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I was trying this tutorial that implements a simple DEX. I am specially interested in the non payable function sell, that exchange custom tokens for ETH.

function sell(uint256 amount) public {
    require(amount > 0, "You need to sell at least some tokens");
    uint256 allowance = token.allowance(msg.sender, address(this));
    require(allowance >= amount, "Check the token allowance");
    token.transferFrom(msg.sender, address(this), amount);
    msg.sender.transfer(amount);
    emit Sold(amount);
}

It is always mandatory for the user to call the approve function of the ERC20 Smart Contract to be sold? Is there any way to do both steps (approve/sell) in a single transaction?

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It is mandatory to invoke approve() for a contract if it uses transferFrom().

Some tokens support a special value of token.approve(spender, uint256(-1)) to indicate approval for every operation.

There's a draft EIP 2612 - permit – 712-signed approvals the introduces permit() which is a sort of approve that can be combined with transferFrom() in a single call. It is recent so not every token has support for it, Uniswap and Dai has something similar.

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