Properly mock external functions in the tests

[aeryz]

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Describe your question in detail.

When writing Unit Tests, I want to be able to mock out the external dependencies but couldn't find a way of doing this without introducing additional logic to the production code or using lambdas. Here is an example:

public entry fun some_function() {
    let (x, y) = some_internal_logic();
    
    some_module::external_function_needs_to_be_mocked(x, y);

    some_more_internal_stuff();
}

Suppose that I want to mock out external_function_needs_to_be_mocked during tests, in other languages where you have Traits and Interfaces, mocking this is pretty trivial but since there are no interfaces in Move, this is not an option.

Even a macro like #[not(test_only)] would've solved this but it's also not there. What I ended up doing is taking out the implementation to a separate function which gets all the functions from some_module as lambda functions:

public entry fun some_function() {
    some_function_impl(|x, y| some_module::external_function_needs_to_be_mocked(x, y));
}

fun some_function(external_function: |u32, u32|) {
    let (x, y) = some_internal_logic();
    
    external_function(x, y);

    some_more_internal_stuff();
}

This way, the test function is able to mock the necessary function however it likes but still, this is far from ideal. I really hope there is a sane way of dealing these kind of situations easily.

What error, if any, are you getting?

No response

What have you tried or looked at? Or how can we reproduce the error?

No response

Which operating system are you using?

Linux (Ubuntu, Fedora, Windows WSL, etc.)

Which SDK or tool are you using? (if any)

N/A

Describe your environment or tooling in detail

No response