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iterator MyIter<T>(q: T) yields (x: T, y: T)
{
}
iterator MyIntIter() yields (x: int, y: int)
{
x, y := 0, 0;
yield;
yield 2, 3;
x, y := y, x;
yield;
}
iterator Naturals(u: int) yields (n: nat)
requires u < 25; // just to have some precondition
ensures false; // never ends
{
n := 0;
while (true)
{
yield n;
n := n + 1;
}
}
method Main() {
var m := new MyIter(12);
assert m.ys == m.xs == [];
var a := m.x;
if (a <= 13) {
print "-- ", m.x, " --\n";
}
var mer := m.MoveNext();
if (mer) {
mer := m.MoveNext();
mer := m.MoveNext(); // error
}
var n := new MyIntIter();
var patience := 10;
while (patience != 0)
invariant n.Valid() && fresh(n._new);
{
var more := n.MoveNext();
if (!more) { break; }
print n.x, ", ", n.y, "\n";
patience := patience - 1;
}
var o := new Naturals(18);
var remaining := 100;
while (remaining != 0)
invariant o.Valid() && fresh(o._new);
{
var more := o.MoveNext();
assert more;
print o.n, " ";
remaining := remaining - 1;
if (remaining % 10 == 0) { print "\n"; }
}
}
// -----------------------------------------------------------
class Cell {
var data: int;
}
iterator IterA(c: Cell)
requires c != null;
modifies c;
{
while (true) {
c.data := *;
yield;
}
}
method TestIterA()
{
var c := new Cell;
var iter := new IterA(c);
var tmp := c.data;
var more := iter.MoveNext();
assert tmp == c.data; // error
}
// -----------------------------------------------------------
iterator IterB(c: Cell)
requires c != null;
modifies c;
yield ensures c.data == old(c.data);
ensures true;
decreases c, c != null, c.data;
{
assert _decreases0 == c;
assert _decreases1 == (c != null);
assert _decreases2 == c.data; // error: c is not protected by the reads clause
var tmp := c.data;
if (*) { yield; }
assert tmp == c.data; // error: c is not protected by the reads clause
c.data := *;
}
method TestIterB()
{
var c := new Cell;
var iter := new IterB(c);
var tmp := c.data;
var more := iter.MoveNext();
if (more) {
assert tmp == c.data; // no prob
} else {
assert tmp == c.data; // error: the postcondition says nothing about this
}
}
// ------------------ yield statements, and_decreases variables ----------------------------------
iterator IterC(c: Cell)
requires c != null;
modifies c;
reads c;
yield ensures c.data == old(c.data);
ensures true;
decreases c, c, c.data;
{
assert _decreases2 == c.data; // this time, all is fine, because the iterator has an appropriate reads clause
var tmp := c.data;
if (*) { yield; }
if (*) { yield; }
assert tmp == c.data; // this time, all is fine, because the iterator has an appropriate reads clause
c.data := *;
}
method TestIterC()
{
var c := new Cell;
var iter := new IterC(c);
var tmp := c.data;
var more := iter.MoveNext();
if (more) {
assert tmp == c.data; // no prob
} else {
assert tmp == c.data; // error: the postcondition says nothing about this
}
iter := new IterC(c);
c.data := 17;
more := iter.MoveNext(); // error: iter.Valid() may not hold
}
// ------------------ allocations inside an iterator ------------------
iterator AllocationIterator(x: Cell)
{
assert _new == {};
var h := new Cell;
assert _new == {h};
SomeMethod();
assert x !in _new;
assert null !in _new;
assert h in _new;
ghost var saveNew := _new;
var u, v := AnotherMethod();
assert u in _new;
if {
case true => assert v in _new - saveNew ==> v != null && fresh(v);
case true => assert !fresh(v) ==> v !in _new;
case true => assert v in _new; // error: it may be, but, then again, it may not be
}
}
static method SomeMethod()
{
}
static method AnotherMethod() returns (u: Cell, v: Cell)
ensures u != null && fresh(u);
{
u := new Cell;
}
iterator DoleOutReferences(u: Cell) yields (r: Cell, c: Cell)
yield ensures r != null && fresh(r) && r !in _new;
yield ensures c != null && fresh(c); // but we don't say whether it's in _new
ensures false; // goes forever
{
var myCells: seq<Cell> := [];
while (true)
invariant forall z :: z in myCells ==> z in _new;
{
c := new Cell;
r := new Cell;
c.data, r.data := 12, 12;
myCells := myCells + [c];
_new := _new - {r}; // remove our interest in 'r'
yield;
if (*) {
_new := _new + {c}; // fine, since 'c' is already in _new
_new := _new + {u}; // error: this does not shrink the set
} else if (*) {
assert c.data == 12; // still true, since 'c' is in _new
assert c in _new; // as is checked here as well
assert r.data == 12; // error: it may have changed
} else {
parallel (z | z in myCells) {
z.data := z.data + 1; // we're allowed to modify these, because they are all in _new
}
}
}
}
method ClientOfNewReferences()
{
var m := new DoleOutReferences(null);
var i := 86;
while (i != 0)
invariant m.Valid() && fresh(m._new);
{
var more := m.MoveNext();
assert more; // follows from 'ensures' clause of the iterator
if (*) {
m.r.data := i; // this change is allowed, because we own it
} else {
m.c.data := i; // this change, by itself, is allowed
assert m.Valid(); // error: ... however, don't expect m.Valid() to survive the change to m.c.data
}
i := i - 1;
}
}
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