Pure functions and permissions

Recall that pure functions have no side effects. Hence, they must not leak any permissions and implicitly transfer back all permissions mentioned in the precondition. While pure functions can require write permission, they cannot modify values, as this would be a side effect. Using wildcard permissions is idiomatic since all permissions held before a pure function call are still held after the call.

The pure and ghost function allZero returns whether all elements of an array behind a pointer are zero. After being allocated with new, the array is filled with zero values, and this can be asserted.

package ghostpure

const N = 42

/*@
ghost
requires forall i int :: 0 <= i && i < len(*s) ==> acc(&s[i], _)
decreases
pure func allZero(s *[N]int) bool {
    return forall i int :: 0 <= i && i < len(*s) ==> s[i] == 0
}
@*/

func client() {
	xs := new([N]int)
	// @ assert acc(xs, 1)
	// @ assert allZero(xs)
	// @ assert acc(xs, 1)
}

Pure functions do not leak any permissions.