This chapter is from the book
This chapter is from the book
This chapter is from the book
1.4 Overview
This section presents a lightning tour of Dart. The goal is to familiarize you with all the core elements of Dart without getting bogged down in detail.
Programming language constructs are often defined in a mutually recursive fashion. It is difficult to present an orderly, sequential explanation of them, because the reader needs to know all of the pieces at once! To avoid this trap, one must first get an approximate idea of the language constructs, and then revisit them in depth. This section provides that approximation.
After reading this section, you should be able to grasp the essence of the many examples that appear in later sections of the book, without having read the entire book beforehand.
Here then, is a simple expression in Dart:
3
It evaluates, unsurprisingly, to the integer 3. And here are some slightly more involved expressions:
3 + 4 (3+4)*6 1 + 2 * 2 1234567890987654321 * 1234567890987654321
These evaluate to 7, 42, 5 and 1524157877457704723228166437789971041 respectively. The usual rules of precedence you learned in first grade apply. The last of these examples is perhaps of some interest. Integers in Dart behave like mathematical integers. They are not limited to some maximal value representable in 32 or 64 bits for example. The only limit on their size is the available memory.1
Dart supports not just integers but floating-point numbers, strings, Booleans and so on. Many of these built-in types have convenient syntax:
3.14159 // A oating-point number `a string' "another string - both double quoted and single quoted forms are supported" 'Hello World' // You've seen that already true false // All the Booleans you'll ever need [] // an empty list [0, 1.0, false, 'a', [2, 2.0, true, \b"]] // a list with 5 elements, the last of which is a list
As the above examples show, single-line comments are supported in Dart in the standard way; everything after // is ignored, up to the end of the line. The last two lines above show literal lists. The first list is empty; the second has length 5, and its last element is another literal list of length 4.
Lists can be indexed using the operator []
[1, 2, 3] [1]
The above evaluates to 2; the first element of a list is at index 0, the second at index 1 and so on. Lists have properties length and isEmpty (and many more we won’t discuss right now).
[1, 2, 3]. length // 3 [].length // 0 [].isEmpty // true ['a'].isEmpty // false
One can of course define functions in Dart. We saw our first Dart function, the main() function of “Hello World”, earlier. Here it is again
main(){
print(`Hello World');
}
Execution of a Dart program always begins with a call to a function called main(). A function consists of a header that gives its name and any parameters (our example has none) followed by a body. The body of main() consists of a single statement, which is a call to another function, print() which takes a single argument. The argument in this case is the string literal ‘Hello World’. The effect is to print the words “Hello World”.
Here is another function:
twice(x) => x * 2;
Here we declare twice with a parameter x. The function returns x multiplied by 2. We can invoke twice by writing
twice(2)
which evaluates to 4 as expected. The function twice consists of a signature that gives its name and its formal parameter x, followed by => followed by the function body, which is a single expression. Another, more traditional way to write twice is
twice(x) {
return x * 2;
}
The two samples are completely equivalent, but in the second example, the body may consist of zero or more statements—in this case, a single return statement that causes the function to compute the value of x*2 and return it to the caller.
As another example, consider
max(x, y){if (x > y) return x; else return y; }
which returns the larger of its two arguments. We could write this more concisely as
max(x, y) => (x > y) ? x : y;
The first form uses an if statement, found in almost every programming language in similar form. The second form uses a conditional expression, common throughout the C family of languages. Using an expression allows us to use the short form of function declarations.
A more ambitious function is
maxElement(a) {
var currentMax = a.isEmpty ?
throw 'Maximal element undefined for empty array' : a[0];
for (var i = 0; i < a.length; i++) {
currentMax = max(a[i], currentMax);
}
return currentMax;
}
The function maxElement takes a list a and returns its largest element. Here we really need the long form of function declaration, because the computation will involve a number of steps that must be sequenced as a series of statements. This short function will illustrate a number of features of Dart.
The first line of the function body declares a variable named currentMax, and initializes it. Every variable in a Dart program must be explicitly declared. The variable currentMax represents our current estimate of the maximal element of the array.
In many languages, one might choose to initialize currentMax to a known value representing the smallest possible integer, typically denoted by a name like MIN INT. Mathematically, the idea of “smallest possible integer” is absurd. However, in languages where integers are limited to a fixed size representation defined by the language, it makes sense. As noted above, Dart integers are not bounded in size, so instead we initialize currentMax to the first element of the list. If the list is empty, we can’t do that, but then the argument a is invalid; the maximal element of an empty list is undefined. Consequently, we test to see if a is empty. If it is, we raise an exception, otherwise we choose the first element of the list as an initial candidate.
