Function RETURNS not returning anything without VIEW

[Newbee2025]

just learned about VIEW function in solidity. I wanted to see if the RETURNS keyword returns something even without VIEW.
this is the code i wrote:

pragma solidity 0.8.7;  

contract Simpl {
function funview(int a) public returns(int b)
{
    b=a*2;
}
}  

in the image, I called the function funview by passing 10. I can see that the value of b has been correctly calculated as 20, but this was not returned by the function. why is that?
expecting guidance from you.

[bobanm]

Short answer

Your frontend [any code outside of EVM] can get function return values only if a function is declared as view or pure.

Full answer

When a function does NOT contain view or pure modifier, it tells EVM that it is a transaction which modifies the state. Under the hood, your frontend library [e.g. Ethers.js] uses low-level RPC method eth_sendTransaction, which costs gas.

On the other hand, functions that DO have view or pure as modifier are read-only, which means they do not modify state. They are executed using eth_call RPC method that does not spend gas. So, they are not transactions, but calls.

Any time your frontend invokes a smart contract function which modifies the state, it will get transaction object as a response, no matter if the function returns a number, or anything. Whatever is the real return value of a such a function is available only to another smart contract which called that function. Or to put it in other words, the return value is not accessible outside of EVM, i.e. in your frontend.

The only way for your frontend to access the return value from a function which is not read-only, is to make the function save that response also to the log by emitting an event, and then read from the log.

To make it more clear with an example, check out this Chainlink VRF demo I pushed yesterday:

https://github.com/bobanm/randomness-logger

If you look inside RandomnessLogger.sol you will find this function:

function requestRandomNumbers (uint32 callbackGasLimit) external {

    uint requestId = vrfCoordinator.requestRandomWords(KEY_HASH, vrfSubscriptionId, MINIMUM_REQUEST_CONFIRMATIONS, callbackGasLimit, NUM_WORDS);
    emit NumberRequested(requestId, block.timestamp, msg.sender);
}

If I made this function return requestId, the value would not have been accessible at all. Any script would get transaction object as response, and because the function is external, it is not accessible by another smart contract. I didn't want to store the requestId in state, so the function emits it through NumberRequested event. Then get-numbers-history.js script retrieves the value from the log:

const filter = RandomnessLogger.filters.NumberRequested()
filter.fromBlock = CONTRACT_BLOCK_DEPLOYED
const logs = await ethers.provider.getLogs(filter)

[Newbee2025]

Ohkay.
I have just started with solidity, so I did not get 100%(event, emit etc.), but I do understand the crux of your answer.
Either I HAVE to use VIEW/PURE or send that variable to a log, and then read from it.

Thanks a lot for your help buddy.

[bobanm]

You got the idea. Your solution will depend on the specific requirements:

1. Does the function response contains data that is immutable?
2. Who needs to use that piece of data? [external vs. smart contract]

It is much cheaper to store data in log, than to write in contract's storage. But it is not possible to read log from a smart contract, only from external code. And obviously, logs are immutable.

Don't overthink it now. Continue with the basics, and these details will clear up as you progress 🙂

Good luck.

[PatrickAlphaC]

Awesome explainer!!