Starting Yul In A Solidity Contract

Yul, as you know by now, is written inside Solidity functions. To start off a Yul code in a Solidity function, you simply declare the assembly keyword, followed by curly braces. Your Yul (Inline Assembly) code can then come inside the curly braces.

assembly {
    // You can now write Yul here.
}

In a proper Solidity function, using the above, you'd have something like this.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    function yulFunction() public {
        assembly {
            // Yul code here.
        }
    }
}

You can have more than one assembly block in a function.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    function yulFunctions() public {
        assembly {
            // Yul code here.
        }

        assembly {
            // Another Yul code here.
        }
    }
}

Variables in Yul are declared with let keyword. A delcared variable that has not been initialized (y below) defaults to 0, 0x0000000000000000000000000000000000000000000000000000000000000000 in bytes32 until it is assigned a value. They are assigned using the := operator. Most importantly, there are no semicolons in Yul code. Whew!

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    function yulFunction() public {
        assembly {
            let x := 5
            let y // Defaults to 0.
            y := 10
        }
    }
}

🚨 Yul does not recognize Solidity global or state variables, it only recognized local variables within functions, function parameters and named return variables.