Exceptions are raised using a throw expression. The keyword throw is followed by another expression that defines the value to be thrown. In Dart, any kind of value can be thrown—it need not be a member of a special Exception type. In this case, we throw a string that describes the problem.
The next line begins a for statement that iterates through the list.2 Every element is compared to currentMax in turn, by calling the max function defined earlier. If the current element is larger than currentMax, we set currentMax to the newly discovered maximal value.
After the loop is done, we are assured that currentMax is the largest element in the list and we return it.
Until now, this tutorial has carefully avoided any mention of terms like object, class or method. Dart allows you to define functions (such as twice, max and maxElement) and variables outside of any class. However, Dart is a thoroughly object-oriented language. All the values we’ve looked at — numbers, strings, Booleans, lists and even functions themselves are objects in Dart. Each such object is an instance of some class. Operations like length, isEmpty and even the indexing operator [] are all methods on objects.
It is high time we learned how to write a class ourselves. Behold the class Point, representing points in the cartesian plane:
class Point {
var x, y;
Point(a, b){x = a; y = b;}
}
The above is an extremely bare-bones version of Point which we will enrich shortly. A Point has two instance variables (or fields) x and y. We can create instances of Point by invoking its constructor via a new expression:
var origin = new Point(0, 0); var aPoint = new Point(3, 4); var anotherPoint = new Point(3, 4);
Each of the three lines above allocates a fresh instance of Point, distinct from any other. In particular, aPoint and anotherPoint are different objects. An object has an identity, and that is what distinguishes it from any other object.
Each instance of Point has its own copies of the variables x and y, which can be accessed using the dot notation
origin.x // 0 origin.y // 0 aPoint.x // 3 aPoint.y // 4
The variables x and y are set by the constructor based on the actual parameters provided via new. The pattern of defining a constructor with formal parameters that correspond exactly to the fields of an object, and then setting those fields in the constructor, is very common, so Dart provides a special syntactic sugar for this case:
class Point {
var x, y;
Point(this.x, this.y);
}
The new version of Point is completely equivalent to the original, but more concise. Let’s add some behavior to Point
class Point {
var x, y;
Point(this.x, this.y);
scale(factor) => new Point(x * factor, y * factor);
}
This version has a method scale that takes a scaling factor factor as an argument and returns a new point, whose coordinates are based on the receiving point’s, but scaled by factor.
aPoint.scale(2).x // 6 anotherPoint.scale(10).y // 40
Another interesting operation on points is addition
class Point {
var x, y;
Point(this.x, this.y);
scale(factor) => new Point(x * factor, y * factor);
operator +(p) => new Point(x + p.x, y + p.y);
}
Now we can write expressions like
(aPoint + anotherPoint).y // 8
The operator + on points behaves just like an instance method; in fact, it is just an instance method with a strange name and a strange invocation syntax.
Dart also supports static members. We can add a static method inside of Point to compute the distance between two points:
static distance(p1, p2) {
var dx = p1.x - p2.x;
var dy = p1.y - p2.y;
return sqrt(dx * dx + dy * dy);
}
The modifier static means this method is not specific to any instance. It has no access to the instance variables x and y, as those are different for each instance of Point. The method makes use of a library function, sqrt() that computes square roots. You might well ask, where does sqrt() come from? To understand that, we need to explain Dart’s concept of modularity.
Dart code is organized into modular units called libraries. Each library defines its own namespace. The namespace includes the names of entities declared in the library. Additional names may be imported from other libraries. Declarations that are available to all Dart programs are defined in the Dart core library which is implicitly imported into all other Dart libraries. However, sqrt() is not one of them. It is defined in a library called dart:math, and if you want to use it, you must import it explicitly.
Here is a complete example of a library with an import, incorporating class Point
library points;
import 'dart:math';
class Point {
var x, y;
Point(this.x, this.y);
scale(factor) => new Point(x * factor, y * factor);
operator +(p) => new Point(x + p.x, y + p.y);
static distance(p1, p2) {
var dx = p1.x - p2.x;
var dy = p1.y - p2.y;
return sqrt(dx * dx + dy * dy);
}
}
We have declared a library called points and imported the library dart:math. It is this import that makes sqrt available inside the points library. Now, any other library that wants to use our Point class can import points.
A key detail to note is that the import clause refers to a string ‘dart:math’. In general, imports refer to uniform resource indicators (URIs) given via strings. The URIs point the compiler at a location where the desired library may be found. The built-in libraries of Dart are always available via URIs of the form ‘dart:˙’, where ˙ denotes a specific library.
There is a lot more to Dart than what we’ve shown so far, but you should have a general idea of what Dart code looks like and roughly what it means. This background will serve you well as we go into details later in the book.