Solidity variable definition: (bool sent, ) - solidity

Question

In Solidity, this sentence:
(bool sent, ) = msg.sender.call{value: _amount}("");
What is the sense of that "," after sent variable?
Any link to offcial documentation?
Many thanks.

Answer 1

It means you do not need to define both the variables to access the return values. You can access either of these variables as follows
(,uint256 alpha) = testFunction(); //access first returned variable
(uint256 beta,) = testFunction(); //access second returned variable
function testFunction() public pure returns(uint256,uint256){
return (1,2);
}

Answer 2

Solidity functions can return multiple variables of different types.
If you only want to keep one variable, then you can declare a variable and then use commas:
function multiValueFunction() public returns (bool, string memory, uint[] memory, uint {
//do something
return (true, "New String", [1,2], 21)
}
function differentFunction() public {
uint numberToKeep;
(,,,numberToKeep) = multiValueFunction();
}
Expected: numberToKeep = 21
Each comma represents the place of a returned variable (that is not kept)

Answer 3

It just means that the function will return two variables and you just want to store the first one.

Answer 4

This kind of syntax (variable type) is called Tuple In Python and other languages.
In Solidity,
they're not considered a proper type but can be used as a shorthand.
See the Official Docs: Destructuring Assignments and Returning Multiple Values
example:
uint256 amount = 10 ether;
require(
amount <= address(this).balance,
"Trying to withdraw more money than the contract has."
);
(bool success, ) = (msg.sender).call{value: amount}("");
require(success, "Failed to withdraw money from contract.");
above code, Roughly, Can be translated with basic if else statements like following:
uint256 amount = 10 ether;
bool success;
if(amount <= address(this).balance) {
success = true;
} else {
success = false;
return "Trying to withdraw more money than the contract has.";
}
if(success) {
(msg.sender).call{value: amount}("")
} else {
return "Failed to withdraw money from contract.";
}
here's another contract as example,
use this link https://remix.ethereum.org/#version=soljson-v0.6.9+commit.3e3065ac.js&optimize=false&gist=83bfbd85ef79387f760154999eb4f192&runs=200&evmVersion=null to play around in remix online
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.5.0 <0.8.0;
contract myContract {
uint index;
function myFunction() public {
( , , uint256 standardDepositAmount, , ) = returnTuple();
require(standardDepositAmount == 3);
// do something...
}
function returnTuple() public pure returns (uint, uint, uint, uint, uint) {
return (1, 2, 3, 4, 5);
}
}