Red Hat

Introduction to Ceylon Part 5

Posted by Gavin King    |    Apr 28, 2011    |    Tagged as Ceylon

This is the fifth installment in a series of articles introducing the Ceylon language. Note that some features of the language may change before the final release.

Narrowing the type of an object reference

In any language with subtyping there is the hopefully occasional need to perform narrowing conversions. In most statically-typed languages, this is a two-part process. For example, in Java, we first test the type of the object using the instanceof operator, and then attempt to downcast it using a C-style typecast. This is quite curious, since there are virtually no good uses for instanceof that don't involve an immediate cast to the tested type, and typecasts without type tests are dangerously non-typesafe.

As you can imagine, Ceylon, with its emphasis upon static typing, does things differently. Ceylon doesn't have C-style typecasts. Instead, we must test and narrow the type of an object reference in one step, using the special if (is ... ) construct. This construct is very, very similar to if (exists ... ) and if (nonempty ... ), which we met earlier.

Object obj = ... ; 
if (is Hello obj) {
    obj.say();
}

The switch statement can be used in a similar way:

Object obj = ... ; 
switch(obj) 
case (is Hello) {
    obj.say();
} 
case (is Person) {
    stream.writeLine(obj.firstName);
} 
else {
    stream.writeLine("Some miscellaneous thing");
}

These constructs protect us from inadvertantly writing code that would cause a ClassCastException in Java, just like if (exists ... ) protects us from writing code that would cause a NullPointerException.

More about union types

We've seen a few examples of how ad-hoc union types are used in Ceylon. Let's just revisit the notion to make sure we completely understand it. When I declare the type of something using a union type X|Y, I'm saying that only expressions of type X and expressions of type Y are assignable to it. The type X|Y is a supertype of both X and Y. The following code is well-typed:

void print(String|Natural|Integer val) { ... }

print("hello");
print(69);
print(-1);

But what operations does a type like String|Natural|Integer have? What are its supertypes? Well, the answer is pretty intuitive: T is a supertype of X|Y if and only if it is a supertype of both X and Y. The Ceylon compiler determines this automatically. So the following code is also well-typed:

Natural|Integer i = ... ;
Number num = i;
String|Natural|Integer val = i;
Object obj = val;

However, num is not assignable to val, since Number is not a supertype of String.

Of course, it's very common to narrow an expression of union type using a switch statement. Usually, the Ceylon compiler forces us to write an else clause in a switch, to remind us that there might be additional cases which we have not handled. But if we exhaust all cases of a union type, the compiler will let us leave off the else clause.

void print(String|Natural|Integer val) {
    switch (val)
    case (is String) { writeLine(val); }
    case (is Natural) { writeLine("Natural: " + val); }
    case (is Integer) { writeLine("Integer: " + val); }
}

Enumerated subtypes

Sometimes it's useful to be able to do the same kind of thing with the subtypes of an ordinary type. First, we need to explicitly enumerate the subtypes of the type using the of clause:

abstract class Hello() 
        of DefaultHello | PersonalizedHello { 
    ...
}

(This makes Hello into Ceylon's version of what the functional programming community calls an algebraic data type.)

Now the compiler won't let us declare additional subclasses of Hello, and so the union type DefaultHello|PersonalizedHello is exactly the same type as Hello. Therefore, we can write switch statements without an else clause:

Hello hello = ... ; 
switch (hello) 
case (is DefaultHello) {
    writeLine("What's your name?");
} 
case (is PersonalizedHello) {
    writeLine("Nice to hear from you again!");
}

Now, it's usually considered bad practice to write long switch statements that handle all subtypes of a type. It makes the code non-extensible. Adding a new subclass to Hello means breaking all the switch statements that exhaust its subtypes. In object-oriented code, we usually try to refactor constructs like this to use an abstract method of the superclass that is overridden as appropriate by subclasses.

However, there are a class of problems where this kind of refactoring isn't appropriate. In most object-oriented languages, these problems are usually solved using the visitor pattern.

Visitors

Let's consider the following tree visitor implementation:

abstract class Node() {
    shared formal void accept(Visitor v);
}
class Leaf(Object val) extends Node() {
    shared Object value = val;
    shared actual void accept(Visitor v) { 
        v.visitLeaf(this); 
    }
}
class Branch(Node left, Node right) extends Node() {
    shared Node leftChild = left;
    shared Node rightChild = right;
    shared actual void accept(Visitor v) { 
        v.visitBranch(this);
    }
}
interface Visitor {
    shared formal void visitLeaf(Leaf l);
    shared formal void visitBranch(Branch b);
}

We can create a method which prints out the tree by implementing the Visitor interface:

void print(Node node) {
    object printVisitor satisfies Visitor {
        shared actual void visitLeaf(Leaf l) {
            writeLine("Found a leaf: " l.value "!");
        }
        shared actual void visitBranch(Branch b) {
            b.leftChild.accept(this);
            b.rightChild.accept(this);
        }
    }
    node.accept(printVisitor);
}

Notice that the code of printVisitor looks just like a switch statement. It must explicitly enumerate all subtypes of Node. It breaks if we add a new subtype of Node to the Visitor interface. This is correct, and is the desired behavior. By break, I mean that the compiler lets us know that we have to update our code to handle the new subtype.

