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This page shows the concept of FlagInterfaces to solve the typesafety problem of the collections api. Note: this is an immature proposal and the syntax will change problably.

A flag interface is an interface with some restrictions it can not have methods and is not an independant type. It can only be used to refine the type of a normal interface or class.

Here a selection of the declarations in the collections api enhanced with FlagInterfaces:

flag CollectionAcces Immutable;
flag Replacable extends Immutable;
flag Reducable extends Immutable;
flag Appendable extends Immutable;
flag Mutable extends Replacable, Reducable, Appendable;

interface Collection@Mutable<E> has CollectionAcces{}
interface List@Mutable<E> extends Collection<E> {}
interface Iterator@Mutable<E> has CollectionAcces {}
interface ListIterator@Mutable<E> extend Iterator<E> {}

<E> boolean contains(Collection@Immutable<E>, E);
<E> int size(Collection@Immutable<E>);
<E> boolean remove(Collection@Reducable<E>, E);
<E> boolean add(Collection@Appendable<E>, E);
<E, CollectionAcces T> Iterator@T<E> iterator(Collection@T<E>);

<E> E get(List@Immutable<E>, int);
<E> E set(List@Replacable<E>, int, E);
<E> E removeAt(List@Reducable<E>, int);
<E> void add(List@Appendable<E>, int, E);
<E, CollectionAcces T> ListIterator@T<E> listIterator(List@T<E>);

class Vector@Mutable<E> implements List<E>{}
class Array@Replacable<E> implements List<E>{}

public void main(String[] args) {
List<String> mylist = new Vector(); //convenience we take the default CollectionAccess type of List here
// List@Mutable<String> mylist = new Vector(); //so this line is the same
mylist.add("abc");
Collection@Immutable<String> coll = mylist; //coll is a immutable view of mylist
coll.remove("abc");//error not allowed
Collection@Appendable<String> c = coll;// error not allowed
}

The same code after removing the syntatic sugar:

interface Immutable{}
interface Replacable extends Immutable{}
interface Reducable extends Immutable{}
interface Appendable extends Immutable{}
interface Mutable extends Replacable, Reducable, Appendable{}

interface Collection<E, T+ | Mutable <: T>{}
interface List<E, T+ | Mutable <: T> extends Collection<E,T> {}
interface Iterator<E, T+ | Mutable <: T>{}
interface ListIterator<E, T+ | Mutable <: T> extend Iterator<E,T> {}

<E> boolean contains(Collection<E,Immutable>, E);
<E> int size(Collection<E,Immutable>);
<E> boolean remove(Collection<E,Reducable>, E);
<E> boolean add(Collection<E,Appendable>, E);
<E, T> Iterator<E,T> iterator(Collection<E,T>);

<E> E get(List<E,Immutable>, int);
<E> E set(List<E,Replacable>, int, E);
<E> E removeAt(List<E,Reducable>, int);
<E> void add(List<E,Appendable>, int, E);
<E, T> ListIterator<E,T> listIterator(List<E,T>);

class Vector<E, T+ | Mutable <: T> implements List<E,T>{}
class Array<E, T+ | Replacable <: T> implements List<E,T>{}

public void main(String[] args) {
List<String,Mutable> mylist = new Vector();
mylist.add("abc");
Collection<String,Immutable> coll = mylist; //coll is a immutable view of mylist
coll.remove("abc");//error not allowed
Collection<String,Appendable> c = coll;// error not allowed
}
An experiment as proof of concept of the way of typing, the commented lines are type errors: package test;
interface I{}
interface J extends I{}
interface K extends J{}

class X<+T | K <: T>{}
class Y<+T | J <: T> extends X<T>{}
void foo(X<I>) = println("foo");
void bar(X<J>) = println("bar");

public void main(String[] args) {
X<K> x = new X();
X<J> y = x;
X<I> z = x;
Y<J> a = new Y();
Y<I> b = a;
//Y<K> c = a;
X<I> d = a;
X<J> e = a;
//X<K> f = a;
a.foo();
a.bar();
b.foo();
//b.bar();
d.foo();
//d.bar();
e.foo();
e.bar();
}

-- ArjanB - 26 Apr 2003

Topic FlagInterfaces . { Edit | Attach | Ref-By | Printable | Diffs | r1.7 | > | r1.6 | > | r1.5 | More }
Revision r1.1 - 26 Apr 2003 - 11:14 GMT - ArjanB
Parents: StdLib?
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