Add with_thunk enhancer for thunk-style dispatch

[demian51]

This adds a new enhancer called with_thunk, which enables dispatching not only plain actions but also thunk functions that take a lager::context.

This allows deferring logic such as conditional dispatch or side effects until runtime, similar to what redux-thunk provides in Redux.

Example usage:

store.dispatch([](lager::context<action> ctx) {
    // perform some effectful logic, then dispatch
    ctx.dispatch(some_action{});
});

[arximboldi]

Thank you a lot for this contribution!

It looks interesting, I'd be happy to merge it. Can you add a comment to with_thunk explaining this enhancer a little bit more?

Some questions:

1. I wonder, how does this affect the covariance that is allowed between action types, i.e. normally one can convert context<A> to context<B> as long as B is a subset of actions of A ?

2. Also, just out of curiosity, what is the use-case that motivated you to implement this? And why was using ctx.loop().post(...) not a viable alternative?

[demian51]

Thanks for the feedback!

1. I believe with_thunk does not affect it. The context is passed through unchanged, so conversions like context<A> to context<B> should still work as before.

2. It was mostly curiosity. While ctx.loop().post(...) can handle similar cases, it requires capturing the context manually. With with_thunk, the context is passed in, so it's easier to reuse named functions and keep the dispatch flow a bit more explicit. That said, I agree it might be overkill for simpler use cases.

Also, I have been thinking that this could make it possible to build something like Redux Toolkit's createAsyncThunk on top, though I'm not sure yet how useful that would really be.

I'll add a brief comment to with_thunk as suggested.

Let me know if you see any issues or if there's a better direction to consider.

[arximboldi]

Cool. Thank you for the clarifications. One last nitpick before merging, can you make the comment have this form, so that it can be referenced in the HTML docs later on:

/*!
 * The comment...
 */

[demian51]

Sure! I've updated the comment to follow the Doxygen format.

Thanks for the thorough review!