Canister called `ic0.trap` with message: 'losing precision'

Developers Motoko
1

What is this error? How to correct it?

Canister called `ic0.trap` with message: 'losing precision'.

Suspected code:

        switch(await CyclesLedger.icrc1_transfer({
            to = {owner = Principal.fromActor(this); subaccount = null};
            fee = null;
            memo = null;
            from_subaccount = ?(Principal.toBlob(user));
            created_at_time = ?(Nat64.fromNat(Int.abs(Time.now())));
            amount = Int.abs(Float.toInt(Float.fromInt(amountToMove) * (1.0 - env.revenueShare)));
        })) {
            case (#Err e) {
                Debug.trap("transfer failed: " # debug_show(e));
            };
            case (#Ok _) {};
        };
        // Pay to the vendor (TODO: Ensure no double fee later.)
        switch(await CyclesLedger.icrc1_transfer({
            to = {owner = revenueRecipient; subaccount = null};
            fee = null;
            memo = null;
            from_subaccount = ?(Principal.toBlob(user));
            created_at_time = ?(Nat64.fromNat(Int.abs(Time.now())));
            amount = Int.abs(Float.toInt(Float.fromInt(amountToMove) * env.revenueShare));
        })) {
            case (#Err e) {
                Debug.trap("transfer failed: " # debug_show(e));
            };
            case (#Ok _) {};
        };

env.revenueShare == 0.05.

2

did you try hardcoding the value instead of using the .env?
also could be, multiple data type conversions in Motoko often leads to these kind of errors

3

I use env/lib.mo module. That’s hardcoding.

“also could be, multiple data type conversions in Motoko often leads to these kind of errors” - no idea what you mean.

4

I mean hardcoding the float, and with data conversions I mean that sometimes (or often times) you have a float that you need to convert to Nat or Int or viceversa. But idk I’m just guessing based on your code.

5

We trap with this error message when you attempt to do a lossy convertion, e.g Nat64.fromNat(n) when n is not in the correct range for a Nat64 [0 .. 2^64). Not sure which operation is raising it here though.

6

Must be these two: Float.toInt(Float.fromInt(....

Motoko has 64-bit Floats, with a 52bit mantissa. When the exponent is not 0, there are $2^ exponent$ possible integers to convert to. My hunch is that this is causing the trouble.

7

I’ve found the error. It was in another part of the program:

I was converting Nat to Nat32 without ensuring 32-bit size.

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