Your First Contract

Here is the token. Function first: a lookup table from a key to a value, with no way to list what is inside it. The name: a mapping. The first maps an address to a number, and that single line holds every WETH balance there has ever been.

The second is a mapping whose values are mappings, so it takes two addresses to reach one number. It is called allowance. Say allowance[Ana][Ben] holds 5. What does that mean?

A function with no name. In this dialect that is the one that runs when ETH arrives and nothing else matches. Modern Solidity writes it as receive(), and you will meet both spellings in real code. Its body calls deposit, so sending ETH straight to the address does the same as asking politely.

One word on line 16 is the permission. payable is the marker that lets ETH arrive alongside a call. Without it the contract refuses the transfer and the whole call fails. The word external answers a different question, who may call this, and says nothing about money.

Three lines and the wrapper works. msg.value is how much ETH came with this call, already added to the contract's balance by the time the code runs. msg.sender is whoever is calling right now. Line 20 adds the one to the other's entry.

Line 21 announces it. Notice what is absent: nothing is sent back to you. The token is the entry in the ledger. Your WETH is a number in a mapping, and that is all any token has ever been.

The reverse trip. First a rule. Function first: a statement that checks a condition and, if it is false, stops everything and rewinds the call as though it had never happened. The name: require.

Then the entry goes down, then the ETH goes out, then the announcement. The order matters more than it looks. Tap each of the three lines to read what it does.

One line, and it is the most elegant thing in the file. totalSupply is how much WETH exists. It does not add up the ledger. It returns the contract's own ETH balance, because those two numbers are the same number by construction.

view promises the function only reads and changes nothing, which is why anyone can call it for free. Ana asks why one WETH is always worth exactly one ETH. What in this contract makes that true?

approve writes a number into the allowance table. The caller is naming someone who may move their WETH and how much. Four lines, one of which is the announcement. It stores a permission and moves nothing at all.

transfer is a single line. It hands the work straight to transferFrom with the source set to you, so the two functions are one machine with two doors. Everything you have read so far now meets in one function.

The longest function, and everything above was preparation for it. Line 48 checks the source has enough. Then, only when the caller is not the source, lines 51 and 52 check the permission and reduce it by what is being spent.

Line 50 holds an oddity worth carrying. uint(-1) is how this dialect writes the largest number it can hold. When an allowance is set to that, the check still runs but the subtraction is skipped, so the permission never runs down. Then two lines move the entry and one announces it.

Look at the whole board. Every line is lit, and you can say what each one does. Paperwork, the box, the receipts, two ledgers, a way in, a way out, a supply that is simply the balance, and a permission system so someone else can move your entry.

That is a real contract, deployed, used by nearly everything, and there is no part of it left that you have not read. One honest question before you go.

This is the shape of the whole season. Real contracts, chosen small first, read line by line, with a prediction before every reveal. The vocabulary grows every week and the method never changes.

It ends with you opening a verified contract you have never seen and reading it cold. Today you read your first, and it was a real one.