Little things that matter in language design: make it do what it looks like it does

Posted Dec 29, 2014 10:26 UTC (Mon) by maryjmcdermott57 (guest, #100380)
In reply to: Little things that matter in language design: make it do what it looks like it does by jem
Parent article: Little things that matter in language design

With your answer, I summed up like this
- with the last expression in a block, add ; is up to me (depend on my intention)

But it's also give me more confusing. So can you explain a little bit more about this code.

if condition {
function1()
} else {
function2()
}
expression3;

With this code above, the if/else is not last expression. And if both function don't return value so we don't need add ; at the end of } of whole if/else. That makes sense (as author said that the language doesn't require that ; if whole if/else doesn't have value). With that knowledge, why we have to add ; at the end of println!("hello"). Because println!("hello") also doesn't return any value

fn main() {
println!("hello") // why we have to add ; at here
println!("world")
}

And if can, please answer me one more question. What about other blocks like match, loop, struct, function declaration? These are also expression as whole or not.

Because I saw function declarations next each other without ; between them. like this

fn foo() {
// ...
} // don't have ; at here
fn bar() {
// ...
}

If they are expression, we have separate them with ; to compile correct, right?

Please bear with me if my questions make you annoying. I'm very confusing about these and I also add question in other places but not get the good answer or some just ignore & vote close topic.

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Little things that matter in language design: make it do what it looks like it does

Posted Dec 29, 2014 10:51 UTC (Mon) by Cyberax (✭ supporter ✭, #52523) [Link] (1 responses)

>fn main() {
>println!("hello") // why we have to add ; at here
>println!("world")
>}

You need ';' because you can't just combine two expressions together. What operation should be used for that?

(never mind the fact that units can't be used in _any_ operation)

Little things that matter in language design: make it do what it looks like it does

Posted Dec 29, 2014 11:21 UTC (Mon) by maryjmcdermott57 (guest, #100380) [Link]

Yes, of course I can't combine 2 expressions together. So what about if/else & expression3. Both are expressions, right? But we can omit the ; at if/else expression in some cases. That makes no sense if we can omit ; at if/else expression but can't do that with println!("hello"). If we want consistent, Rust should force add ; at if/else (after the closing } of else) expression too

Little things that matter in language design: make it do what it looks like it does

Posted Dec 29, 2014 12:50 UTC (Mon) by jem (subscriber, #24231) [Link]

"With this code above, the if/else is not last expression. And if both function don't return value so we don't need add ; at the end of } of whole if/else. That makes sense (as author said that the language doesn't require that ; if whole if/else doesn't have value)."

You don't put a semicolon at the end of } of the whole if/else. This has nothing to do with whether the if should return a value or not – you never put a semicolon there. The if/else returns a value if the last expressions in the if and else branches do not end with a semicolon.

"What about other blocks like match, loop, struct, function declaration? These are also expression as whole or not."

Just like in the if/else case, you don't put a semicolon there. As with the if case, this does not mean these constructs have a value. Rust (mostly) borrows this syntax from C.

I think the best way of thinking about this is not to try to minimize the amount of semicolons. Do not focus all the time on whether you can leave out a semicolon, but instead think "do I want to return the value of the last expression in this block as the value of the block?". If that is the case, then you should leave out the semicolon.