Armin Ronacher's Thoughts and Writings

Such a Little Thing: The Semicolon in Rust

written on Thursday, October 18, 2012

I was very happy to see Rust 0.4 being released yesterday. It's the first language in a really long time that genuinely interests me. It might be the next language besides C, C# and Python that I would really enjoy. The language design strikes such an exciting balance between doing new things and staying familiar enough to feel easy to pick up.

One of the exciting things of Rust is how it learns from other languages before in an experimental way. The language is designed and overthrown as it's being used and seeing the process unfold is very reminiscent of how Python's early days must have looked like.

One of the surprising features of Rust is how it deals with statement termination and it has not been without criticism. Personally: I am absolutely in love with how it does it but I have seen some people being very opposed to it. So here is me explaining why I think it's the best thing since sliced bread.

What Ruby does that Python Lacks

Before we can get to that, we need to do a detour into the dynamic language camp, mostly into the Ruby and Python camps. The languages seem familiar on the surface but they are actually very far apart. In fact, the languages could not be different in many ways. One of the most striking differences is how Ruby and Python deal with methods and statements versus expressions.

In Python everything is an object and that includes functions. In fact calling it a function would be wrong, Python programmers like to call them callables. The reason for that is that there are a lot of things you can call but they behave differently in their lowlevel implementations. That's something you will not notice as a Python novice but it certainly shows up at times, especially when it comes to changing the implementation and exposing unintended side effects.

For instance str() in Python used to be a function and was later converted into a type. dir() in Python is a builtin function whereas quit() is an instance of a quitter type. cgi.escape() is a “function”. On the surface they all work exactly the same, but really the only thing they have in common is that you can call them.

In Ruby there are no functions, instead there are objects that have methods. The idea is that instead of invoking a method on an object you send a message to an object, at least that is the original design. Not having functions has some profound implications. The most obvious one is that without first class functions, functional programming works differently. Ruby developers will generally tell you that functional programming works better in Ruby than it does in Python and will point out that blocks are preferable over anonymous functions. What are blocks? They are basically syntactic sugar to create an object (called a Proc) with a method called call that can be invoked. This proc is then passed to a function in a special parameter.

Let's talk a bit more practice here. Given a list of four numbers, here is how you would calculate the power of two for each item.

First in Python:

>>> def power_it(x):
...  return x ** 2
...
>>> map(power_it, [1, 2, 3, 4])
[1, 4, 9, 16]

Then in Ruby:

>> [1, 2, 3, 4].map { |x| x ** 2 }
=> [1, 4, 9, 16]

The Python version is obviously not the way you would write that in Python. Making a function like this is not very useful. Instead what you do in Python is either using a list comprehension (avoiding the problem) or to use the lambda keyword that allows you to create an unnamed function that has a single expression as body. Let's ignore that for a moment though. The example above shows something very interesting: it shows how different the languages treat expressions.

In Python there are statements and expressions. The syntax allows for statements to contain other statements and expressions to contain other expressions. If an expression is used in the spot where a statement is expected it's wrapped in what the grammar calls an “expression statement”. The purpose of the expression statement is to throw away the resulting value of the expression. This is obvious in both the syntax representation as well as the bytecode.

Take this very benign example:

foo()

On a syntax level this is the representation:

Module(body=[Expr(value=Call(func=Name(id='foo', ctx=Load()),
                             args=[], keywords=[],
                             starargs=None, kwargs=None))])

The call appears on module level, a module has multiple statements in the body. In this case it calls the name “foo” (which is loaded) with no arguments or keyword arguments of any sort. Since it's an expression it is wrapped in an Expr node. This allows an expression to be used there and also then tells the code generator to throw away the result. This would be the bytecode for it:

2           0 LOAD_GLOBAL              0 (foo)
            3 CALL_FUNCTION            0
            6 POP_TOP

It tells the interpreter: load the value for the global variable “foo”, then call it without arguments, then throw away the return value.

This is very different from Ruby. In Ruby many statements are implemented in a way that they are either expressions or at least behave in such a way. The language also very often does not throw away values like Python does. This is helpful because for instance methods return the value of the last expression when they return. Ruby follows that design mantra to ridiculous ways. For instance the following example defines a variable called foo and an empty class Bar where foo contains the last expression within Bar's body:

>> foo = class Bar
>>   42
>> end
=> 42
>> foo
=> 42

(Notice also how the assignment is also an expression that returns the value that was assigned in the expression)

Neat idea huh? But there is also a problem with that, and that's that you could return something as a side effect. For instance imagine you have a function that acts as a setter in ruby:

>> class Foo
>>  def set_x val
>>   @x = val
>>  end
>> end
=> nil
>> f = Foo.new
=> #<Foo:0x007fcaa09b1500>
>> f.set_x 42
=> 42

The setter now returns the value as a side-effect. Since you can just ignore that in Ruby it's generally not a problem, but people might now start to rely on that. Very often people are countering that problem by writing nil at the end of the function to prevent the unintended result:

>> class Foo
>>  def set_x val
>>   @val = val
>>   nil
>>  end
>> end
=> nil
>> f = Foo.new
=> #<Foo:0x007feab11af4e8>
>> f.set_x 42
=> nil

