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Introduction to Ceylon Part 4

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

Sequences

Some kind of array or list construct is a universal feature of all programming languages. The Ceylon language module defines support for sequence types. A sequence type is usually written X[] for some element type X. But this is really just an abbreviation for the union type Empty|Sequence<X>.

The interface Sequence represents a sequence with at least one element. The type Empty represents an empty sequence with no elements. Some operations of the type Sequence aren't defined by Empty, so you can't call them if all you have is X[]. Therefore, we need the if (nonempty ... ) construct to gain access to these operations. 

void printBounds(String[] strings) {
    if (nonempty strings) {
        //strings is a Sequence<String>
        writeLine(strings.first + ".." + strings.last);
    }
    else {
        writeLine("Empty");
    }
}

Note how this is just a continuation of the pattern established for null value handling.

Sequence syntax sugar

There's lots more syntactic sugar for sequences. We can use a bunch of familiar Java-like syntax: 

String[] operators = { "+", "-", "*", "/" };
String? plus = operators[0];
String[] multiplicative = operators[2..3];

Oh, and the expression {} returns a value of type Empty.

However, unlike Java, all these syntactic constructs are pure abbreviations. The code above is exactly equivalent to the following de-sugared code: 

Empty|Sequence<String> operators = Array("+", "-", "*", "/");
Nothing|String plus = operators.value(0);
Empty|Sequence<String> multiplicative = operators.range(2,3);

A Range is also a subtype of Sequence. The following: 

Character[] uppercaseLetters = 'A'..'Z';
Natural[] countDown = 10..0;

Is just sugar for: 

Empty|Sequence<Character> uppercaseLetters = Range('A','Z');
Empty|Sequence<Natural> countDown = Range(10,0);

In fact, this is just a sneak preview of the fact that almost all operators in Ceylon are just sugar for method calls upon a type. We'll come back to this later, when we talk about operator polymorphism.

Iterating sequences

The Sequence interface extends Iterable, so we can iterate a Sequence using a for loop: 

for (String op in operators) {
    writeLine(op);
}

Ceylon doesn't need C-style for loops. Instead, combine for with the range operator ... 

variable Natural fac:=1;
for (Natural n in 1..100) {
    fac*=n;
    writeLine("Factorial " n "! = " fac "");
}

If, for any reason, we need to use the index of each element of a sequence we can use a special variation of the for loop that is designed for iterating instances of Entries: 

for (Natural i -> String op in entries(operators)) {
    writeLine($i + ": " + op);
}

The entries() function returns an instance of Entries<Natural,String> containing the indexed elements of the sequence.

Sequence and its supertypes

It's probably a good time to see some more advanced Ceylon code. What better place to find some than in the language module itself?

Here's how the language module defines the type Sequence: 

shared interface Sequence<out Element>
        satisfies Correspondence<Natural, Element> & 
                  Iterable<Element> & Sized {
    
    doc "The index of the last element of the sequence."
    shared formal Natural lastIndex;
    
    doc "The first element of the sequence."
    shared actual formal Element first;
    
    doc "The rest of the sequence, without the first
         element."
    shared formal Element[] rest;

    shared actual Boolean empty {
        return false;
    }
        
    shared actual default Natural size {
        return lastIndex+1;
    }
    
    doc "The last element of the sequence."
    shared default Element last {
        if (exists Element x = value(lastIndex)) {
            return x;
        }
        else {
            //actually never occurs if 
            //the subtype is well-behaved
            return first; 
        } 
    }

    shared actual default Iterator<Element> iterator() {
        class SequenceIterator(Natural from) 
                satisfies Iterator<Element> {
            shared actual Element? head { 
                return value(from);
            }
            shared actual Iterator<Element> tail {
                return SequenceIterator(from+1);
            }
        }
        return SequenceIterator(0);
    }
    
}

The most interesting operations are inherited from Correspondence, Iterable and Sized: 

shared interface Correspondence<in Key, out Value>
        given Key satisfies Equality {
    
    doc "Return the value defined for the 
         given key."
    shared formal Value? value(Key key);
        
