[question]: how to use the borrow instruction

[dantengsky]

Hi,

I am trying to figure out how to use the borrow instruction.

Based on the code of zkvm/tests/zkvm.rs, I added a simple(failed) test case as follows

#[test]
fn borrow_output() {
    //inputs 10 units, borrows 5 units, outputs two (5 units)
    let flv = Scalar::from(1u64);
    let (preds, scalars) = generate_predicates(3);
    let borrow_prog = Program::build(|p| {
        p.input_helper(10, flv, preds[1].clone()) // stack: Value(10,1)
            .push(Commitment::blinded(5u64))      // stack: Value(10,1), qty(5)
            .var()                                // stack: Value(10,1), qty-var(5)
            .push(Commitment::blinded(flv))       // stack: Value(10,1), qty-var(5),   flv(1)
            .var()                                // stack: Value(10,1), qty-var(5),   flv-var(1)
            .borrow()                             // stack: Value(10,1), Value(-5, 1), Value(5,1)
            .output_helper(preds[0].clone())      // stack: Value(10,1), Value(-5, 1); outputs (5,1)
            .cloak_helper(2, vec![(5u64, flv)])   // stack:  Value(5,1)
            .output_helper(preds[2].clone())      // outputs (5,1)
    });
    build_and_verify(borrow_prog, &vec![scalars[1].clone()]).unwrap();
}

cargo test outputs

thread 'xxxx' panicked at 'called `Result::unwrap()` on an `Err` value: InvalidR1CSProof'

Where did I make a mistake?

Thanks a lot!

[oleganza]

Thanks for reporting, I will look into that!

[oleganza]

Hi, this PR #333 fixes this bug. Will merge soon.

[oleganza]

This is fixed now, thank you again!

[dantengsky]

My apologies for the slow reply, really grateful for your help!