I can already see the argument that is brought up against that example from people doing more Ruby than me which is that nobody writes setters in Ruby. That is correct since ruby as special callback methods for setting attributes. The reason I did not use them is because they have their own semantics attached where the return value of the setter is ignored and overridden with the right hand of the assignment:

>> class Foo
>>  def x= val
>>   @x = val
>>   nil
>>  end
>> end
=> nil
>> f = Foo.new
=> #<Foo:0x007fa17b92ff80>
>> f.x = 42
=> 42

Blocks are not Functions

So let's stick with Ruby for a bit. Often it's argued that blocks are basically just anonymous functions. That however is not the case because they do more things than just functions, and that's important due to how the language works. Notice how we did not use return anywhere in the above examples despite the fact that ruby has return. That's because return is doing something else than just returning the last value when used within a block. A return within a block returns from the calling scope (which is pretty crazy if you think about it):

>> def foo
>>  [1, 2, 3].each { |x| puts x; return 42; }
>> end
=> nil
>> foo
1
=> 42

How does it do that? It users interpreter internal magic by setting a jump point before the invocation. That also means the block behaves differently when returned from the function:

>> def foo
>>  Proc.new { return 42 }
>> end
=> nil
>> p = foo
=> #<Proc:0x007ff25b0efda8@(irb):10>
>> p.call
LocalJumpError: unexpected return
    from (irb):10:in `block in foo'
    from (irb):13:in `call'

So it's pretty clear that blocks are a whole different beast and pretty much require the fact that the last expression is the return value from the block since return is there to have other purposes. Why does return return from the calling method and not the block? That's because of how iteration works in Ruby. Iteration is implemented exactly the other way round compared to Python. In Python iteration works by creating an iterator that can be called to produce more values until an exception is raised. If needs be that iterator keeps a interpreter frame alive in a suspended state (called generators in Python).

In Ruby iteration is implemented by letting something call a block repeatedly until the end of the iteration. The interpreter provides jump points in order to implement skipping or breaking the iteration. A continue is implemented as a form of jumping to the end of the block, a break is implemented by jumping past the call to the iterator function. Without the return it would be very awkward to return something from the function. Imagine a function that returns the first even item from a list:

>> def find_even iterable
>>  iterable.each { |x| return x if x % 2 == 0 }
>> end
=> nil
>> find_even [1, 3, 5, 6]
=> 6

Imagine the non-local return was not available, you would have to rewrite it like this:

>> def find_even iterable
>>  done = false
>>  rv = nil
>>  iterable.each { |x|
>>   if x % 2 == 0
>>    done = true
>>    rv = x
>>    break
>>   end
>>  }
>>  rv
>> end

Back to Rust

Now what does any of this have to do with Rust? Quite a lot actually. If you have not paid much attention to Rust here is a very short primer of what the language is about: compile time verification of a lot of things. The language is designed to catch many errors at compile time. It's operating on a similar low level than C or C++ do so you get direct access to memory if you want, but it will still verify at compile time that you are never dealing with uninitialized memory by accident.

The way it does that is by using different pointer types with different associated semantics. You have pointers where the compiler determines statically that at any point in time there is only one owner, there are pointers with limited garbage collection and there are pointers that are lent memory temporarily. There is a lot more to the language, but that's the most important aspect of it if you have no idea of the language otherwise.

This has a lot of implications of how the language works. For instance it's quite easy to give a loan to memory if you can guarantee that a caller will only temporarily use the memory (for the duration of the call). All you need to do is to verify that the value never persists for longer than the duration of the call. If you think about it: that's how iteration in ruby works as well. You have a block that closes over some variables and that closure lives for as long as the iteration is ongoing.

Rust in fact models it's iteration model very close to ruby and even uses some of the same syntax. The iteration protocol however itself works slightly different by utilizing the return value to indicate a break or continue.

In accordance with Rust's memory model there are different “blocks” (called closures) as well. Here is also where Rust diverges from Ruby. In Ruby a block is a syntactical suffix to a method invocation, in Rust a closure is a syntactical construct that can stand on itself. Closures can either be store on the stack (perfect for things like iteration) or be tracked by the garbage collector or unique in which case only one variable at the time can own the memory.

In this example I'm only talking about stack stored closures which are special in a number of ways. The first and most obvious one is that the syntax looks slightly different (basically like ruby blocks) but also that they can only be passed around, not stored. They additionally also have compiler support which I will come to later.

As mentioned, the syntax for closures is ruby inspired:

/* a closure that takes a variable, creates the power of two and
   returns it */
|x: int| -> int { x ** 2 }

Since Rust has powerful type inference the type annotations can be ignored in places where such a closure is passed as a callback to something else. For instance the map example from above in Rust looks very much like Ruby:

let powers = [1, 2, 3, 4].map(|x| { x ** 2 });

In fact, you can even leave out the braces if they only contain a single expression:

let powers = [1, 2, 3, 4].map(|x| x ** 2);

Looks like it can only hold an expression, but in fact in Rust — like in Ruby — almost everything is an expression. For instance you could do this if you want:

let powers = [1, 2, 3, 4].map(|x| {
    let power = x * 2;
    power
})

Like in Ruby, the last expression returns. But it comes with a twist. Notice how there is a lack of semicolon at the end?

