I have committed to SVN a small (sequential) implementation of lazy,
functional, streams in X10. Streams can be possibly infinite, and are
materialized on demand, and support multiple readers.
Code is at courses/trunk/pppp11/, see package pppp.lib.lazystream.
The code uses thunks (expressions of type ()=>T, for some T) to express
delayed evaluation, and operator overloading to express computations on
streams. It uses static variables and methods to create recursive
(potentially infinite) data-structures.
Note: In X10 2.2, a local variable cannot be initialized with a value
that refers to the variable being initialized. Similarly there are
difficulties when trying to recursively initialize an instance field
because of escaping this. However, static variables and methods can be
used to construct recursive functional data-structures.
For example, the hamming sequence is implemented thus:
/**
* X = 1:omerge(2*X,omerge(3*X,5*X))
*/
public static def h():Stream[Int]=x;
static val x= new StreamImp[Int](1, ()=> (2*h())^(3*h())^(5*h()));
Here * is a pointwise multiplication operator on streams (defined for
types T s.t. T <: Arithmetic[T]), and ^ represents ordered merge
(defined for types T s.t. T <: Ordered[T]).
The sieve of Eratosthenes is expressed thus. Here % is an operator on
streams that takes a (T)=>Boolean filter and passes only those elements
which do not satisfy the filter.
static def primes()=primes(Utils.Gen(2 as Int, (n:Int)=>n+1));
static def multiple(p:Int)=(x:Int):Boolean=>x%p==0;
static def primes(r:Stream[Int]):Stream[Int] = {
val p = r.x(), s=r.y();
new StreamImp[Int](p, ()=>primes(s % multiple(p)))
}
Enjoy.
Vijay
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