gistfile1.java

interface DotPair {}

class ConsList implements DotPair {
  private _first
  def getFirst() {
    _first instanceof Closure ? _first=_first.call() : _first
  }
  private _tail
  def getTail() {
    _tail instanceof Closure ? _tail=_tail.call() : _tail
  }

  ConsList(a, b) {
    _first = a;
    _tail = b;
  }

  def getAt(n) {
    assert n >= 0
    n==0 ? first : tail[n-1]
  }

  String toString() {
    def result = new StringBuilder("(")
    def list
    for (list=this; list instanceof ConsList; list=list.tail) {
      result << list.first.toString()+" ";
    }
    if (list != null) {
        result << ". " + list.toString();
    }
    result << ") ";
  }
}

ExpandoMetaClass.enableGlobally()
List.metaClass.asType = {Class c->
  if (c == ConsList) {
    if  (delegate.size() == 0) {
      return null
    }
    return new ConsList(delegate.first(), delegate.tail() as ConsList)
  }
  else if (c == DotPair) {
    return new ConsList(delegate[0], delegate[1])
  }
}

def zipWith(Closure f, x, y) {
  if (x==null || y==null) {
    null
  }
  else {
    new ConsList(f(x.first, y.first), {zipWith(f, x.tail, y.tail)})
  }
}

assert ([1,2,3] as ConsList).toString() == "(1 2 3 ) "
assert ([1,2] as DotPair).toString() == "(1 . 2) "

fibs = [0, [1, {zipWith({x,y->x+y}, fibs, fibs.tail)}]as DotPair]as DotPair
assert fibs[34] == 5702887