load(options): support both global-level and messager-level options, and add new option LoadFunc

[Kybxd]

• close load(options): support both global-level and messager-level options #261

[codecov]

Codecov Report

Attention: Patch coverage is 96.62921% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.55%. Comparing base (4b09f7c) to head (e3a551d).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
load/load.go	86.36%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   71.28%   71.55%   +0.26%     
==========================================
  Files          83       84       +1     
  Lines       10342    10408      +66     
==========================================
+ Hits         7372     7447      +75     
+ Misses       2404     2394      -10     
- Partials      566      567       +1     

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[wenchy]

Is it a good idea to use *bool to represent field presence? How about other programming languages, such as C++, C#, and TypeScript?

For example, tableau's C++ LoadOptions:
https://github.com/tableauio/loader/blob/7689f47671ae338338933970e251ac9c6ca31f5d/test/cpp-tableau-loader/src/protoconf/load.pc.h#L24-L42

[wenchy]

Use std::optional<bool> (C++17 and later) is a good solution.
Refer https://en.cppreference.com/w/cpp/utility/optional.html)
Example:

#include <optional>

class MyClass {
public:
    std::optional<bool> flag;

    void print() {
        if (flag.has_value()) {
            std::cout << "Flag is present and is " << (flag.value() ? "true" : "false") << "\n";
        } else {
            std::cout << "Flag is not present\n";
        }
    }
};

[wenchy]

In C#, the idiomatic way to represent the presence of a bool field (i.e., whether it has a value or not) is to use a nullable bool, which is bool?.

Explanation

bool can be either true or false.
bool? (nullable bool) can be true, false, or null.
null represents the absence of a value (field not set).

Example

class MyClass
{
    public bool? Flag { get; set; }

    public void Print()
    {
        if (Flag.HasValue)
        {
            Console.WriteLine($"Flag is present and is {Flag.Value}");
        }
        else
        {
            Console.WriteLine("Flag is not present");
        }
    }
}

Usage

var obj = new MyClass();

obj.Print();  // Output: Flag is not present

obj.Flag = true;
obj.Print();  // Output: Flag is present and is True

obj.Flag = false;
obj.Print();  // Output: Flag is present and is False

obj.Flag = null;
obj.Print();  // Output: Flag is not present

Summary
Use bool? (nullable bool) to represent a boolean field that may or may not be present.
null means "not present".
true or false means "present with that value".
This is the direct equivalent of C++'s std::optional<bool>.