How do you read a simple Solidity smart contract?

Solidity in brief

Solidity is a high-level language for writing smart contracts.¹ It uses curly brackets, and it is built to run on the EVM.¹ It borrows ideas from C++, Python and JavaScript.¹

The contract

Below is the first example in Solidity's own documentation. It is called SimpleStorage.²

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.16 <0.9.0;

contract SimpleStorage {
    uint storedData;

    function set(uint x) public {
        storedData = x;
    }

    function get() public view returns (uint) {
        return storedData;
    }
}

Line by line

Line 1: the licence. The first line says the code is shared under the GPL version 3.0 licence.² Licence labels that machines can read matter when publishing code is the normal thing to do.²

Line 2: the version. A pragma is an instruction to the compiler. This one says the code is for Solidity 0.4.16 or newer, but older than 0.9.0.² That stops a future version of the compiler, which might make the code behave differently, from compiling it.²

Line 4: the contract. contract SimpleStorage starts the contract. In Solidity, a contract is code (its functions) and data (its state) at one address.²

Line 5: the stored number. uint storedData; creates a state variable called storedData.² uint means an "unsigned integer" of 256 bits: a whole number that starts at zero and cannot go below it.^2,3 Think of it as one box in a database.² State variables are kept permanently on the blockchain.⁴ Until someone sets it, it holds 0, the default value for a uint.⁵

Lines 7 to 9: set. The set function takes a number, called x, and saves it in storedData.² It is marked public, so it is part of the contract's interface and can be called from outside.⁶ People run a contract's functions by sending transactions, and transactions cost gas.^7,8

Lines 11 to 13: get. The get function hands back the stored number.² It is marked view, which is a promise not to change the contract's state.⁶ Functions like this are often called "getters".⁴

What it lets people do

This contract lets anyone store one number that anyone in the world can read.² Anyone can call set again and overwrite the number. The old number is still kept in the blockchain's history.²

To stop strangers changing a value, a contract needs access rules.² ethereum.org's vending machine example uses a require line, so that only the owner can refill the machine.⁷ A require check runs at the start of a function, and the function only goes ahead if the conditions are met.⁹