Smart contract code is written in human-readable languages, such as Solidity and Vyper. When compiled and stored on the Ethereum Virtual Machine (EVM), it is converted into bytecode, a machine-recognizable format.
Attempting to read and write smart contracts on Ethereum without an interface to translate between human and machine-readable language would make interaction with the smart contracts difficult.
An ABI (Application Binary Interface) serves as an interface that translates human-readable instructions (function calls and parameters) into machine-executable code (calldata) and vice versa.
With the ABI, smart contracts can communicate with other applications and smart contracts. The ABI is produced by a smart contract compiler, outlining the entire scope of functions a smart contract has, the types of arguments those functions take, and the kind of values they return.
For example, when a user wants to initiate an ERC-20 transfer, they will call the function transfer(address _to, uint _value) public. The ABI will send a message in the form of bytecode, such as 0xa9059cbb0000000000000000000000000320b7c5b1c31d43de7ea9d13fbf9ff82067585d00000000000000000000000000000000000000000000000000000000af79e000, to the smart contract for execution.
Understanding what a smart contract is doing is possible without reading the code, simply by referring to the ABI. The ABI for the function call as represented in JSON:
{
"constant":false,
"inputs":[
{
"name":"_to",
"type":"address"
},
{
"name":"_value",
"type":"uint256"
}
],
"name":"transfer",
"outputs":[],
"payable":false,
"stateMutability":"nonpayable",
"type":"function"
}
You can obtain the ABI of a verified contract on Etherscan under the code tab.
For example:
