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method M(n: nat) {
assert 0 <= n;
}
method Main() {
M(25);
M(-25); // error: cannot pass -25 as a nat
}
var f: nat;
method CheckField(x: nat, y: int)
requires 0 <= y;
modifies this;
{
var y: nat := y;
assert 0 <= f;
while (0 < y)
{
f := f + 1;
if (15 < f) {
f := f - 12;
}
y := y - 1;
}
assert 0 <= f;
f := x; // no problem
f := x + 3; // no problem here either
f := x - 3; // error: cannot prove RHS is a nat
}
method Generic<T>(i: int, t0: T, t1: T) returns (r: T) {
if (0 < i) {
var n: nat := 5;
var j := Generic(i-1, n, -4);
assert 0 <= j; // error: the result is an int, not a nat
var q := FenEric(n, -4);
assert 0 <= q; // error: the result is an int, not a nat
}
r := t1;
}
function method FenEric<T>(t0: T, t1: T): T
{
t1
}
datatype Pair<T> = Pr(T, T);
method K(n: nat, i: int) {
match (Pair.Pr(n, i)) {
case Pr(k, l) =>
assert k == n; // fine: although the type of k is int, we know it's equal to n
assert 0 <= k;
assert 0 <= l; // error: l is an int
}
}
datatype List<T> = Nil | Cons(nat, T, List<T>);
method MatchIt(list: List<object>) returns (k: nat)
{
match (list) {
case Nil =>
case Cons(n, extra, tail) =>
var w := MatchIt(tail);
assert 0 <= w;
assert 0 <= n; // fine
assert 0 <= n - 10; // error: possible assertion failure
}
var m := Sum(list);
assert 0 <= m;
k := m;
}
class GenEric<T> {
var f: T;
}
function method GE<T>(d: GenEric<T>): bool { true }
method TestGenEric() {
var ge;
if (ge != null) {
var b := GE(ge);
var n: nat := ge.f; // error: the generic instantiation uses int, not nat
}
}
function method Sum(list: List<object>): nat
{
match list
case Nil => 0
case Cons(x, y, tail) => x + Sum(tail)
}
function BadSum(list: List<object>): nat
{
match list
case Nil => 0
case Cons(x, y, tail) => x + BadSum(tail) - 30 // error: may not be a nat
}
function Abs(x: int): nat
{
if 0 <= x then x else -x
}
// ----- Here are tests that the type of the result value of a function is known by the
// ----- time the well-formedness of the function's specification is checked.
function TakesANat(n: nat): bool
{
n < 29
}
function Naturally(): nat
ensures TakesANat(Naturally()); // the wellformedness of this check requires
{
17
}
function Integrally_Bad(): int
ensures TakesANat(Integrally_Bad()); // error: well-formedness check fails
{
17
}
function Integrally_Good(): int
ensures 0 <= Integrally_Good();
ensures TakesANat(Integrally_Good()); // here, the needed information follows from the preceding ensures clause
{
17
}
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