}
shared interface Iterable<out Element> 
        satisfies Container {
    
    doc "An iterator of values belonging
         to the container."
    shared formal Iterator<Element> iterator();
    
    shared actual default Boolean empty {
        return !(first exists);
    }
    
    doc "The first object."
    shared default Element? first {
        return iterator().head;
    }

}
shared interface Sized 
        satisfies Container {
        
    doc "The number of values or entries 
         belonging to the container."
    shared formal Natural size;
    
    shared actual default Boolean empty {
        return size==0;
    }
    
}
shared interface Container {
        
    shared formal Boolean empty;
    
}

Empty sequences and the Bottom type

Now let's see the definition of Empty: 

object emptyIterator satisfies Iterator<Bottom> {
    
    shared actual Nothing head { 
        return null; 
    }
    shared actual Iterator<Bottom> tail { 
        return this; 
    }
    
}

shared interface Empty
           satisfies Correspondence<Natural, Bottom> & 
                     Iterable<Bottom> & Sized {
    
    shared actual Natural size { 
        return 0; 
    }
    shared actual Boolean empty { 
        return true; 
    }
    shared actual Iterator<Bottom> iterator() {
        return emptyIterator;
    }
    shared actual Nothing value(Natural key) {
        return null;
    }
    shared actual Nothing first {
        return null;
    }
    
}

The special type Bottom represents:

  • the empty set, or equivalently
  • the intersection of all types.

Since the empty set is a subset of all other sets, Bottom is assignable to all other types. Why is this useful here? Well, Correspondence<Natural,Element> and Iterable<Element> are both covariant in the type parameter Element. So Empty is assignable to Correspondence<Natural,T> and Iterable<T> for any type T. That's why Empty doesn't need a type parameter. The following code is well-typed: 

void printAll(String[] strings) {
    variable Iterator<String> i := strings.iterator();
    while (exists String s = i.head) {
        writeLine(s);
    	i := i.tail;
    }
}

Since both Empty and Sequence<String> are subtypes of Iterable<String>, the union type String[] is also a subtype of Iterable<String>.

Another cool thing to notice here is the return type of the first and value() operations of Empty. You might have been expecting to see Bottom? here, since they override supertype members of type T?. But as we saw in Part 1, Bottom? is just an abbreviation for Nothing|Bottom. And Bottom is the empty set, so the union Bottom|T of Bottom with any other type T is just T itself.

The Ceylon compiler is able to do all this reasoning automatically. So when it sees an Iterable<Bottom>, it knows that the operation first is of type Nothing, i.e. it is the value null.

Cool, huh?

Sequence gotchas for Java developers

Superficially, a sequence type looks a lot like a Java array, but really it's very, very different! First, of course, a sequence type Sequence<String> is an immutable interface, it's not a mutable concrete type like an array. We can't set the value of an element: 

String[] operators = .... ; 
operators[0] := "**"; //compile error

Furthermore, the index operation operators[i] returns an optional type String?, which results in quite different code idioms. To begin with, we don't iterate sequences by index like in C or Java. The following code does not compile: 

for (Natural i in 0..operators.size-1) { 
    String op = operators[i]; //compile error 
    ...
}

Here, operators[i] is a String?, which is not directly assignable to String.

Instead, if we need access to the index, we use the special form of for shown above. 

for (Natural i -> String op in entries(operators)) { 
    ...
}

Likewise, we don't usually do an upfront check of an index against the sequence length: 

if (i>operators.size-1) { 
    throw IndexOutOfBoundException();
} 
else {
    return operators[i]; //compile error
}

Instead, we do the check after accessing the sequence element: 

if (exists String op = operators[i]) { 
    return op;
} 
else {
    throw IndexOutOfBoundException();
}

We especially don't ever need to write the following: 

if (i>operators.size-1) { 
    return "";
} 
else {
    return operators[i]; //compile error
}

This is much cleaner: 

return operators[i] ? "";

All this may take a little getting used to. But what's nice is that all the exact same idioms also apply to other kinds of Correspondence, including Entries and Maps.

There's more...

In Part 5 we'll talk about union types and algebraic data types, type switching, and type inference.

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