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Attributes and variables, control structures

This is the third part of the Tour of Ceylon. In the previous leg we learned about classes and met the concept of an attribute. What really makes a class special is that is can hold state—references to other objects. So it's time to learn more about attributes and variables.

Then we're going to skim over some material about control structures (if, switch, for, while and try).

Attributes and local values

In Java, a field of a class is quite easily distinguished from a local variable or parameter of a constructor. Fields are declared directly in the body of a Java class, whereas local variables are declared within the body of the constructor. And they're different kinds of things. A local variable in a constructor doesn't outlive the invocation of the constructor. But a field lives until the object it belongs to is garbage collected.

This distinction is much less meaningful in Ceylon, and often irrelevant.

Is it a field or is it a local variable?

An attribute is really just a value declared in the parameter list or body of the class that happens to be captured by some shared declaration.

Here, count is a block-local variable of the initializer of Counter:

class Counter() {
    variable Integer count = 0;
}

But in this example, count is a shared attribute, visible to clients of the class:

class Counter() {
    shared variable Integer count = 0;
}

The distinction starts to melt away when we consider the next example:

class Counter() {
    variable Integer count = 0;
    shared Integer inc() => ++count;
}

Here, even though count is not shared, it still has a lifecycle that extends beyond the execution of the class initializer. We say that count is captured by the method inc().

This might seem a bit strange at first, but it's really just how the principle of closure works. The same behavior applies to block-local values declared in the body of a function. Functions can't declare shared members, of course, but they can return an object that captures a local variable:

interface Counter {
    shared formal Integer inc();
}

Counter createCounter() {
    variable Integer count = 0;
    object counter satisfies Counter {
        inc() => ++count;
    }
    return counter;
}

Or, as we'll see later, a function can even return a nested function that captures the local variable:

Integer() counter() {
    variable Integer count = 0;
    Integer inc() => ++count;
    return inc;
}

(Don't worry too much about the syntax here—for now all we're interested in is that counter() returns a reference to a function inc() that captures the variable count.)

So even though we'll continue to use the terms "local value" and "attribute" throughout this tutorial, keep in mind that there's no really strong distinction between the things these terms refer to. Any named value might be captured by some other declaration in the same containing scope. A local value is just an attribute that happens to not be captured by anything.

Tip: let expressions

It's possible to declare a local value within an expression using let, for example:

value greeting 
        => let (name = process.arguments.first else "world")
           "Hello ``name``!";

Let expressions are especially useful in combination with tuple and entry destructuring.

Variables

Ceylon encourages you to use immutable references as much as possible. Therefore, immutability is the default! An immutable reference has its value specified when the object is initialized, and is never reassigned.

class Box<Value>(val) {
    shared Value val;
}

Box<String> ref = Box("hello");
print(ref.val);
ref.val = "bar";    //compile error: value is not variable

Note that, just like in Java, we don't have to declare and initialize a reference in one line of code. We can write:

Box<String> ref;
if (leaving) {
    ref = Box("goodbye");
}
else {
    ref = Box("hello");    //ok
}

That's perfectly OK, as long as the compiler can verify that ref only gets assigned once in each conditional path through the code.

But if we want to be able to reassign a new value to a reference that has already been initialized, we need to annotate it variable:

class Box<Value>(val) {
    shared variable Value val;
}

Box<String> ref = Box("hello");
print(ref.val);
ref.val = "goodbye";    //ok
print(ref.val);

Idiomatic Ceylon code uses mutable references relatively less often than in most other languages.

Setters

We've already met the concept of a getter.

If we want to make an attribute defined as a getter mutable, we need to define a matching setter. Usually this is only useful if you have some other internal attribute you're trying to set the value of indirectly.

Suppose our class has the following attributes, intended for internal consumption only, so un-shared:

variable String? firstName = null;
variable String? lastName = null;

(Remember, Ceylon never automatically initializes attributes to null.)

Then we can abstract the attributes using a third attribute defined as a getter/setter pair:

shared String fullName
        => " ".join({ firstName, lastName }.coalesced);

assign fullName {
    value tokens = fullName.split().iterator();
    if (is String first = tokens.next()) {
        firstName = first;
    }
    if (is String last = tokens.next()) {
        lastName = last;
    }
}

A setter is identified by the keyword assign in place of a type declaration. (The type of the matching getter determines the type of the attribute.) Within the body of the setter, the attribute name evaluates to the value being set.

Yes, this is a lot like a Java get/set method pair, though the syntax is significantly streamlined. But since Ceylon attributes are polymorphic, and since you can redefine a reference as a getter or getter/setter pair without affecting clients that call the attribute, you don't need to write getters and setters unless you're doing something special with the value you're getting or setting.

