-else-if-else construct, in presence of simple statements inside if, it is not possible to rewrite it as switch-case-case because specification does not allow simple statements inside case - only inside switch.

For example I wanted to write something like this:

        // use buffered data: start + forward
        case sb.pos <= pos && pos < sb.pos + len64(sb.buf):
                ...

        // emptry request (possibly not hitting buffer - do not let it go to real IO path)
        // `len(p) != 0` is also needed for backward reading from buffer, so this condition goes before
        case len(p) == 0:
                return 0, nil

        // use buffered data: tail + backward
        case posAfter := pos + len64(p);                                // <-- NOTE
                sb.pos < posAfter && posAfter <= sb.pos + len64(sb.buf):
                ....

and could not. It is of course possible to use if-else-if-else chain, but the switch form looks more reasonable, and once again, Effective Go says it is idiomatic to write such chains as switch.

So I propose to extend language specification to allow simple statements inside case.

Thanks beforehand,
Kirill

[bronze1man]

I think a simple statements inside "if" is a bad idea. It too complex to understand that code.I never use that language feature.
So I think this one is a bad idea too.

[dcheney-atlassian]

I think a simple statements inside "if" is a bad idea. It too complex to understand that code.I never use that language feature.

if err := fn(); err != nil {
       // handle error
}

Is quite popular.

[bronze1man]

It Is quite popular indeed.
And we should do independent thinking.So I hate this language feature("if" simple statements).
It just cost a lot energy to understand that code.
Because I need to think is whether this err in a scope, it also too easy to just skip that fn function call.
By the way I hate "if"/"switch"/"for" variable scope language feature too.The php function scope stuff does the best.

I prefer:

err := fn()
if err != nil {
       // handle error
}

[navytux]

I'd like to add: case already supports simple statements under select:

select {
case <-ch1:
    // ...
case x := <-ch2
    // ...use x
    ...
}

which is widely used in go stdlib itself:

kirr@deco:~/src/tools/go/go/src$ git grep -w case |grep ':=' | wc -l
142

For me this is simply language consistency fix, not go2 matter...

[cznic]

case already supports simple statements under select:

Send statement is the only simple statement a CommCase is specified to accept.

[ianlancetaylor]

This makes the switch statement harder to understand when switching on a value. Consider

switch v {
case 0:
case v++; 3:
case 4:
}

While we can define exactly what this means, it is just confusing. The advantage of being able to write the statement does not seem worth the complexity cost.