Added section to manual on interoperable polymorphism
Mike Gerwitz
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doc/interop.texi
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doc/interop.texi
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@ -20,6 +20,7 @@ have adopted ease.js for your own projects.
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@menu
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@menu
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* Using GNU ease.js Classes Outside of ease.js::
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* Using GNU ease.js Classes Outside of ease.js::
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* Prototypally Extending Classes::
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* Prototypally Extending Classes::
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* Interoperable Polymorphism::
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@end menu
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@end menu
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@ -128,3 +129,239 @@ chain:
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ease.js normally prevents this from happening automatically.
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ease.js normally prevents this from happening automatically.
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@end itemize
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@end itemize
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@node Interoperable Polymorphism
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@section Interoperable Polymorphism
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GNU ease.js encourages polymorphism through type checking. In the case of
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@ref{Prototypally Extending Classes,,prototypal subtyping}, type checks will
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work as expected:
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@float Figure, f:typecheck-protosub
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@verbatim
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var Foo = Class( {} );
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function SubFoo() {};
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SubFoo.prototype = Foo.asPrototype();
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SubFoo.constructor = Foo;
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var SubSubFoo = Class.extend( SubFoo, {} );
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// vanilla ECMAScript
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( new Foo() ) instanceof Foo; // true
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( new Subfoo() ) instanceof Foo; // true
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( new SubSubFoo() ) instanceof Foo; // true
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( new SubSubFoo() ) instanceof SubFoo; // true
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// GNU ease.js
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Class.isA( Foo, ( new Foo() ) ); // true
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Class.isA( Foo, ( new SubFoo() ) ); // true
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Class.isA( Foo, ( new SubSubFoo() ) ); // true
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Class.isA( SubFoo, ( new SubSubFoo() ) ); // true
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@end verbatim
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@caption{Type checking with prototypal subtypes of GNU ease.js classes}
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@end float
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Plainly---this means that prototypes that perform type checking for
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polymorphism will accept GNU ease.js classes and vice versa. But this is not
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the only form of type checking that ease.js supports.
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This is the simplest type of polymorphism and is directly compatible with
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ECMAScript's prototypal mode. However, GNU ease.js offers other features
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that are alien to ECMAScript on its own.
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@menu
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* Interface Interop:: Using GNU ease.js interfaces in conjunction with
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vanilla ECMAScript
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@end menu
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@node Interface Interop
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@subsection Interface Interop
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@ref{Interfaces}, when used within the bounds of GNU ease.js, allow for
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strong typing of objects. Further, two interfaces that share the same API
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are not equivalent; this permits conveying intent: Consider two interfaces
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@code{Enemy} and @code{Toad}, each defining a method @code{croak}. The
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method for @code{Enemy} results in its death, whereas the method for
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@code{Toad} produces a bellowing call. Clearly classes implementing these
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interfaces will have different actions associated with them; we would
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probably not want an invincible enemy that croaks like a toad any time you
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try to kill it (although that'd make for amusing gameplay).
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@float figure, f:interface-croak
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@verbatim
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var Enemy = Interface( { croak: [] } ),
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Toad = Interface( { croak: [] } ),
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AnEnemy = Class.implement( Enemy ).extend( /*...*/ ),
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AToad = Class.implement( Toad ).extend( /*...*/ );
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// GNU ease.js does not consider these interfaces to be equivalent
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Class.isA( Enemy, AnEnemy() ); // true
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Class.isA( Toad, AnEnemy() ); // false
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Class.isA( Enemy, AToad() ); // false
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Class.isA( Toad, AToad() ); // true
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defeatEnemy( AnEnemy() ); // okay; is an enemy
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defeatEnemy( AToad() ); // error; is a toad
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function defeatEnemy( enemy )
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{
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if ( !( Class.isA( Enemy, enemy ) ) ) {
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throw TypeError( "Expecting enemy" );
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}
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enemy.croak();
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}
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@end verbatim
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@caption{Croak like an enemy or a toad?}
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@end float
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In JavaScript, it is common convention to instead use @emph{duck typing},
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which does not care what the intent of the interface is---it merely cares
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whether the method being invoked actually exists.@footnote{``When I see a
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bird that walks like a duck and swims like a duck and quacks like a duck, I
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call that bird a duck.'' (James Whitcomb Riley).} So, in the case of the
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above example, it is not a problem that an toad may be used in place of an
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enemy---they both implement @code{croak} and so @emph{something} will
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happen. This is most often exemplified by the use of object literals to
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create ad-hoc instances of sorts:
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@float figure, f:interface-objlit
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@verbatim
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var enemy = { croak: function() { /* ... */ ) },
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toad = { croak: function() { /* ... */ ) };
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defeatEnemy( enemy ); // okay; duck typing
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defeatEnemy( toad ); // okay; duck typing
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// TypeError: object has no method 'croak'
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defeatEnemy( { moo: function() { /*...*/ } } );
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function defeatEnemy( enemy )
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{
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enemy.croak();
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}
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@end verbatim
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@caption{Duck typing with object literals}
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@end float
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Duck typing has the benefit of being ad-hoc and concise, but places the onus
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on the developer to realize the interface and ensure that it is properly
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implemented. Therefore, there are two situations to address for GNU ease.js
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users that prefer strongly typed interfaces:
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@enumerate
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@item
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Ensure that non-ease.js users can create objects acceptable to the
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strongly-typed API; and
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@item
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Allow ease.js classes to require a strong API for existing objects.