Don't ever write code like this in Ceylon:

variable String _name = "";
shared String name => _name;  //pointless getter
assign name => _name=name;    //pointless setter

It's not necessary, and there's never any benefit to it.

Control structures

Ceylon has six built-in control structures. There's nothing much new here for Java or C# developers, so a few quick examples without much additional commentary should suffice.

Gotcha!

First, one "gotcha" for folks coming from other C-like languages: Ceylon doesn't allow you to omit the braces in a control structure. The following doesn't even parse:

if (x > 100) print("big");  //error

You are required to write:

if (x > 100) { print("big"); }

(The reason braces aren't optional in Ceylon is that an expression can begin with an opening brace, for example, {"hello", "world"}, so optional braces in control structures would make the whole grammar ambiguous.)

OK, so here we go with the examples.

If conditionals

The if/else statement is totally traditional:

if (x > 1000) {
    print("really big");
}
else if (x > 100) {
    print("big");
}
else {
    print("small");
}

Later in this tour, we will learn how if can narrow the type of references in its block. We've already seen an example of that, back when we talked about optional types.

Tip: using `then`/`else` instead of `if`

We sometimes use the operators then and else instead of if. Remember, that's Ceylon's replacement for C's ternary operator.

Switch conditionals

The switch/case statement eliminates C's much-criticized "fall-through" behavior and irregular syntax. Each branch of the switch statement is enclosed in braces, just like in any other control structure:

switch (x <=> 100)
case (smaller) { print("smaller"); }
case (equal) { print("one hundred"); }
case (larger) { print("larger"); }

A switch statement may declare a name for the switched value:

switch (name = process.arguments.first)
case (null) {
    print("Hello world!");
}
else {
    print("Hello ``name``!");
}

Each case of a switch must be:

a literal String, Character, or Integer,
an instance of an enumerated type, for example, true, false, or null,
a tuple formed from literals and enumerated instances.

(Don't worry, we'll learn about tuples later in this tour.)

switch (pair)
case ([0,"zero"]) { print("Zero"); }
case ([1, "one"]) { print("One"); }
else { print("something else"); }

A case can handle multiple values, in which case the values are separated by the | symbol:

Character? ch = ... ;
switch (ch)
case (null) {}
case ('.'|'?'|'!') { print("punctuation"); }
case (' '|'\t'|'\n'|'\r'|'\f') { print("whitespace"); }
else { print("something else"); }

As we'll see later, a case may even be:

a type case, or even
a pattern.

And, indeed, type cases are the most common sort of case Ceylon. So we'll have much more to say about switch when we discuss enumerated types.

Assertions

Ceylon also has an assert statement:

assert (length < 10);

Such assertions are good for making statements which you know have to be true, but are not apparent to other readers of the code (including the type checker!). Common uses of assert include things like preconditions, postconditions and class invariants.

If the condition is false at runtime an exception is thrown. The exception message helpfully includes details of the condition which was violated, which is extra important when the assert has more than one condition.

assert (exists arg, !arg.empty);

To customize the assertion message, add a string literal or interpolated string:

"length must be less than 10 but was ``length``"
assert (length < 10);

Where applicable, the typechecker uses asserted type information when checking statements which follow the assertion, for example:

Integer? x = parseInteger("1");
assert (exists x);
// after the assert, x has type Integer instead of Integer?
value y = x + 10;

This is really just the same behavior we saw earlier, only this time it's happening in the middle of a block rather than at the start of an if block. (Don't worry, there's more on this later.)

A value may be declared directly within the assert:

assert (exists x = parseInteger("1"));
value y = x + 10;

Note that, unlike Java's assert, which can be disabled at runtime, Ceylon's assertions are always enabled.

For loops

The for loop allows iteration over the elements of a Ceylon Iterable stream, a Java Iterable, or a Java array.

for (arg in process.arguments) {
    print(arg);
}

The for loop has an optional else block, which is executed when the loop completes normally, rather than via a return or break statement.

variable Boolean minors;
for (p in people) {
    if (p.age < 18) {
        minors = true;
        break;
    }
}
else {
    minors = false;
}

There is no C-style for. Instead, you can use the lengthwise range operator : to produce a sequence of Integers given a starting point and a length:

for (i in min:len) { ... }

Alternatively, you can use the ordinary range operator .. to produce a sequence of Integers given two endpoints:

for (i in min..max) { ... }

There are a couple of other tricks with for that we'll come back to later.

We often use comprehensions or even higher order functions instead of for.

While loops

The while loop is traditional.

value it = names.iterator();
while (!is Finished next = it.next()) {
    print(next);
}

There is no do/while statement.

