1

When creating a smart contract in which a user must provide payment, can an amount of a token be required instead of Ether? Could you enable a user to have an option between Ether or a token?

3

In a smart contract written in solidity, there exists the payable modifier which allows the sender to attach Ether to a transaction which will be accepted by the contract as a fee for providing its service. There is no predefined modifier that would allow the same functionality for any ERC20 compliant token. However, you can achieve this functionality by implementing a workaround. You can accept a payment of amount in the token at tokenAddress as follows:

function provideService() external {
    ERC20Interface tokenContract = ERC20Interface(tokenAddress);
    require(tokenContract.transferFrom(msg.sender, this, amount));
    // provide service here
}

It is necessary that the client has approved the token transfer beforehand. You can find the complete ERC20 Token standard at https://theethereum.wiki/w/index.php/ERC20_Token_Standard for further information.

You could enable the option to pay with Ether or a token by writing separate functions for both, and implementing your application frontend accordingly.

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