The Semicolon!

Now we finally arrive where I wanted to go all the time: the semicolon in Rust. So Rust shares with Ruby that almost everything is an expression, but Rust also has static typing with type inference. That can very much be asking for trouble due to unintended side effects of the last expression. For instance remember how the assignment in Ruby leaks the right hand side in the expression? That would be quite annoying in Rust where the closure would suddenly return a value that is not expected by the caller's callback signature.

Rust has solved that problem currently in an incredible elegant way and that is by giving the presence or absence of the semicolon a meaning.

Trailing semicolon in the last expression in a block means: ignore value (or convert it to “nil” (())), the absence of it means to bubble up that expression.

Before we go further with that I want to point out how amazing semicolons are. I love extreme solutions because they are generally more stable than some wonky ideas in between. Python for instance has a very strong stance on statement termination: newlines terminate statements. C has one as well: semicolons terminate statements. JavaScript is flailing. Are semicolons annoying to type? Probably, I got used to them. But the alternative to semicolons is making line endings significant. Ruby gets away quit well with (what I think is magic or with) some sort of controlled chaos in the grammar.

I really despise what Erlang is doing where the semicolon is not a terminator but a separator and a dot is used as terminator. Why? Because it makes for awkward diffs where you affect the line before if you add a new statement. I understand why Erlang does it, but that does not make it a good idea.

So hereby I declare: I love semicolons and I love languages that take a strong stance on them. Semicolons in Rust have a lot of value.

Since only the last expression can be bubbled anyways the fact that the last semicolon can be present or absent does not even cause a problem in the language grammar. Some semicolon terminated grammars traditionally did not care about the last semicolon in a block anyways (like PHP or CSS for instance).

Iteration Protocol

So why is Rust not requiring an explicit return? Well first of all because it would be ugly, but secondly because it shares part of the iteration protocol with Ruby. For instance here is the Rust version of accepting a list of values and returning the first even one:

fn find_even(vector: &[int]) -> Option<int> {
    for vector.each |x| {
        if *x % 2 == 0 {
            return Some(*x);
        }
    }
    None
}

Since the language is statically typed and the type system is algebraic there is no special null type that can be used for any value. As such the return value from the function is either the value wrapped or None. Also the iterator yields pointers that need to be dereferences, but ignore that part for the moment. The important part is the return. It's inside a stack closure yet it returns from the outer function. How the hell does that work? And what's that for statement. Let's answer the latter question first. The function above could be rewritten like this:

fn find_even(vector: &[int]) -> Option<int> {
    let mut rv: Option<int> = None;
    vector.each(|x| {
        if *x % 2 == 0 {
            rv = Some(*x);
            false
        } else {
            true
        }
    });
    rv
}

As you can see, the iteration callback closure has a return value by itself and that is the indication if the iteration should continue or terminate. The for statement is a neat little syntax abstraction around the iteration protocol that adds the return true and return false for you to make it look nicer. Since the return at that point is up for new use it can be repurposed to mean “return from outer function”, and that's what it does. So Rust, like Ruby benefits greatly from having the return up for other use.

Why is Rust using Ruby style iteration and not Python style iteration? Because it's much easier to understand given the restrictions the type system gives you. It's a small price to pay for what the language gives you in return.

Intermission: Make it Generic

One thing that should be noted here is that unlike the Python or Ruby version this will not work for floating point values. Traditionally that particular example is really hard to implement in languages that have generics and not full templates. This however is still somewhat trivially solvable in Rust due to the support of the language. The following would be a version of find_first_even for arbitrary numbers:

fn find_even<T: Copy num::Num cmp::Eq>(vec: &[T]) -> Option<T> {
    let zero: T = num::from_int(0);
    let two: T = num::from_int(2);
    for vec.each |x| {
        if *x % two == zero {
            return Some(*x);
        }
    }
    return None;
}

The function becomes generic and some trait requirements are defined. The type has to be a number and a number that can be compared with the equivalency operator. Since we are now dealing with arbitrary numbers literals directly are no longer possible. Instead we need to use num::from_int(0) and have that convert into a value of the specific number type before we can use it in the comparison expression. But even with all that extra stuff, it's still very readable code and possible. Something that cannot be said about generics in C# for instance.

The Semicolon Matters

Now given all these things, here is why the semicolon trick is awesome. From the iteration example you see the explicit return is not really an option because it's too important. So what's the downside of always returning the last value? The downside is that you would have to put () (Rust's version of “nil”) in a bunch of functions to fulfil the requirements of the callback's signature since otherwise the type inferred from the function would be the value of the last expression. This would be especially annoying if different branches yield different types. Imagine a callback with an if where the first branch assigns a string and the second assigns an integer. The compiler would in this case complain that the function returns inconsistent types.

And this is why the special behavior of the semicolon in Rust is pretty clever design. Brings the nice effects of everything being an expression like in Ruby into a statically typed environment without becoming a pain to use and unintended side-effects.

This entry was tagged rust and thoughts