Try statements

The try/catch/finally statement works just like Java's:

try {
    message.send();
}
catch (ConnectionException|MessageException e) {
    tx.setRollbackOnly();
}

To handle all Ceylon exceptions, together with all JavaScript exceptions, or all Java exceptions that are subclasses of java.lang.Exception, we can catch the type Exception defined in ceylon.language. If we don't explicitly specify a type, Exception is inferred:

try {
    message.send();
}
catch (e) {  //equivalent to "catch (Exception e)"
    tx.setRollbackOnly();
}

To handle all exceptions, including subtypes of java.lang.Error, we can catch the root exception class Throwable.

The try statement may optionally specify one or more "resource" expressions, just like in Java. The resource must be Destroyable or Obtainable, or a Java AutoCloseable.

try (Transaction(), s = Session()) {
    s.persist(person);
}

There are no Java-style checked exceptions in Ceylon.

Condition lists

Constructs like if, while, and assert accept a condition list. This is one of the most distinctive features of the syntax of Ceylon. A condition list is simply an ordered list of :

boolean expressions,
exists conditions, which we already met,
nonempty conditions, which will make sense when we talk about sequences,
is conditions, of which exists and nonempty conditions are really just abbreviations, which we'll also meet later, and, finally.
negated forms of the above condition types, that is, !exists, !nonempty, and !is.

A condition list is satisfied if (and only if) every one of its conditions is satisfied. Consider the following if statement:

if (exists arg = process.arguments.first, !arg.empty) {
    print("Hello " + arg + "!");
}

The body of the if statement will be executed only if both the conditions are satisfied, that is, only if arg is non-null and non-empty.

The difference between a Ceylon condition list, and an expression formed by combining boolean expressions with &&, is that an exists, nonempty, or is condition actually narrows the type of the value to which the condition is applied.

In our example:

The first condition declares arg, which is inferred to be a String, rather than just a String?, because it forms part of an exists condition.
The second condition, a simple boolean expression, makes use of the fact that arg is already known to be non-null.

The important thing to note is that the compiler lets us use arg in the second condition, and within the body of the if statement, and assigns it the type String, not just String?. We can't achieve this by combining multiple boolean expressions together with &&. (We could do it by nesting two ifs, but that leads to somewhat less readable code, and doesn't work well in while statements or comprehensions.)

Gotcha!

A very common "WTF" moment for programmers new to Ceylon is that there are two quite similar-looking constructs which have different semantics and occur quite differently within the grammar of the language:

The prefix forms exists x, nonempty x, is X x represent conditions that narrow the type of x, and they may only occur directly within a condition list.
On the other hand, the postfix forms x exists, x nonempty, x is X, are simply expressions of type Boolean; they may occur anywhere any other boolean expression may occur, and they don't narrow the type of anything.

By choosing a different syntax for these two different sorts of things, we emphasize the difference in semantics to the reader of the code. It's more obvious that this condition list doesn't narrow the type of whatever:

if (something && whatever exists || somethingElse && whatever exists) { ... }

Whereas this condition list does:

if (something && somethingElse, exists whatever) { ... }

Additionally, the choice of prefix syntax for conditions accommodates the use of a value declaration within the condition, for example:

if (exists whatever = dontCare()) { ... }

Condition lists occur not only in if, switch, while, and assert statements, but also in comprehensions, and in if and switch expressions.

If and switch expressions

It's possible to write an "inline" if or switch within an expression, for example:

value greeting
    => if (exists name = process.arguments.first)
    then "Hello ``name``!" else "Hello world!";

Or:

print(switch (length <=> 100)
      case (smaller) "smaller"
      case (equal) "one hundred"
      case (larger) "larger");

Note that the branches of an if or switch expression are always just expressions, never blocks of code!

Gotcha!

Don't forget the then keyword in your if expression!

There's more...

If you're interested, you can read more about exception handling in Ceylon.

Now that we know enough about classes and their members, we're ready to explore inheritance and refinement (overriding).

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Vert.x

Attributes and variables, control structures

Attributes and local values

Is it a field or is it a local variable?

Tip: let expressions

Variables

Setters

Control structures

Gotcha!

If conditionals

Tip: using `then`/`else` instead of `if`

Switch conditionals

Assertions

For loops

While loops

Try statements

Condition lists

Gotcha!

If and switch expressions

Gotcha!

There's more...

Ceylon

Attributes and variables, control structures

Attributes and local values

Is it a field or is it a local variable?

Tip: let expressions

Variables

Setters

Control structures

Gotcha!

If conditionals

Tip: using then/else instead of if

Switch conditionals

Assertions

For loops

While loops

Try statements

Condition lists

Gotcha!

If and switch expressions

Gotcha!

There's more...

Tip: using `then`/`else` instead of `if`