Abort and Assert

Abort and Assert

return and abort are two control flow constructs that end execution, one for the current function and one for the entire transaction.

More information on return can be found in the linked section

abort

abort is an expression that takes one argument: an abort code of type u64. For example:

abort 42

The abort expression halts execution of the current function and reverts all changes made to global state by the current transaction. There is no mechanism for "catching" or otherwise handling an abort.

Luckily, in Move transactions are all or nothing, meaning any changes to global storage are made all at once only if the transaction succeeds. Because of this transactional commitment of changes, after an abort there is no need to worry about backing out changes. While this approach is lacking in flexibility, it is incredibly simple and predictable.

Similar to return, abort is useful for exiting control flow when some condition cannot be met.

In this example, the function will pop two items off of the vector, but will abort early if the vector does not have two items

use std::vector;
fun pop_twice<T>(v: &mut vector<T>): (T, T) {
    if (vector::length(v) < 2) abort 42;

    (vector::pop_back(v), vector::pop_back(v))
}

This is even more useful deep inside a control-flow construct. For example, this function checks that all numbers in the vector are less than the specified bound. And aborts otherwise

use std::vector;
fun check_vec(v: &vector<u64>, bound: u64) {
    let i = 0;
    let n = vector::length(v);
    while (i < n) {
        let cur = *vector::borrow(v, i);
        if (cur > bound) abort 42;
        i = i + 1;
    }
}

assert

assert is a builtin, macro-like operation provided by the Move compiler. It takes two arguments, a condition of type bool and a code of type u64

assert!(condition: bool, code: u64)

Since the operation is a macro, it must be invoked with the !. This is to convey that the arguments to assert are call-by-expression. In other words, assert is not a normal function and does not exist at the bytecode level. It is replaced inside the compiler with

if (condition) () else abort code

assert is more commonly used than just abort by itself. The abort examples above can be rewritten using assert

use std::vector;
fun pop_twice<T>(v: &mut vector<T>): (T, T) {
    assert!(vector::length(v) >= 2, 42); // Now uses 'assert'

    (vector::pop_back(v), vector::pop_back(v))
}

and

use std::vector;
fun check_vec(v: &vector<u64>, bound: u64) {
    let i = 0;
    let n = vector::length(v);
    while (i < n) {
        let cur = *vector::borrow(v, i);
        assert!(cur <= bound, 42); // Now uses 'assert'
        i = i + 1;
    }
}

Note that because the operation is replaced with this if-else, the argument for the code is not always evaluated. For example:

assert!(true, 1 / 0)

Will not result in an arithmetic error, it is equivalent to

if (true) () else (1 / 0)

So the arithmetic expression is never evaluated!

Abort codes in the Move VM

When using abort, it is important to understand how the u64 code will be used by the VM.

Normally, after successful execution, the Move VM produces a change-set for the changes made to global storage (added/removed resources, updates to existing resources, etc.).

If an abort is reached, the VM will instead indicate an error. Included in that error will be two pieces of information:

• The module that produced the abort (address and name)

• The abort code.

For example

address 0x2 {
module example {
    public fun aborts() {
        abort 42
    }
}
}

script {
    fun always_aborts() {
        0x2::example::aborts()
    }
}

If a transaction, such as the script always_aborts above, calls 0x2::example::aborts, the VM would produce an error that indicated the module 0x2::example and the code 42.

This can be useful for having multiple aborts being grouped together inside a module.

In this example, the module has two separate error codes used in multiple functions

address 0x42 {
module example {

    use std::vector;

    const EMPTY_VECTOR: u64 = 0;
    const INDEX_OUT_OF_BOUNDS: u64 = 1;

    // move i to j, move j to k, move k to i
    public fun rotate_three<T>(v: &mut vector<T>, i: u64, j: u64, k: u64) {
        let n = vector::length(v);
        assert!(n > 0, EMPTY_VECTOR);
        assert!(i < n, INDEX_OUT_OF_BOUNDS);
        assert!(j < n, INDEX_OUT_OF_BOUNDS);
        assert!(k < n, INDEX_OUT_OF_BOUNDS);

        vector::swap(v, i, k);
        vector::swap(v, j, k);
    }

    public fun remove_twice<T>(v: &mut vector<T>, i: u64, j: u64): (T, T) {
        let n = vector::length(v);
        assert!(n > 0, EMPTY_VECTOR);
        assert!(i < n, INDEX_OUT_OF_BOUNDS);
        assert!(j < n, INDEX_OUT_OF_BOUNDS);
        assert!(i > j, INDEX_OUT_OF_BOUNDS);

        (vector::remove<T>(v, i), vector::remove<T>(v, j))
    }
}
}

The type of abort

The abort i expression can have any type! This is because both constructs break from the normal control flow, so they never need to evaluate to the value of that type.

The following are not useful, but they will type check

let y: address = abort 0;

This behavior can be helpful in situations where you have a branching instruction that produces a value on some branches, but not all. For example:

let b =
    if (x == 0) false
    else if (x == 1) true
    else abort 42;
//       ^^^^^^^^ `abort 42` has type `bool`