How to charge fee in an entrypoint?

Getting Started
1

I want to be able to charge a fee through my contract, meaning collecting some tez (amount X) from sp.sender and perform an operation with 50% of X and collect the fee (to another address) for the remainder (50% of X in my assumption as well).
How can I do that?

Thank you!

2

I don’t know if there’s a specific method, but if I were you I will do something like this:

def feeExample(self, receiver):
    #send to receiver
    sp.send(receiver, (sp.amount/2))

    #collect fee
    sp.send(self.data.receiver, (sp.amount/2))

Bear in mind that the division can give you some type-error (Nat or Int), so you have to do more steps.

3

Thank you!
What is one of the recipient is a contract that will use the fee to run its own code?
I think I’m a bit stuck there too

4

If you mean you want to send tez to another contract you can use the contract address as well instead of wallet address.

5

So here’s my code below.
I think there are 2 errors.

  1. With the below code, I get a substraction underflow error. I assume it has to do with the fee as when I remove the related lines, the error is different.
  2. The second error I get is the balance too low error. I assume that it’s trying to transfer from the contract’s balance to the new contract.
    I might be wrong, but the reason I think that’s the case is that the error log tells me the balance is of 0.
        @sp.entrypoint
        def purchaseWithFee(self, address, param, price):
            sp.cast(price, sp.mutez)
            fee = sp.split_tokens(price, 1, 100)
            sp.send(self.data.recipientAddress, fee)
            sp.cast(param, sp.nat)
            contract = sp.contract(sp.nat, address, entrypoint="purchase")
            sp.transfer(
                param,
                price,
                contract.unwrap_some(error="Contract interface doesn't exist"))

Can you help me sort that out?
Thank you!

6

The second error cause is the one you thought, the balance of the contract is too low for the transaction.
About the first i’m not 100% sure but i think the problem is in calculation fee.
Is price more than 100?

7

How would I solve the first error then? Do I need to transfer the funds to the contract and then back?
Can’t I just transfer the balance of sp.amount - fee to the contract I call directly?

For the second, yes, price is a price in mutez. The sp.split_tokens(price, 1, 100) should calculate 1% of the of the price.

So to summarize, I need sp.sender to send price + fee, then transfer fee to a wallet A and price to a contract.

If you know how to achieve that with the new syntax, that would be great.

Thanks!

8 
import smartpy as sp

class FeeTransfer(sp.Contract):
    def __init__(self, admin):
        self.init(admin = admin)

    @sp.entrypoint
    def purchaseWithFee(self, cAddress, param):
        #calculate fee
        fee = sp.split_tokens(sp.amount, 1, 100)
        #send fee
        sp.send(self.data.admin, fee)
        #send price
        c = sp.contract(sp.TNat, cAddress, entrypoint="purchase").open_some()
        sp.transfer(param, (sp.amount - fee) , c)


class ContractExample(sp.Contract):
    def __init__(self):
        self.init( param = sp.nat(0))

    @sp.entrypoint
    def purchase(self, param):
        self.data.param = param
        

@sp.add_test(name = "HTLC")
def testHTLC():
    #set scenario
    sc = sp.test_scenario()
    #create object FeeTransfer
    #create admin
    bob = sp.test_account("bob")
    ft = FeeTransfer(bob.address)
    #create object ContractExample
    ce = ContractExample()
    #start scenario
    sc += ft 
    sc += ce

    #create user
    alice = sp.test_account("alice")

    #first transfer
    ft.purchaseWithFee(sp.record(cAddress = ce.address, param = sp.nat(10))).run(sender = alice, amount = sp.mutez(1000))

I wrote this code on legacy edition. But if suits to you is easily convertible.

I use param as sp.nat just to be faster.

Let me know

1 Like
9

Deleting that here as it might be a better question in the other thread