Method Cascades in Dart

Posted by Gilad Bracha

(UPDATE: Method cascades are now implemented.)

The idea of a cascaded method invocation originates in Smalltalk, and has been proposed for Dart.  The motivation is to make it easier to write more fluent interfaces. 

Usually, fluent interfaces rely on method chaining. Say you want to add a large number of elements to a table:

myTokenTable.add("aToken");

myTokenTable.add("anotherToken");

// many lines elided here

// and here 

// and on and on

myTokenTable.add("theUmpteenthToken");

You might want to write this as

myTokenTable.add("aToken")

            .add("anotherToken").  

            // many lines elided here

            // and here 

            // and on and on

            .add("theUmpteenthToken");

but this requires that add() return the receiver, myTokenTable, instead of the element you just added. The API designer has to plan for this, and it may conflict with other use cases. With cascades, no one needs to plan ahead or make this sort of tradeoff.  The add() method can do its usual thing and return its arguments. However, you can get a chaining effect using cascades:

myTokenTable

  ..add("aToken");

  ..add("anotherToken");

// many lines elided here

// and here 

// and on and on

  ..add("theUmpteenthToken");

Here, ".." is the cascaded method invocation operation.  The ".." syntax invokes a method (or setter or getter) but discards the result, and returns the original receiver instead.

In brief, method cascades provide a syntactic sugar for situations where the receiver of a method invocation might otherwise have to be repeated. Instead of writing:

var address = getAddress();
address.setStreet(“Elm”, “13a”);
address.city = “Carthage”;
address.state = “Eurasia”
address.zip(66666, extended: 6666);
One may write

getAddress()
 ..setStreet(“Elm”, “13a”)
 ..city = “Carthage”
 ..state = “Eurasia”
 ..zip(66666, extended: 6666);

The sugar pays off the more complex the API, the longer the receiver of the cascaded method invocations, and the more methods are being directed toward that receiver. Cascades are expressions, so they also compose better than statements.  A cascade always evaluates to its initial receiver (the details are in the draft specification at the end of this document).

Below you'll find a number of examples, and a draft specification. As always, feedback is welcome. However, I'm about to go on vacation - so don't be offended if it takes a while before I respond to any of your comments.

Examples

The examples below show the use of the construct using a couple of indentation styles. All styles place cascaded access on a separate line, which improves readability.

Example 1

Consider using a (modified) String API:

String s = (new StringBuffer()
..add('Jenny ')
..add('I ')
..add('got ')
..add('your ')
..add('number')
).toString();

Example 2

Another common example would be using a builder API

final addressBook = (new AddressBookBuilder()
..name = 'jenny'
..email = 'jenny867@aol.com'
..phone = (new PhoneNumberBuilder()
  ..number = '867-5309'
  ..label = 'home'
 ).build()
).build();

Example 3

class Point {
num x;
num y;

Point() {
  x = 0;
  y = 0;
}
void scale(num factor) {
 x *= factor;
 y *= factor;
}

// Hack to display debugging output while within a cascade. Naturally you can't use
// the regular print statement.
void log(String msg) {
 print ('logged ($x, $y): $msg');
 }
}

void main(){

num x = 10;
num y = 42; // not used
var p = new Point();
p..log('start')
 ..x = x
 ..scale(10)
 ..log('scaled')
 ..x++
 ..y = x + p.x + p.y;
print('p.x = ${p.x}, p.y = ${p.y}. x = $x');
}


Output:

logged (0, 0): start
logged (100, 0): scaled
p.x = 101, p.y = 111, x = 10

Example 4

class Node {
String key;
Node(this.key);
Node left;
Node right;
}


void main() {
Node right = new Node('e');
Node root = new Node('root')
 ..left = (new Node('a')
   ..left = (new Node('b')
     ..left = new Node('c')
     )
     ..right = Node('d')
 )
 ..right = right;
print(root);
}

Example 5

The following example is perhaps more typical of what you might do in a web application.

var dq = document.query('#mypanel').queryAll('TABLE .firstCol');
dq.classes.remove('firstCol');
dq.style
  ..background = 'red'
  ..border = '2px solid black'
;
dq.nodes.add(new Element.html('<span>This cell is now red</span>')) // ?
;

It might be even nicer if cascades nested, but the nesting makes things hard to read. 

document.query('#mypanel').queryAll('TABLE .firstCol')
..classes.remove('firstCol');
..(style
   ..background = 'red'
   ..border = '2px solid black'

) ..nodes.add(new Element.html('<span>This cell is now red</span>')) // Not legal, just an idea for nesting

Maybe in the future we'll find a way to support such nesting in a truly readable way. For now we defer the issue and stick with one level of cascading.

Example 6

var dq = document.query('#myTable');
var qfc = dq.queryAll('.firstColumn');
qfc.style
   ..background = 'red'
   ..border = '2px solid black'
  ;
qfc.text = 'first column';

dq.queryAll('.lastColumn')
  ..style.background = 'blue'
  ..text = 'last column';

Example 7

view.node.style
..position = 'absolute'
..left = '${_measuredLeft}px'
..top = '${_measuredTop}px'
..width = '${_measuredWidth}px'
..height = '${_measuredHeight}px'
..zIndex = '${layoutParams.layer}'
;

Example 8

front
..beginPath()
..fillStyle = penColor
..arc(tx, ty, penWidth/2+2, 0, PI2, true)
..fill()
..moveTo(wx, wy)
..strokeStyle = "black"
..lineTo(tx, ty)
..closePath()
..stroke()
;

Example 9

document.queryAll('myDiv')
        ..classes.remove('off') // ?
        ..classes.add('on') // ?
;

Example 10

Array access works as well

element.attributes
      ..['foo'] = bar
      ..['baz'] = bla
;

Specification

Here is an initial take on the specification for this proposal. Places where the current spec changes are highlighted in yellow.

A cascaded method invocation has the form e..suffix

where suffix is a sequence of operator, method, getter or setter invocations.

A cascaded method invocation expression of the form e..suffix is equivalent to the expression (t){t.suffix; return t;}(e).

Grammar Changes

topLevelExpression:      assignableExpression assignmentOperator cascade
   | conditionalExpression ('..' cascadeSection)*
   ;

primary:      thisExpression    | super assignableSelector
   | functionExpression
   | literal
   | identifier
   | newExpression
   | constantObjectExpression
   | '(' topLevelExpression ')'    ;


cascadeSection:     (assignableSelector arguments*)+ (assignmentOperator expression)?
   ;