Member "concat" not found or not visible after argument-dependent lookup

Question

I'm trying to concatenate a series of strings but I'm getting the following error:

TypeError: Member "concat" not found or not visible after argument-dependent lookup in bytes storage pointer.
  --> contracts/Swaper0x.sol:28:23:
   |
28 |         return string(bytes(api0xUrl)
   |                       ^ (Relevant source part starts here and spans across multiple lines).


Error HH600: Compilation failed

But my concat() is defined before I call it. What could be the issue? Or the way that I'm calling it is not valid Solidity syntax?

//SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.0;


contract Swaper0x {

    string private api0xUrl = 'https://api.0x.org/swap/v1/quote';
    string private sellStr = '?sellToken=';
    string private buyStr = '&buyToken=';
    string private buyAmountStr = '&buyAmount=';
    

    function concat(bytes memory a, bytes memory b) internal pure returns (bytes memory) {
        return abi.encodePacked(a, b);
    }

    function getRequestSELLBUY(
        string memory _sellToken, 
        string memory _buyToken,
        string memory _buyAmount
    ) internal view returns (string memory) {
           
        return string(bytes(api0xUrl) // ---> error starts here
            .concat(bytes(sellStr))
            .concat(bytes(_sellToken))
            .concat(bytes(buyStr))
            .concat(bytes(_buyToken))
            .concat(bytes(buyAmountStr))
            .concat(abi.encodePacked(_buyAmount)));
    }
}

Answer 1

It fails because the compiler is looking for the concat function as a member of bytes type.

You can use concat as a regular function

return string(concat(bytes(api0xUrl),
                     concat(bytes(sellStr), 
                            ...)));

Another alternative is move concat to a library and use the using for mechanism.

library L {
    function concat(bytes memory a, bytes memory b) internal pure returns (bytes memory) {
        return abi.encodePacked(a, b);
    }
}

contract Swaper0x {
    using L for bytes;  // X.concat(Y) => concat(X, Y)

Now it should work as expected

return string(bytes(api0xUrl) // ---> no error here
        .concat(bytes(sellStr))