Unrecognized early-occurring member function `_f`

[zhfnjust]

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.10;

contract DataLocations {
    uint[3] public arr;
    mapping(uint => address) map;
    struct MyStruct {
        uint foo;
    }
    mapping(uint => MyStruct) myStructs;

    function f() external {
        // call _f with state variables
        _f(arr, map, myStructs[1]);

    }

    function _f(
        uint[3] storage _arr,
        mapping(uint => address) storage _map,
        MyStruct storage _myStruct
    ) internal {
        // do something with storage variables
    }

[xhliu]

Same applies to property & struct, besides function.

Use first, define later.