Intro

From solidity to Yul

function getNumer() external pure returns(uint256) {
    return 42;
}

// in Yul

function getNumber() external pure returns(uint256) {
    uint256 x;
    
    assembly{
        x := 42
    }

    return x;
}

• yul doesn not have semi-colons

• assignment within assembly block uses " := "

• assembly code has visibility on local variables within function scope -> can "see" x, which was defined outside.

Assembly has only 1 type: the 32byte word

function getHex() external pure returns (uint256) {
    uint256 x;
    
    assembly{
        x := 0xa
    }
    
    return x;
}

• on compiling, x will return 10

• Hexadecimal 0xa is decimal 10

• Solidity is interpretating it as a decimal, when casted as uint256

Negative Example: String

function demoString() external pure returns (string memory) {
    string memory myString = "";
    
    assembly{
        myString := "hello world"
    }
    
    return myString;
}

• contract will compile, but calling demoString will result in an error

• myString was initialised as a memory variable -> stored on the heap (not the stack)

• the assignment within the assembly block is trying to assign a value to the pointer on the stack to a location in memory

Psitive Example: String

function demoString() external pure returns (bytes32) {
    bytes32 myString = "";
    
    assembly{
        myString := "hello world"
    }
    
    return myString;
    return string(abi.encode(myString));
}

• if we use return myStringsolidity will return a hexadecimal interpretations

• on abi.ecode we will get back the human readable version

Gotcha! Assembly assumes that the string value is <= 32 bytes. We will get to using longer strings later.