Mantra has type annotations and delegates now!

See http://www.linguamantra.org for more information.

Type annotations

Mantra is not a statically typed language like Java, though you'll see types specified in the code. These types are annotations kind of like "executable documentation". For example how many times have you done this in other dynamically typed languages like Ruby and Python:


f(a): ... # a is an int and f returns a list with blah blah

Mantra integrates such informal type information into the code like a statically typed language, but that information does not affect the validity of the statements in Mantra methods in any way. Mantra uses dynamic typing. "string s = 34;" compiles fine in mantra. The type information can be used to generate pre-and post-conditions that validate incoming and outgoing objects, relieving the programmer of that burden. The type information can also be used for optimization.

Delegates

This feature is essentially the same as a Ruby mixin ('cept no fields)

Delegates behave like abstract superclasses except that a class may delegate to more than one delegate (very much like multiple inheritance). Recall that abstract superclasses add functionality to subclasses and may refer to methods that don't have implementations in the abstract superclass. A subclass "includes" all of the methods and fields from the superclass except for those methods overridden in the subclass. A delegate differs only in that you cannot define fields and you do not have to provide abstract method declarations for method you call. References to methods become self.methodName() rather than the usual implied this.methodName().

Philosophically, delegates differ from abstract base classes in an important way: delegates are just code they do not talk about identity whereas the superclass/subclass relationship is normally one of identity ("is a" relationship). For example, a manager "is an" employee and a house cat "is a" feline (well, some people think they are human). In contrast, an input stream is not a kind of StreamSupport object, but it does want to reuse that code. Delegates in mantra provides the ability to share code without polluting the organization of the class hierarchy.

My desire for something like delegates came up when I realized that the exact same code for stream comparison should be included in all input streams. How do you do that without sharing the base class or duplicating code? One way is to can create a separate class containing the comparison code (call it a delegate). This code can be shared by multiple classes (the delegators) as long as they have pointers to an instance of the support (delegate) class. The support code reaches methods in the delegator through the use of a "back pointer". The following code illustrates the necessary infrastructure:


class Delegator {
    Delegate d = new Delegate(this);
    void foo() { bar(); }
    void bar() { d.bar(); } // delegator to other class
}

class Delegate {
    Delegator self;
    public Delegate(Delegator self) { this.self = self; }
    void bar() { System.out.println("bar()"); }
    void duh() { self.bar(); } // invokes bar from Delegator
}

All of that infrastructure is a hassle to do manually, but it works efficiently and clearly. Mantra delegates does all of this automatically.