In Ceylon, we can achieve the same effect, with less verbosity, by enumerating the subtypes of Node in its definition, and using a switch:

abstract class Node() of Leaf | Branch {}
class Leaf(Object val) extends Node() {
    shared Object value = val;
}
class Branch(Node left, Node right) extends Node() {
    shared Node leftChild = left;
    shared Node rightChild = right;
}

Our print() method is now much simpler, but still has the desired behavior of breaking when a new subtype of Node is added.

void print(Node node) {
    switch (node)
    case (is Leaf) {
        writeLine("Found a leaf: " node.value "!");
    }
    case (is Branch) {
        print(node.leftChild);
        print(node.rightChild);
    }
}

Typesafe enumerations

Ceylon doesn't have anything exactly like Java's enum declaration. But we can emulate the effect using the of clause.

shared class Suit(String name) 
        of hearts | diamonds | clubs | spades 
        extends Case(name) {}
        
shared object hearts extends Suit("hearts") {} 
shared object diamonds extends Suit("diamonds") {} 
shared object clubs extends Suit("clubs") {} 
shared object spades extends Suit("spades") {}

We're allowed to use the names of object declarations in the of clause if they extend the language module class Case.

Now we can exhaust all cases of Suit in a switch:

void print(Suit suit) {
    switch (suit)
    case (hearts) { writeLine("Heartzes"); }
    case (diamonds) { writeLine("Diamondzes"); }
    case (clubs) { writeLine("Clidubs"); }
    case (spades) { writeLine("Spidades"); }
}

(Note that these cases are ordinary value cases, not case (is...) type cases.)

Yes, this is a bit more verbose than a Java enum, but it's also slightly more flexible.

For a more practical example, let's see the definition of Boolean from the language module:

shared abstract class Boolean(String name) 
        of true | false 
        extends Case(name) {}
shared object false extends Boolean("false") {}
shared object true extends Boolean("true") {}

And here's how Comparable is defined. First, the typesafe enumeration Comparison:

doc "The result of a comparison between two
     Comparable objects."
shared abstract class Comparison(String name) 
        of larger | smaller | equal 
        extends Case(name) {}
doc "The receiving object is exactly equal 
     to the given object."
shared object equal extends Comparison("equal") {}
doc "The receiving object is smaller than 
     the given object."
shared object smaller extends Comparison("smaller") {}
doc "The receiving object is larger than 
     the given object."
shared object larger extends Comparison("larger") {}

Now, the Comparable interface itself:

shared interface Comparable<in Other> 
        satisfies Equality
        given Other satisfies Comparable<Other> {
    
    doc "The <=> operator."
    shared formal Comparison compare(Other other);
    
    doc "The > operator."
    shared Boolean largerThan(Other other) {
        return compare(other)==larger;
    }
    
    doc "The < operator."
    shared Boolean smallerThan(Other other) {
        return compare(other)==smaller;
    }
    
    doc "The >= operator."
    shared Boolean asLargeAs(Other other) {
        return compare(other)!=smaller;
    }
    
    doc "The <= operator."
    shared Boolean asSmallAs(Other other) {
        return compare(other)!=larger;
    }
    
}

Type inference

So far, we've always been explicitly specifying the type of every declaration. I think this generally makes code, especially example code, much easier to read and understand.

However, Ceylon does have the ability to infer the type of a locals or the return type of a local method. Just place the keyword local in place of the type declaration.

local hello = DefaultHello();
local operators = { "+", "-", "*", "/" };
local add(Natural x, Natural y) { return x+y; }

There are some restrictions applying to this feature. You can't use local:

  • for declarations annotated shared,
  • for declarations annotated formal,
  • when the value is specified later in the block of statements,
  • for methods with multiple return statements, or
  • to declare a parameter.

These restrictions mean that Ceylon's type inference rules are quite simple. Type inference is purely right-to-left and top-to-bottom. The type of any expression is already known without needing to look to any types declared to the left of the = specifier, or further down the block of statements.

  • The inferred type of a local declared local is just the type of the expression assigned to it using = or :=.
  • The inferred type of a method declared local is just the type of the returned expression.

Type inference for sequence enumeration expressions

What about sequence enumeration expressions like this:

local sequence  = { DefaultHello(), "Hello", 12.0 };

What type is inferred for sequence? You might answer: Sequence<X> where X is the common superclass or super-interface of all the element types. But that can't be right, since there might be more than one common supertype.

The answer is that the inferred type is Sequence<X> where X is the union of all the element expression types. In this case, the type is Sequence<DefaultHello|String|Float>. Now, this works out nicely, because Sequence<T> is covariant in T. So the following code is well typed:

local sequence  = { DefaultHello(), "Hello", 12.0 }; //type Sequence<DefaultHello|String|Float>
Object[] objects = sequence; //type Empty|Sequence<Object>

As is the following code:

local nums = { 12.0, 1, -3 }; //type Sequence<Float|Natural|Integer>
Number[] numbers = nums; //type Empty|Sequence<Number>

What about sequences that contain null? Well, do you remember the type of null from Part 1 was Nothing?

local sequence = { null, "Hello", "World" }; //type Sequence<Nothing|String>
String?[] strings = sequence; //type Empty|Sequence<Nothing|String>
String? s = sequence[0]; //type Nothing|Nothing|String which is just Nothing|String

It's interesting just how useful union types turn out to be. Even if you only very rarely explicitly write code with any explicit union type declaration (and that's probably a good idea), they are still there, under the covers, helping the compiler solve some hairy, otherwise-ambiguous, typing problems.

There's more...

A more advanced example of an algebraic datatype is shown here.

In Part 6 we'll explore Ceylon's generic type system in more depth.

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