Good Elixir TDD resources?

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At the risk of stating the obvious:

  • When dealing with function values nothing much changes. Pass in your fake function instead of the real function and have the fake do what it needs to do.

  • However Elixir doesn’t have Objects. In JavaScript Objects conflate (mutable) state and behaviour. In Elixir state is stored in immutable data structures while behaviour resides in modules.

So it’s:

someValue = state.method(...params); // spread params array over arguments

versus

new_state = Module.function(old_state, params) # params is a parameter key list

So in Elixir the “fake object” situation may require a fake data structure (though at times you can simply use the real one) and will require a fake module, which means that your mockable boundary cannot hard code the module so that we can write:

new_state = apply(Module, :function, [old_state, params])

with the rough JavaScript equivalent:

someValue = (state.method).apply(state, params);

Now in Elixir that Module value has to come from somewhere and typically that is resolved at compile time (something that JavaScript doesn’t have) by grabbing the Module from the compilation environment - i.e. compilation in preparation for running tests versus running the application.

This is essentially what Mocking and Explicit Contracts and Mox are based on.

Consequently choosing mockable boundaries in Elixir is a much more deliberate process (hence “Explicit Contracts”) than it typically is in JavaScript with the ad hoc creation of injectable spies (where state has customized behaviour already bound to it).

(Another example)

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