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@end enumerate
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These two are closely related and rely on the same underlying concepts.
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@menu
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* Object Interface Compatibility:: Using vanilla ECMAScript objects where
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type checking is performed on GNU ease.js
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interfaces
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* Building Interfaces Around Objects:: Using interfaces to validate APIs of
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ECMAScript objects
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@end menu
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@node Object Interface Compatibility
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@subsubsection Object Interface Compatibility
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It is clear that GNU ease.js' distinction between two separate interfaces
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that share the same API is not useful for vanilla ECMAScript objects,
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because those objects do not have an API for implementing interfaces (and if
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they did, they wouldn't be ease.js' interfaces). Therefore, in order to
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design a transparently interoperable system, this distinction must be
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removed (but will be @emph{retained} within ease.js' system).
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The core purpose of an interface is to declare an expected API, providing
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preemptive warnings and reducing the risk of runtime error. This is in
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contrast with duck typing, which favors recovering from errors when (and if)
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they occur. Since an ECMAScript object cannot implement an ease.js interface
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(if it did, it'd be using ease.js), the conclusion is that ease.js should
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fall back to scanning the object to ensure that it is compatible with a
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given interface.
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A vanilla ECMAScript object is compatible with an ease.js interface if it
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defines all interface members and meets the parameter count requirements of
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those members.
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@float Figure, f:interface-compat
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@verbatim
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var Duck = Interface( {
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quack: [ 'str' ],
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waddle: [],
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} );
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// false; no quack
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Class.isA( Duck, { waddle: function() {} } );
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// false; quack requires one parameter
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Class.isA( Duck, {
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quack: function() {},
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waddle: function() {},
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} );
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// true
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Class.isA( Duck, {
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quack: function( str ) {},
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waddle: function() {},
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} );
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// true
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function ADuck() {};
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ADuck.prototype = {
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quack: function( str ) {},
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waddle: function() {},
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};
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Class.isA( Duck, ( new ADuck() ) );
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@end verbatim
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@caption{Vanilla ECMAScript object interface compatibility}
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@end float
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@node Building Interfaces Around Objects
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@subsubsection Building Interfaces Around Objects
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A consequence of @ref{Object Interface Compatibility,,the previous section}
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is that users of GNU ease.js can continue to use strongly typed interfaces
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even if the objects they are interfacing with do not support ease.js'
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interfaces. Consider, for example, a system that uses @code{XMLHttpRequest}:
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@float Figure, f:interface-xmlhttp
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@verbatim
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// modeled around XMLHttpRequest
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var HttpRequest = Interface(
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{
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abort: [],
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open: [ 'method', 'url', 'async', 'user', 'password' ],
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send: [],
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} );
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var FooApi = Class(
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{
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__construct: function( httpreq )
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{
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if ( !( Class.isA( HttpRequest, httpreq ) ) )
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{
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throw TypeError( "Expecting HttpRequest" );
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}
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// ...
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}
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} );
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FooApi( new XMLHttpRequest() ); // okay
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@end verbatim
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@caption{Building an interface around needed functionality of
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XMLHttpRequest}
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@end float
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This feature permits runtime polymorphism with preemptive failure instead of
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inconsistently requiring duck typing for external objects, but interfaces for
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objects handled through ease.js.
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